Loops

Theory

Loops allow you to execute a block of code repeatedly. C provides three loop constructs: for, while, and do-while. Choosing the right loop depends on your use case.

for loop is best when you know the number of iterations in advance:

for (initialization; condition; increment) {
    // body
}

The initialization runs once at the start. The condition is checked before each iteration. The increment runs after each iteration. All three parts are optional.

for (int i = 0; i < 5; i++) {
    printf("%d ", i);  // 0 1 2 3 4
}

while loop runs as long as a condition is true. It checks the condition before executing the body — if the condition is initially false, the body never runs.

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

do-while loop is similar to while but guarantees at least one execution — the condition is checked after the body runs.

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

Loop control statements:

• break — exits the current loop immediately
• continue — skips the rest of the current iteration and goes to the next
• goto — jumps to a labeled statement (use sparingly)

Infinite loops run forever. Common patterns: while(1), for(;;). Use them intentionally (e.g., server loops) and ensure there's a break condition.

Nested loops place one loop inside another. The inner loop runs completely for each iteration of the outer loop. Useful for multi-dimensional data, patterns, and tables.

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

Common loop use cases:

• Iterating through arrays
• Input validation (repeat until valid)
• Generating patterns and tables
• Counting and accumulating sums

Practical Examples

Exercises

Enter a number: 10\nSum: 55\nAverage: 5.50

#include <stdio.h>\n\nint main() {\n  // Nested loops for pyramid\n  return 0;\n}

1\n12\n123\n1234\n12345

Enter a number: 17\n17 is a prime number.

Mini Quiz

Mini Project