Loops: while, for, and accumulation

Repeating work

• A loop runs the same block of code many times, so you don't copy it by hand.
• C has three loops: while, for, and do-while. They all repeat while a condition is true.
• The condition is checked each time around. When it becomes false, the loop stops.

while loops

• while (test) { ... } checks the test first, then runs the block if it is true.
• Something inside the block must eventually make the test false, or the loop never ends.
• Use while when you don't know in advance how many times you will repeat.

for loops

• A for loop puts three parts in one line: for (start; test; step).
• for (int i = 0; i < 5; i++) means: start i at 0, keep going while i < 5, add 1 each time.
• Use for when you are counting a known number of times.

#include <stdio.h>

int main(void) {
    int total = 0;
    for (int i = 1; i <= 4; i++) {
        total = total + i;     // accumulate: 1, then 3, then 6, then 10
    }
    printf("%d\n", total);     // 10
    return 0;
}

Accumulation and counting

• An accumulator is a variable that builds up a result across the loop, like total above.
• A counter adds 1 each time something happens, to count matches.
• break; stops the loop early; continue; skips to the next round.

Common mistakes

• Do not put a ; right after for(...) — it makes an empty loop.
• Change the condition so the loop can end.

Now you try

• Set up your accumulator or counter before the loop, update it inside, and use it after.
• For the function tasks, do not write a main — the checker provides one.