Different ways to Initialize all members of an array to the same value in C

An array is a collection of data that holds fixed number of values of same type. For example: if you want to store marks of 100 students, you can create an array for it.

int num[100];

How to declare an array in C?

Data_type  array_name[size_of_array];

For example,

float num[10];

Below are some of the different ways in which all elements of an array can be initialized to the same value:

  1. Initializer List: To initialize an array in C with the same value, the naive way is to provide an initializer list. We use this with small arrays.

    int num[5] = {1, 1, 1, 1, 1};
    

    This will initialize the num array with value 1 at all index.
    We may also ignore the size of the array:

    int num[ ] = {1, 1, 1, 1, 1}
    

    The array will be initialized to 0 in case we provide empty initializer list or just specify 0 in the initializer list.

    int num[5] =  { };                // num = [0, 0, 0, 0, 0]
    int num[5] =  { 0 };             // num = [0, 0, 0, 0, 0]
    
  2. Designated Initializer: This initializer is used when we want to initialize a range with the same value. This is used only with GCC compilers.
    [ first . . . last ] = value;
    int num[5]={ [0 . . . 4 ] = 3 };               // num = { 3, 3, 3, 3, 3}
    

    We may also ignore the size of array:

    int num[  ]={ [0 . . . 4 ] = 3 };               // num = { 3, 3, 3, 3, 3}
    
  3. Macros: For initializing a huge array with the same value we can use macros.
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    #include<stdio.h>
      
    #define x1 1
    #define x2 x1, x1
    #define x4 x2, x2
    #define x8 x4, x4
    #define x16 x8, x8
    #define x32 x16, x16
      
    int main(void)
    {
    // array declaration
    int num[] = { x32, x8, x4, x1};
    int size = sizeof(num)/ sizeof(int);    // 32+8+4+1= 45
      
    printf("The size of the array is %d\n", size);
    printf("The value of element in the array at index 5 is %d "
                                        num[4]);
      
    return 0;
      
    }
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    Output:
    The size of the array is 45
    The value of element in the array at index 5 is 1
    
  4. Using For Loop: We can also use for loop to initialize an array with the same value.
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    #include<stdio.h>
      
    int main(void)
    {
        int size = 6;
        int val = 1;
      
        // array declaration
        int arr[size];
        int i;
      
        // initializing array elements
        for (i = 0; i < size ; i++){
            arr[i] = val;
        }
      
         // print array
         printf("The array is:");
         for (i = 0; i < size ; i++){
             printf("%d ", arr[i]);
        }
      
        return 0;
    }
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    Output:
    The array is:1 1 1 1 1 1
    

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