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4.1 — Blocks (compound statements)

Blocks (compound statements)

A block of statements, also called a compound statement, is a group of statements that is treated by the compiler as if it were a single statement. Blocks begin with a { symbol, end with a } symbol, and the statements to be executed are placed in between. Blocks can be used any place where a single statement is allowed. No semicolon is needed at the end of a block.

You have already seen an example of blocks when writing functions:

Blocks can be nested inside of other blocks. As you have seen, the if statement executes a single statement if the condition is true. However, because blocks can be used anywhere a single statement can, we can instead use a nested block of statements to make the if statement execute multiple statements if the condition is true!

If the users enters the number 3, this program prints:

Enter an integer: 3
3 is a positive integer (or zero)
Double this number is 6

If the user enters the number -4, this program prints:

Enter an integer: -4
-4 is a negative integer
The positive of this number is 4

It is even possible to put blocks inside of blocks inside of blocks:

There is no practical limit to how many nested blocks you can have. However, it is generally a good idea to try to keep the number of nested blocks to at most 3 (maybe 4) blocks deep. If your function has a need for more, it’s probably time to break your function into multiple smaller functions!

Summary

Blocks allow multiple statements to be used wherever a single statement can normally be used. They are extremely useful when you need a set of statements to execute together.

4.1a -- Local variables, scope, and duration
Index
3.x -- Chapter 3 comprehensive quiz

38 comments to 4.1 — Blocks (compound statements)

  • Aditi

    Hi!

    In the third program , what result would we get if we enter a negative integer ? Who don’t we have an else statement for negative integers ? Also , isn’t it necessary that every if statement needs to be followed by an else statement ?

    • Aditi

      I think the reason is because as we have taken data type as int, which is already unsigned and accepts only positive integral values. But if we had already specified the data type earlier , why did we need an If statement for accepting only positive numbers? Even if we did not have an if statement for accepting positive integral values , and the user entered a negative number , wouldn’t we get an error in either case ?

    • nascardriver

      Hi Aditi!

      > what result would we get if we enter a negative integer
      No output

      > isn’t it necessary that every if statement needs to be followed by an else statement
      No, you don't need an else-statement

      > we have taken data type as int, which is already unsigned
      An int is signed by default

      Run the program yourself, play around with some numbers and place breakpoints to get a better understanding of what's happening.

  • Ali Dahud

    Hi Alex!

    could you go through my code and correct the mistakes i made?
    oh and please explain this piece:
            tempNum = larger;
            larger = smaller;
            smaller = tempNum;

    • nascardriver

      Hi Ali!

      I've added comments addressing the issue with your code.
      Try correcting it and post your progress, if you have any questions feel free to ask.

      > please explain this piece
      It's swapping the values of @larger and @smaller. Imagine you have three baskets in front of you, one contains a red apple, one contains a green apple and one is empty.
      You want the red apple to be in the green apple's basket and the green apple to be in the red apple's basket.
      You are only allowed to hold one apple at a time.

      PS: Please post your code in the corresponding lesson next time, this quiz is in lesson 4.1a

      • Ali Dahud

        isn't that some kind of sorting?

        • nascardriver

          No it's not, you only want two elements to change places.

          EDIT: Well, technically, it might be sorting since you now have two elements in a specific order, but I would call it sorting when it's just two elements and not even a list/array.

          • Ali Dahud

            i remember the bubble sorting used a method like this. or at least some kind of like this

            • Alex

              Yes, many sorting methods work by swapping elements until the list of items is sorted. Bubble sort swaps adjacent elements. Selection sort swaps once per iteration.

              Generally, when we transpose the value of two elements, we call it a swap. We call it a sort when we put all the elements in a list or array into some sorted order.

              • Ali Dahud

                Hi Alex, how many tutorials do you recommeend me on reading daily?

                • Alex

                  I don't have a recommendation on this.

                  It really depends entirely on your learning capability, motivation, and how much time you want to spend experimenting with each topic on your own.

              • i have the time to read as much as i want and my learning capability is pretty big i guess. with each topic i guess 20 minutes-1 hour or 1,5 hour depending if it has coding quizzes in it. so how  much should i read? oh and i lack motivation i guess so how should i motivate myself? for example i start watching videos, news while im supposed to learn.

  • Amir

    Hi Alex. The Output of the first program has a mistake!

    This is the code :

    below it we have this output :

       Enter an integer: 3
       3 is a positive integer
       Double this number is 6

    I guess it should be like this :

       Enter an integer: 3
       3 is a positive integer (or zero)
       Double this number is 6

    Thank You! : ))

  • Nick

    Thank you so much for making all these.

    On the 2ed code example if you put in 0 won't it fail when it gets to

  • Jack Oz

    Alex thank you so much your time, you help many people. Are you familiar with socket programming in C++?

    Best regards,
    Jack

  • Rajeg

    Hey Alex,

    maybe you should add a special case for 0 on your second program, now it says that 0 is a negative integer.

  • Jim

    Alex,

    On line 18 of the second program you have:
    cout << "The positive of this number is " << -value << endl;.

    Since you entered -4 for the input, how does the program know to print out 4 (positive)? when no math is ever done?

    Does this somehow say take value and multiply it times -1 ?
    In math we learned (-4 * -1)= 4.

  • Mr D

    In the last code example above, you're missing a semi-colon at the end of the line(4):

  • techsavvy....aye

    A minor note in the last program
    it should be just value instead of nValue shouldn't it?
    In block inside of block inside of block!
       🙂

  • Todd

    Typo.

    I would change the title,

    "4.1 — Blocks (compound statements) and local variables"

    to the title referenced on your main page

    "4.1 — Blocks (compound statements)"

    since you don't talk about local variables here.

  • Zidane

    The best tutorials I've ever seen 😛

  • posentel

    It may be helpful to point out that since a block replaces a single statement (that ends with a semicolon), the block supercedes the use of a semicolon. No semicolon is necessary to end a block.

  • Fluke

    Now i see a benefit from declare_variable_when_needed instead of all at the top of the function.
    Cheers 🙂

  • davidv

    Hi Alex,

    Fabulous work, first of all.

    I have a question: how does "using namespace std" work with the nesting? It seems that once written in a block, it is valid for the sub-blocks as well. If it is so, wouldn't it be easier to simply declare it at the beginning rather than several times inside functions?
    Thanks.

  • learning c++

    Hey Alex when you read this would like to suggest adding something like a review at the end of every section just quickly reminding people like for example on the variables section 1 it would say all the variable types with quick meanings and examples maybe.

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