8.3 — Common if statement problems

This lesson is a continuation of lesson 8.2 -- If statements and blocks. In this lesson, we’ll take a look at some common problems that occur when using if-statements.

Nested if-statements and the dangling else problem

It is possible to nest if-statements within other if-statements:

#include <iostream>

int main()
{
    std::cout << "Enter a number: ";
    int x{};
    std::cin >> x;

    if (x >= 0) // outer if statement
        // it is bad coding style to nest if statements this way
        if (x <= 20) // inner if statement
            std::cout << x << " is between 0 and 20\n";

    return 0;
}

Now consider the following program:

#include <iostream>

int main()
{
    std::cout << "Enter a number: ";
    int x{};
    std::cin >> x;

    if (x >= 0) // outer if-statement
        // it is bad coding style to nest if statements this way
        if (x <= 20) // inner if-statement
            std::cout << x << " is between 0 and 20\n";

    // which if statement does this else belong to?
    else
        std::cout << x << " is negative\n";

    return 0;
}

The above program introduces a source of potential ambiguity called a dangling else problem. Is the else-statement in the above program matched up with the outer or inner if-statement?

The answer is that an else-statement is paired up with the last unmatched if-statement in the same block. Thus, in the program above, the else-statement is matched up with the inner if-statement, as if the program had been written like this:

#include <iostream>

int main()
{
    std::cout << "Enter a number: ";
    int x{};
    std::cin >> x;

    if (x >= 0) // outer if statement
    {
        if (x <= 20) // inner if statement
            std::cout << x << " is between 0 and 20\n";
        else // attached to inner if statement
            std::cout << x << " is negative\n";
    }

    return 0;
}

This causes the above program to produce incorrect output:

Enter a number: 21
21 is negative

To avoid such ambiguities when nesting if-statements, it is a good idea to explicitly enclose the inner if-statement within a block. This allows us to attach an else-statement to either the inner or the outer if-statement without ambiguity:

#include <iostream>

int main()
{
    std::cout << "Enter a number: ";
    int x{};
    std::cin >> x;

    if (x >= 0)
    {
        if (x <= 20)
            std::cout << x << " is between 0 and 20\n";
        else // attached to inner if statement
            std::cout << x << " is greater than 20\n";
    }
    else // attached to outer if statement
        std::cout << x << " is negative\n";

    return 0;
}

The else-statement within the block attaches to the inner if-statement, and the else-statement outside of the block attaches to the outer if-statement.

Flattening nested if-statements

Nested if-statements can often be flattened by either restructuring the logic or by using logical operators (covered in lesson 6.8 -- Logical operators). Code that is less nested is less error prone.

For example, the above example can be flattened as follows:

#include <iostream>

int main()
{
    std::cout << "Enter a number: ";
    int x{};
    std::cin >> x;

    if (x < 0)
        std::cout << x << " is negative\n";
    else if (x <= 20) // only executes if x >= 0
        std::cout << x << " is between 0 and 20\n";
    else // only executes if x > 20
        std::cout << x << " is greater than 20\n";

    return 0;
}

Here’s another example that uses logical operators to check multiple conditions within a single if-statement:

#include <iostream>

int main()
{
    std::cout << "Enter an integer: ";
    int x{};
    std::cin >> x;

    std::cout << "Enter another integer: ";
    int y{};
    std::cin >> y;

    if (x > 0 && y > 0) // && is logical and -- checks if both conditions are true
        std::cout << "Both numbers are positive\n";
    else if (x > 0 || y > 0) // || is logical or -- checks if either condition is true
        std::cout << "One of the numbers is positive\n";
    else
        std::cout << "Neither number is positive\n";

    return 0;
}

Null statements

A null statement is an expression statement that consists of just a semicolon:

if (x > 10)
    ; // this is a null statement

Null statements do nothing. They are typically used when the language requires a statement to exist but the programmer doesn’t need one. For readability, null statements are typically placed on their own lines. We’ll see examples of intentional null statements later in this chapter, when we cover loops.

Null statements are rarely intentionally used with if-statements. However, they can unintentionally cause problems for new (or careless) programmers. Consider the following snippet:

if (nuclearCodesActivated()); // note the semicolon at the end of this line
    blowUpTheWorld();

In the above snippet, the programmer accidentally put a semicolon on the end of the if-statement (a common mistake since semicolons end many statements). This unassuming error compiles fine, and causes the snippet to execute as if it had been written like this:

if (nuclearCodesActivated())
    ; // the semicolon acts as a null statement
blowUpTheWorld(); // and this line always gets executed!

for x in [0, 1, 2]:
  pass               # To be completed in the future

#define PASS

void foo(int x, int y)
{
    if (x > y)
        PASS;
    else
        PASS;
}

int main()
{
    foo(4, 7);

    return 0;
}

Operator== vs Operator= inside the conditional

Inside your conditional, you should be using operator== when testing for equality, not operator= (which is assignment). Consider the following program:

#include <iostream>

int main()
{
    std::cout << "Enter 0 or 1: ";
    int x{};
    std::cin >> x;
    if (x = 0) // oops, we used an assignment here instead of a test for equality
        std::cout << "You entered 0\n";
    else
        std::cout << "You entered 1\n";

    return 0;
}

This program will compile and run, but will produce the wrong result in some cases:

Enter 0 or 1: 0
You entered 1

In fact, this program will always produce the result You entered 1. This happens because x = 0 first assigns the value 0 to x, then evaluates to the value of x, which is now 0, which is Boolean false. Since the conditional is always false, the else-statement always executes.