C :: Array With Only Zeros - Add Value To All Of Previous Values

For my assignment I have to have an array with only zeros.

Code:
int a[20] = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 }; Then I need to send it into a function that makes the array like this

Code: int a[20] = {0,1,2,3,4,5,6, ... , 19}

Which I have done here

Code:
int initialize(int a[], int n) {
int m = 0;
int i;
printf("

[Code] ....

Now I need to do the following with the array. I need to take whatever value is in each position and add that value to all of the previous values. like this.

Code:
a[3] = a[3] + a[2] + a[1] + a[0]

only for every a[i] I know that I can code this the long way, but I just can't see to be able to find out how to do this a better way.