The while Loop

Syntax and Structure

The while loop in C is one of the easiest and the fundamental loop type in C that allows us to repeat a block of code based on the condition it checks. Following is the syntax of the while loop in C:

while (condition) statement;

Let’s understand the syntax properly:

• while:  this keyword marks the beginning of the loop and is mandatory.
• condition:  an expression that evaluates to either true or false. 
• statement:  this will execute as long as the condition remains true. 

As long as the condition remains true, the statement following the closing parentheses will execute repeatedly. 

The syntax allows us to add only one statement, but most often we may want to execute a bunch of statements repeatedly. For this, we can do a trick. We can use what is called a compound statement in C. 

A compound statement is a set of statements enclosed within curly braces. In C language, a group of statements enclosed within curly braces are considered as a single statement. So, technically, the above syntax can be replaced by the following:

while (condition) 
{
    // Bunch of statements to execute repeatedly
}

Now, we can execute a couple of statements repeatedly within the loop body.

How Does It Work?

There are three things involved in the successful execution of the while loop:

1. Initialization (before the loop):  you typically declare and initialize a loop counter variable before the loop starts. 
2. Condition:  the loop counter variable is utilized here and usually compared with some other value. If the expression evaluates to true, then the control transfers to the loop body immediately. Otherwise, the statement following the loop will execute.
3. Increment/Decrement:  within the loop body, there can be either an increment or decrement statement that either increments or decrements the counter variable. This ensures that the condition eventually becomes false, terminating the loop.

So, in a nutshell

• The counter variable is initialized.
• The while condition is checked.
• If the condition is true, the loop body will execute.
• If the condition is false, the statement following the loop will execute.
• Within the loop body, apart from the statements we want to repeat, there can be an increment or decrement statement that increments or decrements the counter variable.
• The while condition is again checked, and the process repeats.

If the process is still unclear, the following flowchart will make it crystal clear:

Example 1:  Printing the number from 1 to 10

#include <stdio.h>

int main() {
    int counter = 1; // counter variable initialization
    while (counter <= 10) { // condition checking
        printf("%d\n", counter);
        counter++; // incrementing the counter variable
    }
    return 0;
}

In this example program:

• The variable counter is initialized to 1.
• Then, the condition “counter <= 10” is checked. It evaluates to true.
• The loop body will execute which means the “prinf()” function will display the current value of the counter variable which is 1 and then the counter is incremented by 1. Now, it becomes 2.
• The condition “counter <= 10” is again checked. This time also the condition is true.
• The loop body will execute, and in the same way, it continues to execute until the counter variable holds 11 which eventually makes the condition false, and terminates the loop.

Example 2:  Summing numbers until user enters 0

#include <stdio.h>

int main() {
    int sum = 0, number;

    printf("Enter the number (0 to stop): ");
    scanf("%d", &number);

    while (number != 0) {
        sum += number;
        printf("Enter the number (0 to stop): ");
        scanf("%d", &number);
    }
    printf("The sum is %d", sum);
    return 0;
}

Output:

Enter the number (0 to stop): 10
Enter the number (0 to stop): 20
Enter the number (0 to stop): 30
Enter the number (0 to stop): 40
Enter the number (0 to stop): 50
Enter the number (0 to stop): 0
The sum is 150

In this example,

• The variable sum is initialized to 0 and the number variable is the counter variable which is not yet initialized. In the next step, we will ask the user to provide the value for the number variable.
• Now, we are asking the user to enter the number which will eventually be stored in the number variable.
• Let’s assume that the user has entered the number 10. Then, the while condition “number != 0” evaluates to true. This means the loop body will execute.
• The statement “sum += number” will execute which is equivalent to “sum = sum + number”. The initial value of sum is zero, therefore, after execution of the statement, the variable sum will hold value 10.
• Again, within the loop body, we are asking the user to enter the number. Let’s assume this time the user has entered 20.
• The condition “number != 0” evaluates to true.
• The statement “sum += number” will execute. The current value of sum is 10 and that of the number variable is 20. This means this time, the variable sum will hold value 30.
• Again we ask the user to enter the next number. The process will continue this way until the user enters 0.
• Once the user enters 0, the while condition becomes false and the statement following the while loop which is “printf(“The sum is %d”, sum);” will be executed. This prints the final value of the sum.

Common Mistakes and Best Practices

First, we will discuss what are some of the most common mistakes people make in the while loop and then we will proceed with some of the best practices which every programmer should follow.

Common Mistakes in while Loops

1. Infinite loops:  if you forget to increment/decrement the loop counter variable, then the loop condition will never become false and the loop will run forever. In some cases, infinite loops are useful (we learn more about this in our upcoming tutorials) but mostly, we may need to terminate our loop through the loop counter variable.
2. Incorrect condition:  the loop condition decides whether to enter or exit the loop. An incorrect condition may lead to unexpected behavior. It may be possible that the loop will run a number of times which was unexpected or we may not even enter the loop in the first place.
3. Uninitialized loop variables:  never forget to initialize the loop counter variable. Doing so will lead to unexpected behaviors.
4. Off-by-one errors:  make sure that the correct relational operator (==, != , <=, >=, <, >) is used in the loop condition. For instance, it may be possible that instead of less than or equal to (<=) operator, you may use less than (<) operator, and that would lead to off-by-one error as you will miss one value due to this.

Best Practices

1. Clear and concise loop condition:  the loop condition must be easy to understand and should reflect the desired behavior.
2. Meaningful variable names:  Choose names for your variables that make sense and describe what they are. For example, instead of just using “var = 30,” you could use “age = 30.” This makes it clear that the number 30 represents someone’s age.
3. Comments:  add comments to explain the purpose of the loops you write. 
4. Proper indentation:  improper indentation may lead to confusion and decrease the readability of your code significantly. Try to indent your code the way it should.

Level-Up Your Understanding

while (n > 0) {
    d = n % 10;
    s += d;
    n /= 10;
}

num = 1;
while (num < 10) {
  printf("%d ", num);
  num++;
}

#include <stdio.h>

int main() 
{
    int x = 0;
    while (++x < 5) {
        printf("%d", x);
    }
}

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