Loop Patterns and Nested Loops in C

Nested loops are loops placed inside other loops. They are commonly used for working with 2D data and printing patterns.

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Nested Loop Syntax

for (int i = 1; i <= outer; i++) {
    for (int j = 1; j <= inner; j++) {
        /* body */
    }
}

The inner loop completes all its iterations for each single iteration of the outer loop.

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Pattern Programs

Right-Angled Triangle of Stars

for (int i = 1; i <= 5; i++) {
    for (int j = 1; j <= i; j++) {
        printf("* ");
    }
    printf("\n");
}
/*
*
* *
* * *
* * * *
* * * * *
*/

Inverted Triangle

int n = 5;
for (int i = n; i >= 1; i--) {
    for (int j = 1; j <= i; j++) {
        printf("* ");
    }
    printf("\n");
}

Number Triangle

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

Pyramid of Stars

int n = 5;
for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= n - i; j++) printf(" ");
    for (int j = 1; j <= 2 * i - 1; j++) printf("*");
    printf("\n");
}

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Multiplication Table

for (int i = 1; i <= 10; i++) {
    for (int j = 1; j <= 10; j++) {
        printf("%4d", i * j);
    }
    printf("\n");
}

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Loop Control in Nested Loops

/* break only exits the innermost loop */
for (int i = 0; i < 3; i++) {
    for (int j = 0; j < 3; j++) {
        if (j == 1) break;   /* exits inner loop only */
        printf("%d,%d ", i, j);
    }
}
/* Output: 0,0 1,0 2,0 */

To break out of nested loops, use a flag variable or goto:

int done = 0;
for (int i = 0; i < 5 && !done; i++) {
    for (int j = 0; j < 5; j++) {
        if (i == 2 && j == 2) { done = 1; break; }
        printf("%d,%d ", i, j);
    }
}