5.10 — std::cin, extraction, and dealing with invalid text input

Most programs that have a user interface of some kind need to handle user input. In the programs that you have been writing, you have been using std::cin to ask the user to enter text input. Because text input is so free-form (the user can enter anything), it’s very easy for the user to enter input that is not expected.

As you write programs, you should always consider how users will (unintentionally or otherwise) misuse your programs. A well-written program will anticipate how users will misuse it, and either handle those cases gracefully or prevent them from happening in the first place (if possible). A program that handles error cases well is said to be robust.

In this lesson, we’ll take a look specifically at ways the user can enter invalid text input via std::cin, and show you some different ways to handle those cases.

std::cin, buffers, and extraction

In order to discuss how std::cin and operator>> can fail, it first helps to know a little bit about how they work.

When we use operator>> to get user input and put it into a variable, this is called an “extraction”. The >> operator is accordingly called the extraction operator when used in this context.

When the user enters input in response to an extraction operation, that data is placed in a buffer inside of std::cin. A buffer (also called a data buffer) is simply a piece of memory set aside for storing data temporarily while it’s moved from one place to another. In this case, the buffer is used to hold user input while it’s waiting to be extracted to variables.

When the extraction operator is used, the following procedure happens:

  • If there is data already in the input buffer, that data is used for extraction.
  • If the input buffer contains no data, the user is asked to input data for extraction (this is the case most of the time). When the user hits enter, a ‘\n’ character will be placed in the input buffer.
  • operator>> extracts as much data from the input buffer as it can into the variable (ignoring any leading whitespace characters, such as spaces, tabs, or ‘\n’).
  • Any data that can not be extracted is left in the input buffer for the next extraction.

Extraction succeeds if at least one character is extracted from the input buffer. Any unextracted input is left in the input buffer for future extractions. For example:

If the user enters “5a”, 5 will be extracted, converted to an integer, and assigned to variable x. “a\n” will be left in the input stream for the next extraction.

Extraction fails if the input data does not match the type of the variable being extracted to. For example:

If the user were to enter ‘b’, extraction would fail because ‘b’ can not be extracted to an integer variable.

Validating input

The process of checking whether user input conforms to what the program is expecting is called input validation.

There are three basic ways to do input validation:

  • Inline (as the user types)
    • Prevent the user from typing invalid input in the first place.
  • Post-entry (after the user types)
    • Let the user enter whatever they want into a string, then validate whether the string is correct, and if so, convert the string to the final variable format.
    • Let the user enter whatever they want, let std::cin and operator>> try to extract it, and handle the error cases.

Some graphical user interfaces and advanced text interfaces will let you validate input as the user enters it (character by character). Generally speaking, the programmer provides a validation function that accepts the input the user has entered so far, and returns true if the input is valid, and false otherwise. This function is called every time the user presses a key. If the validation function returns true, the key the user just pressed is accepted. If the validation function returns false, the character the user just input is discarded (and not shown on the screen). Using this method, you can ensure that any input the user enters is guaranteed to be valid, because any invalid keystrokes are discovered and discarded immediately. Unfortunately, std::cin does not support this style of validation.

Since strings do not have any restrictions on what characters can be entered, extraction is guaranteed to succeed (though remember that std::cin stops extracting at the first non-leading whitespace character). Once a string is entered, the program can then parse the string to see if it is valid or not. However, parsing strings and converting string input to other types (e.g. numbers) can be challenging, so this is only done in rare cases.

Most often, we let std::cin and the extraction operator do the hard work. Under this method, we let the user enter whatever they want, have std::cin and operator>> try to extract it, and deal with the fallout if it fails. This is the easiest method, and the one we’ll talk more about below.

A sample program

Consider the following calculator program that has no error handling:

This simple program asks the user to enter two numbers and a mathematical operator.

Enter a double value: 5
Enter one of the following: +, -, *, or /: *
Enter a double value: 7
5 * 7 is 35

Now, consider where invalid user input might break this program.

First, we ask the user to enter some numbers. What if they enter something other than a number (e.g. ‘q’)? In this case, extraction will fail.

Second, we ask the user to enter one of four possible symbols. What if they enter a character other than one of the symbols we’re expecting? We’ll be able to extract the input, but we don’t currently handle what happens afterward.

Third, what if we ask the user to enter a symbol and they enter a string like “*q hello”. Although we can extract the ‘*’ character we need, there’s additional input left in the buffer that could cause problems down the road.

Types of invalid text input

We can generally separate input text errors into four types:

  • Input extraction succeeds but the input is meaningless to the program (e.g. entering ‘k’ as your mathematical operator).
  • Input extraction succeeds but the user enters additional input (e.g. entering ‘*q hello’ as your mathematical operator).
  • Input extraction fails (e.g. trying to enter ‘q’ into a numeric input).
  • Input extraction succeeds but the user overflows a numeric value.

Thus, to make our programs robust, whenever we ask the user for input, we ideally should determine whether each of the above can possibly occur, and if so, write code to handle those cases.

Let’s dig into each of these cases, and how to handle them using std::cin.

Error case 1: Extraction succeeds but input is meaningless

This is the simplest case. Consider the following execution of the above program:

Enter a double value: 5
Enter one of the following: +, -, *, or /: k
Enter a double value: 7

In this case, we asked the user to enter one of four symbols, but they entered ‘k’ instead. ‘k’ is a valid character, so std::cin happily extracts it to variable op, and this gets returned to main. But our program wasn’t expecting this to happen, so it doesn’t properly deal with this case (and thus never outputs anything).

The solution here is simple: do input validation. This usually consists of 3 steps:

1) Check whether the user’s input was what you were expecting.
2) If so, return the value to the caller.
3) If not, tell the user something went wrong and have them try again.

Here’s an updated getOperator() function that does input validation.

As you can see, we’re using a while loop to continuously loop until the user provides valid input. If they don’t, we ask them to try again until they either give us valid input, shutdown the program, or destroy their computer.

Error case 2: Extraction succeeds but with extraneous input

Consider the following execution of the above program:

Enter a double value: 5*7

What do you think happens next?

Enter a double value: 5*7
Enter one of the following: +, -, *, or /: Enter a double value: 5 * 7 is 35

The program prints the right answer, but the formatting is all messed up. Let’s take a closer look at why.

When the user enters “5*7” as input, that input goes into the buffer. Then operator>> extracts the 5 to variable x, leaving “*7\n” in the buffer. Next, the program prints “Enter one of the following: +, -, *, or /:”. However, when the extraction operator was called, it sees “*7\n” waiting in the buffer to be extracted, so it uses that instead of asking the user for more input. Consequently, it extracts the ‘*’ character, leaving “7\n” in the buffer.

After asking the user to enter another double value, the “7” in the buffer gets extracted without asking the user. Since the user never had an opportunity to enter additional data and hit enter (causing a newline), the output prompts all get run together on the same line, even though the output is correct.

Although the above problem works, the execution is messy. It would be better if any extraneous characters entered were simply ignored. Fortunately, that’s easy to do:

Since the last character the user entered must be a ‘\n’, we can tell std::cin to ignore buffered characters until it finds a newline character (which is removed as well).

Let’s update our getDouble() function to ignore any extraneous input:

Now our program will work as expected, even if we enter “5*7” for the first input -- the 5 will be extracted, and the rest of the characters will be removed from the input buffer. Since the input buffer is now empty, the user will be properly asked for input the next time an extraction operation is performed!

Error case 3: Extraction fails

Now consider the following execution of the calculator program:

Enter a double value: a

You shouldn’t be surprised that the program doesn’t perform as expected, but how it fails is interesting:

Enter a double value: a
Enter one of the following: +, -, *, or /: Enter a double value: 

and the program suddenly ends.

This looks pretty similar to the extraneous input case, but it’s a little different. Let’s take a closer look.

When the user enters ‘a’, that character is placed in the buffer. Then operator>> tries to extract ‘a’ to variable x, which is of type double. Since ‘a’ can’t be converted to a double, operator>> can’t do the extraction. Two things happen at this point: ‘a’ is left in the buffer, and std::cin goes into “failure mode”.

Once in ‘failure mode’, future requests for input extraction will silently fail. Thus in our calculator program, the output prompts still print, but any requests for further extraction are ignored. The program simply runs to the end and then terminates (without printing a result, because we never read in a valid mathematical operation).

Fortunately, we can detect whether an extraction has failed and fix it:

That’s it!

Let’s integrate that into our getDouble() function:

Note: Prior to C++11, a failed extraction would not modify the variable being extracted to. This means that if a variable was uninitialized, it would stay uninitialized in the failed extraction case. However, as of C++11, a failed extraction due to invalid input will cause the variable to be zero-initialized. Zero initialization means the variable is set to 0, 0.0, “”, or whatever value 0 converts to for that type.

Error case 4: Extraction succeeds but the user overflows a numeric value

Consider the following simple example:

What happens if the user enters a number that is too large (e.g. 40000)?

Enter a number between -32768 and 32767: 40000
Enter another number between -32768 and 32767: The sum is: 32767

In the above case, std::cin goes immediately into “failure mode”, but also assigns the closest in-range value to the variable. Consequently, x is left with the assigned value of 32767. Additional inputs are skipped, leaving y with the initialized value of 0. We can handle this kind of error in the same way as a failed extraction.

Note: Prior to C++11, a failed extraction would not modify the variable being extracted to. This means that if a variable was uninitialized, it would stay uninitialized in the failed extraction case. However, as of C++11, an out-of-range failed extraction will cause the variable to be set to the closest in-range value.

Putting it all together

Here’s our example calculator with full error checking:


As you write your programs, consider how users will misuse your program, especially around text input. For each point of text input, consider:

  • Could extraction fail?
  • Could the user enter more input than expected?
  • Could the user enter meaningless input?
  • Could the user overflow an input?

You can use if statements and boolean logic to test whether input is expected and meaningful.

The following code will test for and fix failed extractions or overflow:

The following will also clear any extraneous input:

Finally, use loops to ask the user to re-enter input if the original input was invalid.

Note: Input validation is important and useful, but it also tends to make examples more complicated and harder to follow. Accordingly, in future lessons, we will generally not do any kind of input validation unless it’s relevant to something we’re trying to teach.

5.11 -- Introduction to testing your code
5.9 -- Random number generation

219 comments to 5.10 — std::cin, extraction, and dealing with invalid text input

  • JMEllis

    Ah! I see. I guess there isn't a neater way :(

    Thanks :)

  • JMEllis


    Love the site. Anyway, I'm just trying to understand std::cin a bit better. Could you take a look at the following version of the getDouble() function?

    #include <limits>
    #include <iostream>

    double getDouble() {
        double a;
        do {
            std::cout << "Enter a number: ";
            std::cin >> a;
            std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
        } while (;
        return a;

    This works if you enter an appropriate double value, but goes into an infinite loop if you don't. The question is why?

    Thanks :)

    • extraction fails
      ignore fails, because the stream is in a bad state
      fail() returns true, the loop loops
      clear() clears the error, the stream is in a good state
      cin tries to extract the same character that made it fail before

  • Helliarc

    Awesome! I feel more confident now!  I built a calculator on my own during the "operators" lessons, but was having trouble figuring out error checking on the double types inputs.  Before this lesson, I did figure out how to check the operator on my own, this is what I did to check the operator before this lesson!  Thanks a ton:

  • Arthur

    here is my calculator modified again from what I learn in this lesson. I am curious if the for loop I used is generally accepted as an alternative to a while loop?

    I also am curious if the example given for the calculator with full error correction is  missing a divide by zero error ? it seems to me that

    4 / 0  input would error out?

    I am wondering about std::cout << " \n \r " type commands for taking control of console output but am unsure what that type of command it is called, I thought I saw a list but cant seem to find it in the tutorial.

    • Hi Arthur!

      * Line 23: Should be

      It does the same, but is easier to understand.

      * @main

      * Line 127: Looks like a null statement, merge it with line 126.

      • Arthur

        awesome thanks for taking the time that is way more efficient. with what you did @ main here I am wondering if the same should be applied to initializing other variables , merging lines 9-10, lines 32-33, lines 70-73? as well as a lot of changes in the ball drop ch5 quiz. I see the URL is on the topic of formatting in code::blocks, I find that formatting questions tend to be shut down as 'opinion' so when I try to find suggestion it's either a closed thread or a google bible, which translates as there is no right way or there is only one way lol. I still need to learn code::blocks, will bookmark that thread for reference and tinker with the suggestion there and looking at your example try a smaller tab setting. when I can actually read more complicated programs I'll spend some time looking at opensource examples, that will give me a better understanding of what reads well and why.

        • Arthur

          I guess 9-10, 32-33, 70-73 cant be merged the variables cant be initialized with a call for input.

        • > I guess 9-10, 32-33, 70-73 cant be merged

          I don't care how you format your code for the most part. As you said, it's personal preference.
          What I'm asking for is consistent formatting. Every formatting convention requires consistent formatting. There's no point in having a convention when you don't follow it half of the time. The auto-formatter will take care of this for you. Set it up to your liking and add a keyboard shortcut.
          I'm talking about things like line 94-116, 71-72, 171.

          • Arthur

            I see what you mean, especially for the two do while loops , with misplaced brackets and a while floating off into the distance :-D , I will look in to how to setup/use auto formatting.

  • Jon

    Hello! Just a quick note, I think in Error Case 3 in the code section under where you say "Let’s integrate that into our getDouble() function:", it's missing another std::cin.ignore(32767,'\n'); statement in between lines 15 and 16. Looks like it's back in there in the final example.

  • Faruk Hodzic

    Hi your tutorials have been very helpful.Thanks for the good c++ tutorials.I would like if someone could look at my code for the calculator and say what they think and feedback is much appreciated.




    • Hi Faruk!

      * Initialize your variables with uniform initialization
      * Don't pass 32767 to @std::cin.ignore. Pass @std::numeric_limits<std::streamsize>::max()
      * Misleading indention. Line 40ff is unreachable. Use curly brackets and the auto-formatting feature of your editor.
      * Use ++prefix unless you need postfix++

  • Alex A

    Hi Alex,

    i believe that there is a linguistic mistake in this chapter as following the explanation of the extraction operator ">>" you have put
    "Extraction succeeds if at least one character can be extracted from the input buffer. Any unextracted input is left in the input buffer for future extractions. For example:"
    now this is a bit misleading as the term at least would imply that if i had declared an integer variable then the user input "abc1" while the characters abc cannot be extracted and assigned to an integer variable the "1" can.
    I have just quickly tested this with std::cin on my compiler and it doesnt seem to pick up the 1 but silently fail, so i presume that it is the linguistics here that are incorrect (or do not make it clear at least to myself a native english speaker with a degree in english language so im sure im not alone!)

    loving the tutorials so far :)

    • Alex

      I changed "can be" to "is", since in the case of "abc1" as input for an integer, no characters are extracted. Hopefully that's enough to clarify.

  • Shri

    I'm on c++ 11 or above.

    When running the program Alex has mentioned in Error Case 4,

    I get the following output (which is dissimilar to Alex's):


    Enter a number between -32768 and 32767: 32769
    Enter another number between -32768 and 32767: The sum is: 32767
    Program ended with exit code: 0

    Please help to understand this behavior.

    When I edit the program to:
    int main()
        std::int8_t x { 0 }; // x is 16 bits, holds from -32768 to 32767
        std::cout << "Enter a number between -128 and 127: ";
        std::cin >> x;
        std::int16_t y { 0 }; // y is 16 bits, holds from -32768 to 32767
        std::cout << "Enter another number between -32768 and 32767: ";
        std::cin >> y;
        std::cout << "The sum is: " << x + y << '\n';
        return 0;

    I get the following output:


    Enter a number between -128 and 127: 130
    Enter another number between -32768 and 32767: The sum is: 79
    Program ended with exit code: 0

    OUTPUT1 and OUTPUT2 seem to be having different kind of logic.
    In OUTPUT1, variable x was assigned the maximum value that could be assigned to an int16_t variable when cin extracted an "overflow" value.
    But the same is not seen in OUTPUT2.
    Shouldn't this be consistent irrespective of the variable type?

    • Alex

      1) It looks like the way std::cin and operator<< handle invalid inputs changed in C++11. Now instead of leaving the variable alone, it always initializes it with some value. I've updated the lesson text.
      2) int8_t is typically a typedef for char, and chars tend to have different handling than int. Avoid int8_t.

  • Shri

    For the above program, if the input given after "Enter a number" is anything between "5a" to "5f" the output is as shown below :

    Enter a number
    Enter a character
    Program ended with exit code: 0

    But if the input given after "Enter a number" is anything more than "5f" like "5g" or "5h" the output is correct as shown below:
    Enter a number
    Enter a character
    Program ended with exit code: 0

    I'm not sure why this is so. NOTE: the issue is seen for "5i" too.

  • Shri

    I have this code :

    I get this output:
    Enter a number
    Enter an alphabet
    Program ended with exit code: 0

    Even if I have initialized variable "integer" to "1". When I enter "d5" as the input for the first cin, how does "0" get assigned to variable "integer"?

    • Shri

      I now understand that a failed extraction leads to "zero-initialization" which is zero assignment in this case, due to which I get the output posted in my previous comment.

  • Rasikko

    When using either while(1) or while(true), I get a warning that the condition is constant. Obviously because by design the loop is supposed to be infinite for this sort of thing.. but my compiler(VC2013) isn't having any of that. How to get around this?

  • Clapfish

    Hi Alex,

    In section 'Error case 1: Extraction succeeds but input is meaningless':

    "2) If so, return the value to the user."

    Did you mean to say:

    "2) If so, return the value to the caller."?

  • Piyush

    Do these statements change the range of values the variable can store.
    int16_t  a;
    int8_t  b;
    int32_t  c;
    It is good if they do. I have not tried these declarations before

    • Yes they do. The number after "int" is the number of bits the type can store.
      @std::int8_t can store 2^8 values
      @std::int16_t can store 2^16 values
      @std::int32_t can store 2^32 values

      Note however, that these types aren't guaranteed to be implemented by the compiler. See the "Types" list over here for alternatives

  • I'm in love with alex's tutor

    Selam Dear Mr Alex/Nas
    Depending on the quiz at section 6.9a I was trying to write a full code that fulfills all the lows and dealing with invalid text input. As you see the below code i expected to have an out put that loops the question again and again if the input is invalid or fails. and print the how much names i am gonna register if not fails, however what i have seen is different from my expectation.To save your time i don't write the out put c/s you already know how it outputs. So Please tell where  my mistake was started? It takes me a lot of time to fix and i cant so far.

    God bless you both!!!!!!!!

    • Hi!

      * Don't use "using namespace"
      * Initialize your variables with uniform initialization
      * Line 13 should be moved behind into an else block, because it won't work if extraction failed.
      * Line 21: cannot return true here, because you cleared all errors beforehand

      • I'm in love with alex's tutor

        Dear Mr. Nas!
        Thank you so much!
        I have learnt more from your answers. It works with do{} while(!length);
        Stay blessed Sir.

  • Robert

    This reads weird:

    When we use operator>> to get user input and put it into a variable, this is called an “extraction”. operator>> is accordingly called the extraction operator when used in this context.


    When we use the >> operator to get user input and put it into a variable, this is called an “extraction”. The >> operator is accordingly called the extraction operator when used in this context.

  • Andi

    Hi Alex,
    Just a minor fix:
    The indent of the curly bracket in line 14 of the function under "Types of invalid text input - Error case 1: Extraction succeeds but input is meaningless" and line 46 of the code under "Putting it all together" should be one tab less. :)

  • I'm in love with alex's tutor

    Dear Mr Alex/Nas
    My code below is working to me very nicely  since i entered '+' operator (NOTE:-I did that for demo purpose only)
    But please i need you to revise  my usage for while statement and waiting for your regretful comment if there is a batter or shortest way than mine. Please focus  only on a while statement. (to save your time)

    stay bless more and more!!!!!!!!!!!!

    • Hi!

      There's nothing wrong with your loop.

      * Line 7, 26, 30: Initialize your variables with uniform initialization
      * Line 13, 18, 28, 32: Use @std::numeric_limits<std::streamsize>::max(). See the documentation of @std::basic_istream::ignore
      * Line 19: Double line feed
      * Line 38: Don't use @system. If you want to pause the program for whatever reason, use @std::cin.get

      • I'm in love with Alex's tutorial

        Dear Mr.Nas!
        I have no words for you both! I really blessed to have you!
        I have taken all your great comments. I am enjoying programming b/s of you!!!!
        Always my prayer great  God to be with you!!!!

      • I'm in love with alex's tutor

        Dear Mr Alex/NAS!
        Kindly and very thank fully ,This is my last question for this section.
        In the above example ("putting it all together") in the 1st function double get Double() What was the reason for declaring std::cin.ignore(32767,'\n'); after "else" (line 21). B/s i think it increases the number of errors and "try again" statements by one.
        For example in the current function definition above,if the user enters double number 3.0 but if there was extraneous input at the buffer (Eg. '\n') the input fails and the function asks the user to input again.
        But if we declare std::cin.ignore(32767,'\n');as an input validation  after the cin>>x; (line ten), it deletes extraneous input ('\n') and pass the user's double value to be extracted that enables the if( statement to be false and execute what we want at the 1st time the user inputs.
        Would  you explain more on what was preferable cin.ignore() usage from using it after else(line 21) or after Cin>> extraction(line ten).
           Thank you so much in advance for your usual and constant kindness my dears!!!!!!

        • @std::basic_istream::ignore cannot clear the stream when it's in a bad state.
          If you called it on a bad stream before calling @std::basic_istream::clear it wouldn't do anything. But you don't want to call @std::basic_istream::clear on a stream that is in a good state. Alex' code is correct.

          • I'm in love with alex's tutor

            DEar Mr Nas!
            I becoming to love questioning you b/s i have gotten so beloved answers whenever i asked you.
            Always i am thank full for your perpetual kindness, Sir!

  • Bullwinkle

    Great tutorial.  Thank you so much for your time and effort!  Excellent information.

    In this particular article, I'm not sure I agree with the statement, "However, parsing strings and converting ... is only done in rare cases."  It's probably true, but shouldn't be (necessarily).  Playing the "semantics police" here... there are some of us who believe that if you ask for numeric input and the user enters non-digit characters, that should be flagged as a format error and cause a reprompt.  For example:

    This is "probably" a typo... should the value be 1245, 12345 (# = <shift>3), or some other value?  Rather than allow 12 to be successfully extracted and "#45" discarded, the entire input should be rejected.

    Checking for this was a simple task in straight 'C'... the 2nd parameter of strtol(3) (i.e. **endp) allows the caller to examine the character that caused the numeric conversion to terminate.  If it isn't an ASCII NUL ('\0'), then it is a format error (assuming you've already stripped off trailing whitespace).  Additionally, the 3rd parameter of strtol(3) (i.e. the radix) allows you to restrict the notion of a valid digit.  For example, if I ask for a decimal value, I don't want to read something like 0x5D6.  I merely set the radix to 10 and go from there.

    [Note: Of course, when I say that it's a "simple" task, I'm ignoring the fact that you have to use fgets(3) to read in a line of input, strip off trailing whitespace, etc.  Which is why I don't disagree when you say parsing strings can be challenging. :-)]

    So... is there a simple way to accomplish these sorts of format checks in C++ "on the fly", without using getline() first and then parsing the string directly?  I don't see how (consider the input string "123  456" for example, which has intermediate whitespace)... but then, I wouldn't be here if I was a C++ guru, right? :-))  Any insights appreciated.

    • Hi Bullwinkle!

      @std::strtol and it's variants are still present in cpp. But they aren't required when dealing with streams, because those have built-in functionality for parsing.

      > "#45" discarded
      It's not discarded, it remains in the stream, allowing you to check for what you described using @std::istream::peek.

      std::basic_istream::peek -

  • Arush Agarampur

    I just wanted to ask, I usually see people use

    for cin.ignore. Is that better, or is it the same as this?

    • Hi Arush!

      What you mean is

      Yes, that's not only better, but it's the only correct way. Passing this value to @std::cin.ignore causes it to skip all input until it hits the delimiter (Usually a line feed). Passing any other value will stop skipping characters after that value has been reached and there could potentially be more input in the stream, causing your program to malfunction.
      Alex uses 32767, because it's easier, you should use

      std::basic_istream::ignore -

  • Donlod

    When asking the user for input, the following code results in having to make an input at least twice.

    Using the debugger it seems that the line

    is  causing the problem. After it is executed i have to make an input. Then i have to make an input again for the actual cin >> number.

    • nascardriver

      Hi Donlod!

      @std::cin.clear clears the error flag. There's no reason to call it when no error occurred.
      @std::cin.ignore clears the input buffer. There's no reason to call it when there's no trailing data in the buffer.
      Change these things and your problem is gone.

      I can't find it in the documentation, but it seems as if calling functions that read from the input buffer when the input buffer is empty wait for input to be made.

      • Donlod

        Thanks for the reply.
        So is it possible to do this the "clean way"?
        I came up with two possible solutions:

        But if i remember correctly, we should avoid using break, continue in loops, right? Also i think the code is more self-explanatory when actually having a clearer loop condition.

        And this:

        Here i do not like the fact to check twice for

        • nascardriver

          > we should avoid using break, continue in loops, right?
          I can't find a rule for this and I don't see anything wrong with using break/continue.

          Your first solution is the better one, because, as you already said, you're checking twice in the second solution. There are three issues with it though
          * You're using an int for the while-loop's condition
          * You're using a magic number
          * You have duplicate code

          I don't think there's a nicer solution to this, but since this code is independent of the input extraction itself we can write a wrapper function for it so we don't need to repeat this all over the place.

          • Jack

            Would seem better to rename @cleanStream to something like notCleanStream, considering you are returning true if extraction fails. If extraction fails you do not have a cleanStream. Either that or change the function to return false if the extraction failed so the while loop reads as


            • nascardriver

              It's the verb "clean", not the adjective.

  • Joe

    Hi guys,

    I am having an issue with extraction handling.
    My issue is that it is not working.

    this is a snip of what I am trying and it does not handle my failures.  Anything I type, gets passed.

    • nascardriver

      Hi Joe!

      * Initialize your variables.
      * Don't use goto.

      Anything you can type can be converted to a char.
      Letters? Sure
      Numbers? Sure, a char is a number.
      Special characters? Yep, those fit in a char
      Strings? The first character can be extracted.

      If you only want to allow a specific subset of characters you'll need to filter them manually. eg. for only allowing lower case:

      Have a look at the ascii table to see the order of characters

      • Joe

        Thank you again nascardriver.  I really appreciate the help and input.

        Tell me, what is the item with which you initialized char c with? \x00

        • nascardriver

          \x interprets the next two characters as a hexadecimal ascii code. So '\x00' is just 0, you could use 0, I use '\x00' when I use a char to hold a character and 0 when it holds a number, do what you like.

      • Joe

        Nevermind, I figured it out.  Thank you again.

        How about this?

        I know its clunky, but I have more to learn then I can cut it to better size.

        • Joe

          actually, this works

          • nascardriver

            Keep in mind that 32767 doesn't have any special effect to @std::cin.ignore. In a real program std::numeric_limits<std::streamsize>::max() should be used, this causes @std::cin.ignore to ignore everything up to the next newline.

  • Peter Baum

    "Error Case 3", code line 15:

    it wasn't clear to me why x was "good" after this failure.

    • nascardriver

      Hi Peter!

      • Peter Baum

        Hi nascardriver!  (Are you really a nascardriver?)

        I think the problem for me was the comment wording and the way it was laid out.  Perhaps the following will be clearer to some people:

  • WiseFool

    I have a question.  In this section we used, cin.clear(), and cin.ignore() functions, but where can I look to see what other similar associated functions that there might be for std::cin, and any other #include library commands, types or classes, and what they do and how they work?  (Also, I'm just curious - when we use std::cin, we don't need to add .function() or parenthetical parameters, so how does it just "know" to take input using the >> operator?  Is this just something built in to compilers to treat certain library things the same way they would a built-in operation?)  Thank you in advance for an answer.

    • nascardriver

      Hi WiseFool!

      cppreference and cplusplus have good documentations and examples to many functions.

    • Peter Baum

      A great question.  It would be nice to include some of this information about documentation, as well as a little summary table of these functions/methods within the lesson.

  • Nick

    How's this for error handling (and code writing i.e any bad habits or better way to do it)?

    • nascardriver

      Hi Nick!

      I had to include two additional header to allow proper use of NaN. The same could be achieved by defining an arbitrary value to be used as NaN.

  • Sirius

    Very minor nitpick with the ordering of the error handling explanations; the 3rd and 4th single line error descriptions match up to the 4th and 3rd explanations. I.e.:

    "Input extraction succeeds but the user overflows a numeric value." - 3rd in list
    "Input extraction fails (e.g. trying to enter ‘q’ into a numeric input)." - 4th in list

    Then below:

    "Error case 3: Extraction fails" - matches 4th in list above
    (...some text...)
    "Error case 4: Extraction succeeds but the user overflows a numeric value" - matches 3rd in list above
    (...some text...)

    Sorry if I haven't made this very clear, or if it's intentional.

  • Michael

    Hi Alex,
    why ignore up to 32767 characters? what's special about this number?

    • Alex

      It's the largest 2-byte signed integer. We need a number larger than 1 just in case there are multiple characters to ignore. So what should we pick? 2? But what if there are 3 characters? 3? What if there are 4? So we pick the largest integer value that's guaranteed to fit in an integer.

  • Raj

    when the user enters 5*7 , y does *7/n remains in buffer and not only *7..why has the new line character appeared in the buffer without user sending it in cin?

    • Alex

      When you hit enter on your keyboard, that key is represented as a '\n' character in ASCII. Therefore, \n is in the buffer because you put it there. :)

  • Kewal

    What if I want to accept integers from the user until he enters space or presses enter twice, extraction doesn't fail in this case, how can I do it?

    • Alex

      This is an odd request, and I'm not sure why you'd want to do this. But my first thought is to read things in character by character, and reconstruct the integers the user is entering from the number characters.

  • Prashanth

    Hey, why do you use the number 32,767 exactly in cin.ignore() function. Does the number has to do anything with 'int' datatype?

  • antiriad7

    We use Enter button to confirm our input. Is there any other ways to confirm our input and make our program go to next step (I am trying to confirm input without trailing '\n')?

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