Pass the readings with their count

The desk has three readings to pass to a function. In a parameter list, const int readings[] is adjusted to a pointer to the first element. That pointer does not carry the array's length, and sizeof readings inside the function measures the pointer rather than the whole array. Pass the element count separately. The const qualifier prevents writes through this parameter.

#include <stdio.h>

int first_reading(const int readings[], size_t count) {
    if (count == 0) {
        return 0;
    }
    return readings[0];
}

int main(void) {
    int readings[] = {18, 20, 22};
    size_t count = sizeof readings / sizeof readings[0];
    printf("First: %d\n", first_reading(readings, count));
    return 0;
}

sizeof counts elements in main, where readings is an array. It would not do that inside first_reading. Change the first value and predict the output. The next exercise uses the count to visit every element without reading past the end.