The following statement should determine if x is not greater than 20 what is wrong with it if x > 20

Conditional code flow is the ability to change the way a piece of code behaves based on certain conditions. In such situations you can use if statements.

The if statement is also known as a decision making statement, as it makes a decision on the basis of a given condition or expression. The block of code inside the if statement is executed is the condition evaluates to true. However, the code inside the curly braces is skipped if the condition evaluates to false, and the code after the if statement is executed.

Syntax of an if statement

if (testCondition) {
   // statements
}

A simple example

Let’s look at an example of this in action:

#include <stdio.h>
#include <stdbool.h>

int main(void) {
    if(true) {
        printf("Statement is True!\n");
    }

    return 0;
}

Output:

Statement is True!

If the code inside parenthesis of the if statement is true, everything within the curly braces is executed. In this case, true evaluates to true, so the code runs the printf function.

if..else statements

In an if...else statement, if the code in the parenthesis of the if statement is true, the code inside its brackets is executed. But if the statement inside the parenthesis is false, all the code within the else statement's brackets is executed instead.

Of course, the example above isn't very useful in this case because true always evaluates to true. Here's another that's a bit more practical:

#include <stdio.h>

int main(void) {
    int n = 2;

    if(n == 3) { // comparing n with 3
        printf("Statement is True!\n");
    } 
    else { // if the first condition is not true, come to this block of code
        printf("Statement is False!\n");
    }

    return 0;
}

Output:

Statement is False!

There are a few important differences here. First, stdbool.h hasn’t been included. That's okay because true and false aren't being used like in the first example. In C, like in other programming languages, you can use statements that evaluate to true or false rather than using the boolean values true or false directly.

Also notice the condition in the parenthesis of the if statement: n == 3. This condition compares n and the number 3. == is the comparison operator, and is one of several comparison operations in C.

Nested if...else

The if...else statement allows a choice to be made between two possibilities. But sometimes you need to choose between three or more possibilities.

For example the sign function in mathematics returns -1 if the argument is less than zero, +1 if the argument is greater than zero, and returns zero if the argument is zero.

The following code implements this function:

if (x < 0)
   sign = -1;
else
   if (x == 0)
      sign = 0;
   else
      sign = 1;

As you can see, a second if...else statement is nested within else statement of the first if..else.

If x is less than 0, then sign is set to -1. However, if x is not less than 0, the second if...else statement is executed. There, if x is equal to 0, sign is also set to 0. But if x is greater than 0, sign is instead set to 1.

Rather than a nested if...else statement, beginners often use a string of if statements:

if (x < 0) {
   sign = -1;
}
   
if (x == 0) {
   sign = 0;
}
   
if (x > 0) {
   sign = 1;
}

While this works, it's not recommended since it's unclear that only one of the assignment statements (sign = ...) is meant to be executed depending on the value of x. It's also inefficient – every time the code runs, all three conditions are tested, even if one or two don't have to be.

else...if statements

if...else statements are an alternative to a string of if statements. Consider the following:

#include <stdio.h>

int main(void) {
    int n = 5;

    if(n == 5) {
        printf("n is equal to 5!\n");
    } 
    else if (n > 5) {
        printf("n is greater than 5!\n");
    }

    return 0;
}

Output:

n is equal to 5!

If the condition for the if statement evaluates to false, the condition for the else...if statement is checked. If that condition evaluates to true, the code inside the else...if statement's curly braces is run.

Comparison Operators

Logical Operators

We might want a bit of code to run if something is not true, or if two things are true. For that we have logical operators:

For example:

#include <stdio.h>

int main(void) {
    int n = 5;
    int m = 10;

    if(n > m || n == 15) {
        printf("Either n is greater than m, or n is equal to 15\n");
    } 
    else if( n == 5 && m == 10 ) {
        printf("n is equal to 5 and m is equal to 10!\n");
    } 
    else if ( !(n == 6)) {
        printf("It is not true that n is equal to 6!\n");
    }
    else if (n > 5) {
        printf("n is greater than 5!\n");
    }

    return 0;
}

Output:

n is equal to 5 and m is equal to 10!

An important note about C comparisons

While we mentioned earlier that each comparison is checking if something is true or false, but that's only half true. C is very light and close to the hardware it's running on. With hardware it's easy to check if something is 0 or false, but anything else is much more difficult.

Instead it's much more accurate to say that the comparisons are really checking if something is 0 / false, or if it is any other value.

For example, his if statement is true and valid:

if(12452) {
    printf("This is true!\n")
}

By design, 0 is false, and by convention, 1 is true. In fact, here’s a look at the stdbool.h library:

#define false   0
#define true    1

While there's a bit more to it, this is the core of how booleans work and how the library operates. These two lines instruct the compiler to replace the word false with 0, and true with 1.