Copies and scope

Functions work on copies

When you call f(a), the function gets a copy of a's value. Changing it inside the function doesn't touch a in main. (Pointers, in module 9, are how a function changes the caller's variable.)

Variables made inside a function exist only while it runs. Two functions can each have their own i without clashing.

Did it change?

#include <stdio.h>

void addTen(int x) {
    x = x + 10;
}

int main() {
    int a = 5;
    addTen(a);
    printf("%d\n", a);
    return 0;
}

What does this print?

• 5
• 15
• 10

Exercise: Write isPrime()

Write int isPrime(int n) returning 1 for a prime and 0 otherwise (1 and below are not prime).

Read a number k, then print every prime from 2 to k on one line, separated by spaces.

Exercise: Greatest common divisor

Write int gcd(int a, int b) with Euclid's method: while b isn't 0, replace (a, b) with (b, a % b); then a is the answer.

Read two positive numbers and print their gcd.