Initialization of Pointer Arrays in C Programming

Discover how to initialize an array of pointers in C programming. Learn how pointers store the addresses of variables and how you can use them effectively by creating arrays of pointers. This guide covers the initialization process with examples, helping you to manage memory addresses efficiently in your C programs.

Initialization of Pointer Arrays in C

A pointer is a special type of variable that stores the address of another variable. To define a pointer variable, you use the * symbol before the variable name. Just like regular variables, you can also create an array of pointers, where each index in the array holds the address of an array type.

How to Initialize an Array of Pointers in C?

You can initialize a pointer variable at the time of its declaration by assigning it the address of an existing variable. Here's an example:

int x = 25;
int *y = &x;

By default, all variables, including pointer variables, belong to the auto storage class. This means a pointer variable may store an unpredictable or random memory address, which can lead to undefined behavior, such as segmentation faults. Therefore, it's good practice to initialize pointers to NULL if you don’t have a specific value to assign at the time of declaration:

int *ptr = NULL;

An array of pointers stores addresses in each of its elements. The type of the array should match the type of the target variable.

Initialize Array of Pointers Using the Static Keyword

You can also use the static keyword to initialize an array of pointers, which will automatically set each element to 0.

Example

#include <stdio.h>

int main() {
    static int *ptr[5];

    for (int i = 0; i < 5; i++) {
        printf("ptr[%d] = %d\n", i, ptr[i]); 
    }
    
    return 0;
}

ptr[0] = 0
ptr[1] = 0
ptr[2] = 0
ptr[3] = 0
ptr[4] = 0

Initialize Array of Integer Pointers

In this example, we declare an array of integer pointers and store the addresses of three integer variables.

Example

#include <stdio.h>

int main() {
    int a = 15, b = 25, c = 35;
    int *ptr[3] = {&a, &b, &c};

    for (int i = 0; i < 3; i++) {
        printf("ptr[%d]: address: %d value: %d\n", i, ptr[i], *ptr[i]);
    }
    
    return 0;
}

ptr[0]: address: 6422050 value: 15
ptr[1]: address: 6422046 value: 25
ptr[2]: address: 6422042 value: 35

Initialize Array of Pointers by Direct Addresses

You can store the address of each element of a normal array in the corresponding element of a pointer array.

Example

#include <stdio.h>

int main() {
    int arr[] = {15, 25, 35};
    int *ptr[3] = {&arr[0], &arr[1], &arr[2]};

    for (int i = 0; i < 3; i++) {
        printf("ptr[%d]: address: %d value: %d\n", i, ptr[i], *ptr[i]);
    }
    
    return 0;
}

ptr[0]: address: 6422042 value: 15
ptr[1]: address: 6422046 value: 25
ptr[2]: address: 6422050 value: 35

Traversing an Array with Its Base Address

When you obtain the base address of an array (for example, &arr[0]), you can access the addresses of its subsequent elements since the pointer increments by the size of the data type.

Thus, with just the base address (the name of the array represents the address of its 0th element), you can traverse the array.

Example 1

Here's an example:

#include <stdio.h>

int main() {
    int arr[] = {15, 25, 35};
    int *ptr = arr;

    for (int i = 0; i < 3; i++) {
        printf("ptr[%d]: address: %d value: %d\n", i, ptr + i, *(ptr + i));
    }
    
    return 0;
}

ptr[0]: address: 6422036 value: 15
ptr[1]: address: 6422040 value: 25
ptr[2]: address: 6422044 value: 35

Example 2: Traversing a 2D Array Using a Pointer Array

In this example, we have a 2D array. The address of the first element of each row is stored in a pointer array. When traversing, we can use the address stored in each element of the pointer array, which points to the first element of the corresponding row, incremented to fetch the values in each row.

#include <stdio.h>

int main() {
    // 2D array
    int arr[3][4] = {
        {10, 20, 30, 40},
        {50, 60, 70, 80},
    };

    int ROWS = 2, COLS = 4;
    int i, j;

    // Pointer
    int (*ptr)[4] = arr;

    // Print the elements of the array via pointer ptr
    for (i = 0; i < ROWS; i++) {
        for (j = 0; j < COLS; j++) {
            printf("%d ", *(ptr[i] + j));
        }
        printf("\n");
    }
    
    return 0;
}

10 20 30 40 
50 60 70 80 

Example 3

In this case, we don't need a pointer array, as we can use the name of this 2D array as its base pointer, incrementing it row and column-wise to access the elements in the given 2D array:

#include <stdio.h>

int main() {
    // 2D array
    int arr[3][4] = {
        {10, 20, 30, 40},
        {50, 60, 70, 80},
    };

    int ROWS = 2, COLS = 4;
    int i, j;

    // Pointer
    int *ptr = arr;

    // Print the elements of the array via pointer ptr
    for (i = 0; i < ROWS; i++) {
        for (j = 0; j < COLS; j++) {
            printf("%d  ", *(ptr + i * COLS + j));
        }
        printf("\n");
    }
    
    return 0;
}

10  20  30  40  
50  60  70  80  

Initialize Array of Character Pointers (String)

In C, a string is an array of characters. Since the name of an array also represents the address of its 0th element, you can declare a string like this:

char arr[] = "Goodbye";

Using pointer notation, you can assign a string to a character pointer like this:

char *arr = "Goodbye";

You can then declare an array of character pointers to store multiple strings:

char *arr[3] = {"string1", "string2", "string3"};

Example

Here’s an example with an array of character pointers to store names of programming languages:

#include <stdio.h>

int main() {
    char *langs[10] = {
        "JAVA", "RUST", "GO",
        "SWIFT", "HTML", "KOTLIN", "C#",
        "REACT", "NODE", "PHP"
    };

    for (int i = 0; i < 10; i++) {
        printf("%s\n", langs[i]);
    }

    return 0;
}

JAVA
RUST
GO
SWIFT
HTML
KOTLIN
C#
REACT
NODE
PHP

In this program, langs is a pointer to an array of 10 strings. If langs[0] points to address 5000, then langs + 1 will point to address 5004, which stores the pointer to the second string. Therefore, we can also use the following loop variation to print the array of strings:

for (int i = 0; i < 10; i++) {
    printf("%s\n", *(langs + i));
}

Initialization of Dynamic Array of Pointers

You can also use the malloc() function to declare and initialize an array of pointers dynamically.

Example

#include <stdio.h>
#include <stdlib.h>

int main() {
    int *arr = (int *)malloc(sizeof(int) * 5); 

    for (int i = 0; i < 5; i++) {
        arr[i] = i + 5;
    }
    for (int x = 0; x < 5; x++) {
        printf("%d %d\n", x, arr[x]);
    }
    
    free(arr);  // Free the allocated memory
    return 0;
}

0 5
1 6
2 7
3 8
4 9

You can also request user input to assign values to the elements in the pointer array:

for (int i = 0; i < 5; i++) {
    scanf("%d", &arr[i]);
}