Arrays and Loops

The visit pattern

int a[5] = {4, 8, 15, 16, 23};
int sum = 0;
for (int i = 0; i < 5; i++) {
    sum += a[i];
}

Three canonical walks: compute a value from all elements (sum/max), transform in place (a[i] = a[i] * 2), and build a new array from an old one.

The identity: last index is size-1

int a[5];
for (int i = 0; i <= 5; i++) {   // BUG: i == 5 is out of bounds
    a[i] = i;
}

i < size is the loop condition for arrays. i <= size writes one element past the end — sometimes it "works", sometimes it corrupts a neighbor, always it is undefined behavior.

Counting and finding

int count = 0, found = -1;
for (int i = 0; i < 5; i++) {
    if (a[i] > 10) count++;          // count matches
    if (a[i] == 15) found = i;       // remember the index
}

found = -1 as "not present" is a C idiom you will use everywhere.