Lesson 6.3. For loop in C

The for loop repeats a sequence of instructions like the while and do..while loops. The for loop is particularly suitable for loops that need to run a given number of times. Typically, to count or to browse indexed data (such as arrays for example):

for ( i=0 ; i<100 ; i++) {
    //...
}

Syntax

The for loop expects 3 parameters:

• initialization: initialize one or several variables ;
• test: leave the loop as soon as the test is false;
• iterator: a calculation before going to the next step (which generally increases the loop index).

for ( initialization ; test ; iterator ) {
    // ...
    Block of instructions
    // ...
}

As with the while and do..while the for loop repeats the sequence of instructions as long as the condition is true.

Here is the flowchart for the for loop:

Note the arrangement of the blocks in the flowchart. The configuration is similar to the while loop. The body of the loop may not be executed if the test is false from the start.

Each parameter is optional, we can write for(;;);, which is equivalent to while(1);.

Example 1

Here is an example of a for loop that displays all integers between 0 and 42:

int i;
// Display integers between 0 and 42
for ( i=0 ; i<=42 ; i++) {
    printf ("%d ", i);    
}

Example 2

Here is an example of a for loop that displays the integers between -27 and 15 with a step of 3 :

int i;
// Displays the integers between -27 and 15 with a step of 3
for ( i=-27 ; i<=15 ; i+=3) {
  printf ("%d ", i);    
}

Exercise 1

Write a program that displays all integers between 0 and 49 inclusive:

int i;
// Display integers from 0 to 49
// COMPLETE HERE

#include <stdio.h> int main(void) { int i; // Display integers from 0 to 49 // COMPLETE HERE }

Exercise 2

Write a program that displays all integers in descending order from 49 to 0 inclusive:

int i;
// Displays all integers in descending order from 49 to 0 inclusive:
// COMPLETE HERE

#include <stdio.h> int main(void) { int i; // Displays all integers in descending order from 49 to 0 inclusive: // COMPLETE HERE }

Exercise 3

Write a program that displays all integers from 0 to 256 with a step of 16 :

int i;
// Displays all integers from 0 to 256 with a step of 16
// COMPLETE HERE

#include <stdio.h> int main(void) { int i; // Displays all integers from 0 to 256 with a step of 16 // COMPLETE HERE }

Exercise 4

Write a program that displays all powers of 2 from 1 to 1024 :

int i;
// Displays all powers of 2 from 1 to 1024
// COMPLETE HERE

Here is the expected output:

1 2 4 8 16 32 64 128 256 512 1024

#include <stdio.h> int main(void) { int i; // Displays all powers of 2 from 1 to 1024 // COMPLETE HERE }

Exercise 5

Write the code that displays a square with n letters on each side containing the letters of the alphabet.

• The first line starts with the letter A.
• The second line starts with the letter B
• The third line starts with the letter C.
• and so on.

Here is an example for a square of size 6x6:

ABCDEF
BCDEFG
CDEFGH
DEFGHI
EFGHIJ
FGHIJK

#include <stdio.h> int main(void) { int l,c,n=6; // COMPLETE HERE return 0; }

Quiz

What does the following code display?

for ( i=0 ; i<5 ; i++) 
    printf ("%d ",i);

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What does the following code display?

for ( i=0 ; i<=5 ; i++) 
    printf ("%d ",i);

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What does the following code display?

for ( i=5 ; i>=0 ; i--) 
    printf ("%d ",i);

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What does the following code display?

for ( i=5 ; i>0 ; i--); 
    printf ("%d ",i);

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What does the following code display?

int i,j;
for ( i=0 ; i<4 ; i++)
    for ( j=0 ; j<3 ; j++)
        printf ("%d ", j);

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What does the following code display?

int i,j;
for ( i=0 ; i<4 ; i++) {
    for ( j=0 ; j<i ; j++)
        printf ("%d ", j);
    printf ("- ");
}

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See also

• Learn C programming
• Lesson 1.1. History of C programming language
• Lesson 1.2. First program
• Lesson 1.3. Compilation
• Lesson 1.4. Preprocessor directives
• Lesson 1.5. Which compiler to choose?
• Lesson 1.6. Flowchart
• Lesson 2.1. Types and variables
• Lesson 2.2. Integers
• Lesson 2.3. Floating point numbers
• Lesson 2.4. Characters
• Lesson 2.5. Variables initialization
• Lesson 2.6. Ariane flight 501
• Lesson 3.1. Arithmetic operators
• Lesson 3.2. modulo
• Lesson 3.3. Operators and types
• Lesson 3.4. Type casting
• Lesson 3.5. Bitwise operators
• Lesson 3.6. Bitwise operators details
• Lesson 3.7. Shifting operators
• Lesson 3.8. Assignment operators
• Lesson 3.9. Increment and decrement operators
• Lesson 3.10. Comparison operators
• Lesson 3.11. Logical operators
• Lesson 3.12. Operator precedence
• Lesson 4.1. printf
• Lesson 4.2. scanf
• Lesson 4.3. putchar
• Lesson 5.1. Conditional branching (if...else)
• Lesson 5.2. Nested if and indentation
• Lesson 5.3. Checking intervals
• Lesson 5.4. Ternary Operator ?:
• Lesson 5.5. switch..case statement
• Lesson 5.6. Break keyword in switch..case
• Lesson 6.1. do..while loop in C
• Lesson 6.2. While loop in C
• Lesson 6.4. How to choose the right loop in C ?
• Lesson 6.5. Exercices on loops in C
• Lesson 7.1. Bit masking
• Lesson 7.2. Bit masking, how to clear a bit in C?
• Lesson 7.3. Bit masking, how to set a bit in C?
• Lesson 7.4. Bit masking, how to toggle a bit in C?
• Lesson 7.5. Bit masking, how to check a single bit in C?
• Lesson 7.6. Overview of bit masking
• Lesson 8.1. Basic syntax of C functions
• Lesson 8.2. Calling C functions
• Lesson 8.3. Void keyword
• Lesson 8.4. Return keyword
• Lesson 8.5. Variable scope
• Lesson 8.6. Global variables
• Lesson 8.7. Static variables
• Lesson 8.8. Random numbers in C
• Lesson 8.9. Mathematical functions in C
• Lesson 9.1. Syntax of C arrays
• Lesson 9.2. Array initialization
• Lesson 9.3. Multidimensional arrays
• Lesson 9.4. How C arrays are stored in memory?
• Lesson 9.5. Arrays and functions
• Lesson 9.6. Exercises on C arrays
• Lesson 10.1. Strings in C
• Lesson 10.2. Null terminated string
• Lesson 10.3. The string.h library
• Lesson 10.4. String and functions
• Lesson 11.1. Introduction to pointers in C
• Lesson 11.2. Syntax of C pointers
• Lesson 11.3. Dynamic memory allocation
• Lesson 11.4. Incrementing pointers
• Lesson 11.5. Pointers as function argument in C
• Lesson 12.1. Introduction to structures in C
• Lesson 12.2. Properties of structures in C
• Lesson 12.3. Structures and pointers
• Lesson 12.4. Structures and functions
• Lesson 13.1. Recursive functions in C
• Lesson 13.2. Depth of recursive functions
• Lesson 13.2. Mutual recursion
• Lesson 14.1. Additional exercises

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