An array is a sequence of data items of the same type.

float marks[100];

It's possible to initialize an array during declaration. 

Example:

void main()

{

    int marks[10] = {1,2,3,4,5,6,7,8,9,0}, 

         i , 

         n = 10, 

         sum = 0;

    float average;

    for(i=0; i<n; ++i)

    {

       sum += marks[i];

    }

    average = (float)sum/n;

    printf("Average = %f", average);

}

Output:

 Average = 4.500000