How to make array 1*x and sum up its digits? - c

Question

So i have this type of problem. How to make an array 1*x and then sum up its digits together. I wrote down something like this for now. Any ideas? Thank you.
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
/* run this program using the console pauser or add your own getch, system("pause") or input loop */
int main(int argc, char *argv[])
{
int a,i,w,j,m;
int s[a];
printf("How many digits do you want to sum up\n");
scanf("%d",&a);
for(i=0;i<a;i++)
{
printf("Enter numer %d: ",i);
scanf("%d",&s[i]);
}
for(j=0;j<a;j++)
{
m=s[j]+s[j++];
}
printf("\n %d",m);
return 0;
}

Answer 1

The problems of your code are:
int a;
int s[a];
Here a is uninitialized.So,array size is unknown which is incorrect.And,instead of this
m=s[j]+s[j++];
you should do like this :
m += s[j];
One more thing,you have initialize m = 0 before starting to add.
I've Changed your program to this:
#include <stdio.h>
int main(int argc, char *argv[]) {
int a,i,m = 0;
//First get the array size
printf("How many digits do you want to sum up\n");
scanf("%d",&a);
//Then declare the array with the size (a)
int s[a];
for(i = 0; i < a; i++){
printf("Enter numer %d: ",i);
scanf("%d",&s[i]);
m += s[i];
}
printf("\n %d",m);
return 0;
}

Answer 2

Using uninitialized variable is undefined behaviour.
int s[a];
The above statement defines an array s of size a but the value of a is unpredictable since it is uninitialized and contains garbage. The size of the array must be known when it defined and it remains the same throughout its lifetime. You cannot resize your array by changing the value of a here. You can use dynamic memory allocation using malloc.
Further, the following statement again invokes undefined behaviour -
m=s[j]+s[j++];
That's because it violates the the following rule stated in the C99 standard §6.5 ¶2
Between the previous and next sequence point an object shall have its
stored value modified at most once by the evaluation of an expression.
Furthermore, the prior value shall be read only to determine the value
to be stored.