Pointers

🏠 The house and address analogy

The variable name age is just a convenient label for humans. Internally, the computer only works with the address 0x1004. The name exists only in your source code — it disappears when the program compiles.

#include <stdio.h>
 
int main() {
    int age = 25;
    float salary = 50000.0;
    char grade = 'A';
 
    printf("Value of age:    %d\n", age);
    printf("Address of age:  %p\n", &age);     /* %p prints an address */
 
    printf("Value of salary: %f\n", salary);
    printf("Address of salary: %p\n", &salary);
 
    printf("Value of grade:  %c\n", grade);
    printf("Address of grade: %p\n", &grade);
 
    return 0;
}

/* Sample output (addresses will differ on your machine):
Value of age:      25
Address of age:    0x7fff5fbff5ac
Value of salary:   50000.000000
Address of salary: 0x7fff5fbff5a8
Value of grade:    A
Address of grade:  0x7fff5fbff5a7
*/

p stores the address of age (0x1004). p itself also lives somewhere in memory (0x2008). Following p leads you to age.

#include <stdio.h>
 
int main() {
    int age = 25;
    int *p;          /* declare a pointer to int — the * here means "pointer to" */
 
    p = &age;        /* store the address of age inside p */
 
    printf("age = %d\n", age);      /* 25 — direct access */
    printf("&age = %p\n", &age);    /* address of age */
    printf("p = %p\n", p);          /* p holds the same address */
    printf("*p = %d\n", *p);        /* dereference p — go to that address, get the value */
 
    /* you can also CHANGE age through the pointer */
    *p = 30;                         /* go to address p, write 30 there */
    printf("age is now: %d\n", age); /* age is now: 30 */
 
    return 0;
}

#include <stdio.h>
 
int main() {
    int   num    = 100;
    float price  = 9.99;
    char  letter = 'Z';
 
    int   *ip = &num;      /* int pointer — points to an int */
    float *fp = &price;    /* float pointer — points to a float */
    char  *cp = &letter;   /* char pointer — points to a char */
 
    printf("num    via pointer: %d\n",  *ip);  /* 100 */
    printf("price  via pointer: %f\n",  *fp);  /* 9.990000 */
    printf("letter via pointer: %c\n",  *cp);  /* Z */
 
    return 0;
}

#include <stdio.h>
 
int main() {
    int arr[4] = {10, 20, 30, 40};
    int *p = arr;  /* p points to arr[0] — array name IS the address of first element */
    int i;
 
    printf("Using pointer arithmetic to walk the array:\n");
    for (i = 0; i < 4; i++) {
        printf("p+%d = address %p, value = %d\n", i, p+i, *(p+i));
    }
 
    /* arr[i] and *(p+i) are identical — two ways to say the same thing */
    printf("\narr[2]   = %d\n", arr[2]);    /* 30 */
    printf("*(p + 2) = %d\n", *(p + 2));    /* 30 — same thing */
 
    return 0;
}

#include <stdio.h>
 
/* WRONG: pass by value — cannot change the original */
void tryDouble_wrong(int n) {
    n = n * 2;  /* only changes the local copy — original untouched */
}
 
/* CORRECT: pass by reference — can change the original */
void tryDouble_correct(int *n) {
    *n = *n * 2;  /* dereference n to reach the original variable */
}
 
int main() {
    int x = 10;
 
    tryDouble_wrong(x);
    printf("After wrong:   x = %d\n", x);   /* x = 10 — unchanged */
 
    tryDouble_correct(&x);                   /* pass the ADDRESS of x */
    printf("After correct: x = %d\n", x);   /* x = 20 — changed! */
 
    return 0;
}

#include <stdio.h>
 
void swap(int *a, int *b) {
    int temp = *a;  /* save value at address a */
    *a = *b;        /* write value at b into address a */
    *b = temp;      /* write saved value into address b */
}
 
int main() {
    int x = 100, y = 200;
 
    printf("Before: x = %d, y = %d\n", x, y);  /* x=100, y=200 */
    swap(&x, &y);
    printf("After:  x = %d, y = %d\n", x, y);  /* x=200, y=100 */
 
    return 0;
}

#include <stdio.h>
 
/* when you write int arr[], C treats it as int *arr internally */
void doubleAll(int *arr, int n) {
    int i;
    for (i = 0; i < n; i++) {
        arr[i] = arr[i] * 2;  /* modifies the ORIGINAL array */
    }
}
 
int main() {
    int numbers[5] = {1, 2, 3, 4, 5};
    int i;
 
    doubleAll(numbers, 5);  /* pass array name = pass address of first element */
 
    for (i = 0; i < 5; i++) {
        printf("%d ", numbers[i]);
    }
    /* Output: 2 4 6 8 10 — original array was modified */
 
    return 0;
}

#include <stdio.h>
 
int main() {
    int *p = NULL;  /* p does not point to anything yet */
 
    /* ALWAYS check before dereferencing */
    if (p != NULL) {
        printf("Value: %d\n", *p);
    } else {
        printf("Pointer is NULL — nothing to read\n");
    }
 
    /* Assigning a real address later */
    int age = 25;
    p = &age;
 
    if (p != NULL) {
        printf("Value: %d\n", *p);  /* Value: 25 */
    }
 
    return 0;
}