C for Loop | How to Write Iterations in C

Why Do We Need Loops? — The Pain of Repetition

What if loops didn't exist? Let's try writing "print 1 to 5" without any loop, and see how painful programming would be.

❌ Without loops

#include <stdio.h>

int main(void) {
    printf("1\n");
    printf("2\n");
    printf("3\n");
    printf("4\n");
    printf("5\n");
    return 0;
}

Five times is barely manageable, but… what if you needed to print 1 to 100?

Imagine this: You're told to print 1 to 100. Would you really write printf("1\n"); printf("2\n"); ... printf("100\n"); for 100 lines? And if the requirement changed to "print only multiples of 3", you'd have to rewrite everything.

✅ With a loop

#include <stdio.h>

int main(void) {
    for (int i = 1; i <= 5; i++) {
        printf("%d\n", i);
    }
    return 0;
}

Just 3 lines! And if you want to go up to 100, just change 5 to 100. Even 10,000 iterations fit in the same amount of code.

Feel the power of loops with this slider

Change the number of iterations. See how many lines you'd need without a loop.

Iterations N = 5

❌ Without loop

5 lines

You need to write printf N times

✅ With loop

3 lines

Same code no matter what N is

It gets worse: what if the logic changes?

Computing "the sum of 1 to 10" without a loop is also painful.

// Without loop: manually add each number
int sum = 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10;

// With loop
int sum = 0;
for (int i = 1; i <= 10; i++) sum += i;

The essence of loops: "Repeat the same pattern, with a slightly different value each time." It's one of the most important concepts in programming — you'll see it in almost every program.

Even worse: If the number of iterations depends on user input (e.g., "sum from 1 to N where N is entered at runtime"), you can't write it without a loop. You don't know how many printf to write until the program runs.

break & continue — loop control

break

Exits the loop immediately. All remaining iterations are skipped.

for (int i=0; i<10; i++) {
    if (i == 5) break;
    printf("%d ", i);
}
// Output: 0 1 2 3 4

continue

Skips the rest of the current iteration and goes on to the next one. The loop itself keeps running.

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

Caution: break and continue only affect the innermost loop. break inside a nested loop does not exit the outer loop.

🔍 continue in depth — behaviour, misconceptions, readability

continue means "skip the rest of this iteration, go to the next one". Unlike break, the loop does not exit. Let's trace the execution step by step.

📋 continue execution trace

for (int i = 0; i < 5; i++) {
    if (i == 2) continue;    // ← when i==2, skip the rest
    printf("%d ", i);     // not reached after continue
}

Iteration 1 (i=0): i==2 false → printf → prints 0 → i++ makes i=1
Iteration 2 (i=1): i==2 false → printf → prints 1 → i++ makes i=2
Iteration 3 (i=2): i==2 true → continue → printf skipped → i++ makes i=3 (key point)
Iteration 4 (i=3): i==2 false → printf → prints 3
Iteration 5 (i=4): i==2 false → printf → prints 4 → i=5 terminates

💡 Key point: after continue, the for's update clause i++ is still executed. That's why i=2 skipped but i=3 follows.

🔴 Misconception ① "continue is like break, right?"

They're opposites. Mixing them up changes behaviour completely.

	break	continue
Whole loop	Exit (done)	Continue (next iteration)
Rest of this iteration	Skip	Skip
for's update `i++`	Not executed	Executed
When to use	"done searching"	"skip just this element"

🔴 Misconception ② continue in a while can cause an infinite loop

for loops update the counter in the header — it always runs. But in a while loop you update the counter yourself inside the body, and continue can skip it. That's a classic infinite-loop trap.

int i = 0;
while (i < 5) {
    if (i == 2) continue;  // ❌ never reaches i++ → stuck at i=2 forever
    printf("%d ", i);
    i++;
}

Fix: bump the counter before continuing:

if (i == 2) { i++; continue; }  // ✅ safe

💡 continue vs wrapping in an if — readability trade-off

"Skip certain items" can be written with continue or with an if. Which reads better depends on the situation.

✅ continue wins: early skip pattern

for (int i=0; i<n; i++) {
    if (arr[i] < 0) continue;  // skip negatives
    if (arr[i] == 0) continue; // skip zeros too

    // long main processing for positives...
    process(arr[i]);
    total += arr[i] * 2;
}

Filtering out unwanted cases up front keeps the main block un-indented.

⚠️ Same thing with if gets nested

for (int i=0; i<n; i++) {
    if (arr[i] > 0) {  // invert & wrap
        process(arr[i]);
        total += arr[i] * 2;
    }
}

One simple condition is fine. Multiple skip conditions → if-nesting becomes deep.

Rule of thumb:
• 1 simple skip + short body → if
• Multiple skips + long body → continue (guard-clause style)
Same idea as early return in functions — see if Statement.

Lesson 11: for Loop

Why Do We Need Loops? — The Pain of Repetition

❌ Without loops

✅ With a loop

Feel the power of loops with this slider

It gets worse: what if the logic changes?

for loop — a fixed number of iterations

Iteration-count visualizer

Step execution — for loop

Variable state

Standard output

break & continue — loop control

🔍 continue in depth — behaviour, misconceptions, readability

Try it yourself — loops

Related Lessons

Review Quiz

Q1. How many times does `for (int i=0; i<5; i++)` execute?

Q2. What happens with `while(1)`?

Q3. What is the difference between do-while and while?

Recommended Books to Deepen Your Understanding

Why Do We Need Loops? — The Pain of Repetition

❌ Without loops

✅ With a loop

Feel the power of loops with this slider

It gets worse: what if the logic changes?

for loop — a fixed number of iterations

Iteration-count visualizer

Step execution — for loop

Variable state

Standard output

break & continue — loop control

🔍 continue in depth — behaviour, misconceptions, readability

Try it yourself — loops

Related Lessons

Review Quiz

Q1. How many times does for (int i=0; i<5; i++) execute?

Q2. What happens with while(1)?

Q3. What is the difference between do-while and while?

Recommended Books to Deepen Your Understanding

Q1. How many times does `for (int i=0; i<5; i++)` execute?

Q2. What happens with `while(1)`?