Memory and Addresses

Memory is a long street of houses

The computer's memory is like a very long street. Every house has a number, called its address. When you create a variable, C picks a house for it and stores the value inside.

Think of it like this: int age = 25; is like moving the number 25 into house number 1000. The variable's name is age, its value is 25, and its address is 1000.

The & operator: "the address of"

You have already used & with scanf. &age means "the address of age". You can print an address with %p:

#include <stdio.h>

int main() {
    int age = 25;
    printf("Value: %d\n", age);
    printf("Address: %p\n", (void *) &age);
    return 0;
}

Output (on your own computer with gcc – the simple C runner in the browser editor can't run this one) (the address will be different on every computer and every run)

Value: 25
Address: 0x7ffdcb9a0ea4

Addresses are usually shown in hexadecimal (with letters a–f). You never need to know the actual number – what matters is that every variable has one.

Why care about addresses?

• They let a function change a variable that belongs to someone else (that's how scanf works!).
• They let you work with arrays and strings efficiently.
• They let you ask for new memory while the program runs (Module 11).

Remember: age is the value. &age is where that value lives in memory.