Q&A

Using for loops in C

By AlgoInsight Team2026-06-07c

Overview

The for loop is C's primary counting loop, ideal when you know (or can express) how many times to iterate. Its three-part header puts the loop's setup, continuation test, and step in one place, making counting loops compact and readable.

graph TD
    A[init: runs once] --> B{condition true?}
    B -->|yes| C[execute body]
    C --> D[update]
    D --> B
    B -->|no| E[exit loop]
    style B fill:#1e1b4b,color:#fff

Anatomy

for (initialization; condition; update) {
    // loop body
}

1. initialization - runs once before the loop (e.g. int i = 0).
2. condition - checked before each iteration; loop runs while it's true.
3. update - runs after each iteration (e.g. i++).

Basic Examples

// Print 0 through 4
for (int i = 0; i < 5; i++) {
    printf("%d\n", i);
}

// Count down
for (int i = 10; i > 0; i--) {
    printf("%d ", i);          // 10 9 8 ... 1
}

// Step by 2
for (int i = 0; i <= 10; i += 2) {
    printf("%d ", i);          // 0 2 4 6 8 10
}

Note: declaring the loop variable inside the for (int i = 0) requires C99 or later; older C89 requires declaring i before the loop.

Iterating an Array

int arr[] = {10, 20, 30, 40, 50};
int n = sizeof(arr) / sizeof(arr[0]);   // number of elements
for (int i = 0; i < n; i++) {
    printf("%d ", arr[i]);
}

The sizeof(arr)/sizeof(arr[0]) idiom computes array length (works only on a real array in scope, not a pointer).

All Parts Are Optional

Any of the three header parts can be omitted:

// Infinite loop (empty condition is always true)
for (;;) {
    // ... use break to exit
}

// Initialization done elsewhere, no update in header
int i = 0;
for (; i < 5;) {
    printf("%d ", i);
    i++;                       // update inside body
}

Multiple Variables (comma operator)

for (int i = 0, j = 9; i < j; i++, j--) {
    printf("%d %d\n", i, j);   // converge from both ends
}

The comma operator lets you initialize and update more than one variable.

break and continue

for (int i = 0; i < 10; i++) {
    if (i == 5) break;         // exit the loop entirely
    if (i % 2 == 0) continue;  // skip to the next iteration
    printf("%d ", i);          // prints odd numbers < 5: 1 3
}

• break - leave the loop immediately.
• continue - skip the rest of the body and go to the update/condition.

Nested Loops

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

Common for grids/matrices; complexity multiplies (here O(n²)).

Why This Matters in Interviews

C interviews use for loops to check fundamentals: the init/condition/update order and when each runs (init once; condition before each iteration; update after), the array-length idiom, that all header parts are optional (infinite loops, for(;;)), the comma operator for multiple variables, and break/continue. A frequent gotcha: the condition is checked before the first body execution, so a false condition means the body never runs.

Common Pitfalls

• Off-by-one errors: < vs <= changes the iteration count and can overrun arrays.
• Declaring int i inside for on a C89 compiler (use C99+ or declare beforehand).
• Modifying the loop variable inside the body, causing unexpected counts.
• Using sizeof(arr)/sizeof(arr[0]) on a pointer (e.g. an array passed to a function decays to a pointer) - gives the wrong length.
• Forgetting the update, creating an infinite loop.

References

• C for statement: https://en.cppreference.com/w/c/language/for
• C language reference: https://en.cppreference.com/w/c/language

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