Initialize an Array in C

An initializer list initializes elements of an array in the order of the list.

For example, consider the below snippet:

int arr[5] = {1, 2, 3, 4, 5};

This initializes an array of size 5, with the elements {1, 2, 3, 4, 5} in order.

This means that arr[0] = 1, arr[1] = 2, and so on.

We don’t need to initialize all the elements 0 to 4. We can even do only from indices 0 to 2.

The following code is also valid:

int arr[5] = {1, 2, 3};

But now, arr[4] and arr[5] will still remain garbage values, so you need to be careful!

If you’re using an initializer list with all elements, you don’t need to mention the size of the array.

// Valid. Size of the array is taken as the number of elements
// in the initializer list (5)
int arr[] = {1, 2, 3, 4, 5};
// Also valid. But here, size of a is 3
int a[] = {1, 2, 3};

If you want to initialize all the elements to 0, there is a shortcut for this (Only for 0). We can simply mention the index as 0.

#include <stdio.h>
int main() {
    // You must mention the size of the array, if you want more than one
    // element initialized to 0
    // Here, all 5 elements are set to 0!
    int arr[5] = {0};
    for (int i=0; i<5; i++) {
        printf("%d\n", arr[i]);
    }
    return 0;
}

Output

0
0
0
0
0

If you’re using multi-dimensional arrays, you can still initialize them all in one block, since arrays are stored in a row-wise manner.

#include <stdio.h>
int main() {
    int arr[3][3] = {1,2,3,4,5,6,7,8,9};
    for (int i=0; i<3; i++)
        for (int j=0; j<3; j++)
            printf("%d\n", arr[i][j]);
    return 0;
}

Output

1
2
3
4
5
6
7
8
9

A similar method can also be used for other datatypes, like float, char, char*, etc.

#include <stdio.h>
int main() {
    // Array of char* elements (C "strings")
    char* arr[9] = { "Hello", [1 ... 7] = "JournalDev", "Hi" };
    for (int i=0; i<9; i++)
        printf("%s\n", arr[i]);
    return 0;
}

Output

Hello
JournalDev
JournalDev
JournalDev
JournalDev
JournalDev
JournalDev
JournalDev
Hi

Remember, this method with [1 … 7] = “Journaldev” might not work with all compilers. I work on GCC in Linux.

We can also use the for loop to set the elements of an array.

#include <stdio.h>
int main() {
    // Declare the array
    int arr[5];
    for (int i=0; i<5; i++)
        arr[i] = i;
    for (int i=0; i<5; i++)
        printf("%d\n", arr[i]);
    return 0;
}

Output

0
1
2
3
4

If you’re using gcc as your C compiler, you can use designated initializers, to set a specific range of the array to the same value.

// Valid only for gcc based compilers
// Use a designated initializer on the range
int arr[9] = { [0 ... 8] = 10 };

Note that there is a space between the numbers and there are the three dots. Otherwise, the compiler may think that it is a decimal point and throw an error.

#include <stdio.h>
int main() {
    int arr[9] = { [0 ... 8] = 10 };
    for (int i=0; i<9; i++)
        printf("%d\n", arr[i]);
    return 0;
}

Output (for gcc only)

10
10
10
10
10
10
10
10
10

We can also combine this with our old initializer list elements!

For example, I am setting only array index arr[0], arr[8] as 0, while the others are designated initialized to 10!

#include <stdio.h>
int main() {
    int arr[9] = { 0, [1 ... 7] = 10, 0 };
    for (int i=0; i<9; i++)
        printf("%d\n", arr[i]);
    return 0;
}

Output

0
10
10
10
10
10
10
10
0

In this article, we learned how we could initialize a C array, using different methods.

For similar articles, do go through our tutorial section on C programming!