Pointers and Functions

When you pass a variable to a function, the function receives a copy. Pointers let us do something special: pass by reference

First, let's look at the problem. This function tries to swap two numbers but fails

#include <stdio.h>

void swap(int a, int b) {
    int temp = a;
    a = b;
    b = temp;
}

int main(void) {
    int x = 10, y = 20;
    swap(x, y);
    printf("x = %d, y = %d\n", x, y);
    return 0;
}

Output

x = 10, y = 20

Nothing happened! The function swapped its local copies, but the original x and y never changed. The copies were destroyed when the function ended

The fix: pass pointers instead of values

#include <stdio.h>

void swap(int *a, int *b) {
    int temp = *a;
    *a = *b;
    *b = temp;
}

int main(void) {
    int x = 10, y = 20;
    swap(&x, &y);
    printf("x = %d, y = %d\n", x, y);
    return 0;
}

Output

x = 20, y = 10

Now it works. We passed the addresses of x and y. The function followed those addresses and changed the real values. This is pass by reference, the most important use of pointers

This pattern shows up everywhere in C. Want a function to modify a variable? Pass it the address

#include <stdio.h>

void double_it(int *n) {
    *n = *n * 2;
}

int main(void) {
    int score = 50;
    double_it(&score);
    printf("%d\n", score);   // 100
    return 0;
}

Another common use: functions that need to return multiple values. A C function can only return a single thing, but with pointers we can "return" as many as we want

#include <stdio.h>

void min_max(int arr[], int n, int *min, int *max) {
    *min = arr[0];
    *max = arr[0];
    for (int i = 1; i < n; i++) {
        if (arr[i] < *min) *min = arr[i];
        if (arr[i] > *max) *max = arr[i];
    }
}

int main(void) {
    int numbers[] = {3, 7, 1, 9, 4};
    int lo, hi;
    min_max(numbers, 5, &lo, &hi);
    printf("Min: %d, Max: %d\n", lo, hi);
    return 0;
}

Output

Min: 1, Max: 9

The function "returns" both the minimum and the maximum through pointers. This is idiomatic C. You'll see it everywhere

Mission: The Signal Amplifier

At the computing center, a signal translator reads a raw value off a magnetic tape. The amplifier needs to triple it before sending it onward, but direct access to the variable isn't allowed, only through a pointer.

Read an int signal from input
Write a function triple_it that takes a pointer to int and triples the value it points to
Call triple_it(&signal) from main
Print the result

Example

Input

Output

Example

Input

Output