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C For Loop

Most programming languages including C support the for keyword for constructing a loop. In C, the other loop-related keywords are while and do-while. Unlike the other two types, the for loop is called an automatic loop, and is usually the first choice of the programmers.

The for loop is an entry-controlled loop that executes the statements till the given condition. All the elements (initialization, test condition, and increment) are placed together to form a for loop inside the parenthesis with the for keyword.

The syntax of the for loop in C programming language is −

for (init; condition; increment){
   statement(s);
}

Here is how the control flows in a "for" loop −

The init step is executed first, and only once. This step allows you to declare and initialize any loop control variables. You are not required to put a statement here, as long as a semicolon appears.

Next, the condition is evaluated. If it is true, the body of the loop is executed. If it is false, the body of the loop does not execute and the control jumps to the next statement just after the "for" loop.

After the body of the "for" loop executes, the control flow jumps back up to the increment statement. This statement allows you to update any loop control variables. This statement can be left blank, as long as a semicolon appears after the condition.

The condition is now evaluated again. If it is true, the loop executes and the process repeats itself (body of loop, then increment step, and then again the condition). After the condition becomes false, the "for" loop terminates.

The following flowchart represents how the for loop works −

Developers prefer to use for loops when they know in advance how many number of iterations are to be performed. It can be thought of as a shorthand for while and do-while loops that increment and test a loop variable.

The for loop may be employed with different variations. Let us understand how the for loop works in different situations.

This is the most basic form of the for loop. Note that all the three clauses inside the parenthesis (in front of the for keyword) are optional.

#include <stdio.h>

int main(){
   int a;

   // for loop execution
   for(a = 1; a <= 5; a++){
      printf("a: %d\n", a);
   }

   return 0;
}

Run the code and check its output −

a: 1
a: 2
a: 3
a: 4
a: 5

The initialization step can be placed above the header of the for loop. In that case, the init part must be left empty by putting a semicolon.

#include <stdio.h>

int main(){
   int a = 1;

   // for loop execution
   for( ; a <= 5; a++){
      printf("a: %d\n", a);
   }
   return 0;
}

You still get the same output −

a: 1
a: 2
a: 3
a: 4
a: 5

You can also put an empty statement in place of the increment clause. However, you need to put the increment statement inside the body of the loop, otherwise it becomes an infinite loop.

#include <stdio.h>

int main(){

   int a;

   // for loop execution
   for(a = 1; a <= 5; ){
      printf("a: %d\n", a);
      a++;
   }
   return 0;
}

Here too, you will get the same output as in the previous example −

a: 1
a: 2
a: 3
a: 4
a: 5

You can also omit the second clause of the test condition in the parenthesis. In that case, you will need to terminate the loop with a break statement, otherwise the loop runs infinitely.

#include <stdio.h>

int main(){
   int a;

   // for loop execution
   for(a = 1; ; a++){
      printf("a: %d\n", a);
      if(a == 5)
      break;
   }
   return 0;
}

On executing this code, you will get the following output −

a: 1
a: 2
a: 3
a: 4
a: 5

There may be initialization of more than one variables and/or multiple increment statements in a for statement. However, there can be only one test condition.

#include <stdio.h>

int main(){
   int a, b;

   // for loop execution
   for(a = 1, b = 1; a <= 5; a++, b++){
      printf("a: %d b: %d a*b: %d\n", a, b, a*b);
   }

   return 0;
}

When you run this code, it will produce the following output −

a: 1 b: 1 a*b: 1
a: 2 b: 2 a*b: 4
a: 3 b: 3 a*b: 9
a: 4 b: 4 a*b: 16
a: 5 b: 5 a*b: 25

You can also form a decrementing for loop. In this case, the initial value of the looping variable is more than its value in the test condition. The last clause in the for statement uses decrement operator.

The following program prints the numbers 5 to 1, in decreasing order −

#include <stdio.h>
int main(){
   int a;

   // for loop execution
   for(a = 5; a >= 1; a--){
      printf("a: %d\n", a);
   }

   return 0;
}

Run the code and check its output −

a: 5
a: 4
a: 3
a: 2
a: 1

For loop is well suited for traversal of one element of an array at a time. Note that each element in the array has an incrementing index starting from "0".

#include <stdio.h>

int main(){
   int i;
   int arr[] = {10, 20, 30, 40, 50};

   // for loop execution
   for(i = 0; i < 5; i++){
      printf("a[%d]: %d\n", i, arr[i]);
   }

   return 0;
}

When you run this code, it will produce the following output −

a[0]: 10
a[1]: 20
a[2]: 30
a[3]: 40
a[4]: 50

The following program computes the average of all the integers in a given array.

#include <stdio.h>
int main(){
   int i;
   int arr[] = {10, 20, 30, 40, 50};
   int sum = 0;
   float avg;

   // for loop execution
   for(i=0; i<5; i++){
      sum += arr[i];
   }
   avg = (float)sum / 5;
   printf ("Average = %f", avg);

   return 0;
}

Run the code and check its output −

Average = 30.000000

The following code uses a for loop to calculate the factorial value of a number. Note that the factorial of a number is the product of all integers between 1 and the given number. The factorial is mathematically represented by the following formula −

x! = 1 * 2 * . . . * x

Here is the code for computing the factorial −

#include <stdio.h>

int main(){

   int i, x = 5;
   int fact = 1;

   // for loop execution
   for(i=1; i<= x; i++){
      fact *= i;
   }
   printf("%d != %d", x, fact);

   return 0;
}

When you run this code, it will produce the following output −

5! = 120

The for loop is ideally suited when the number of repetitions is known. However, the looping behaviour can be controlled by the break and continue keywords inside the body of the for loop. Nested for loops are also routinely used in the processing of two dimensional arrays.

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