Pascal and C: different output

Question

1st question on StackOverflow...I have 2 programs, one in C and one in Pascal, that should give me the same output but they don't.

Pascal:

program ssgf108;
uses crt;
var n,q,r,s,i:integer;
var a:array[1..10] of integer;

begin
        a[1]:=3;
        a[2]:=-5;
        a[3]:=12;
        a[4]:=7;
        a[5]:=-4;
        a[6]:=-3;
        a[7]:=2;
        a[8]:=8;
        a[9]:=4;
        a[10]:=10;
        readln(n);
        q:=0;
        r:=0;
        s:=1;
        for i:=1 to n do
        begin
                q:=i*a[i];
                r:=i+a[i];
                s:=s+q*r;
        end;
        writeln(q,' ',r,' ',s);
end.

C:

#include <stdio.h>

int main(){
        int a[10],n,q,r,s,i;
        scanf("%d",&n);
        a[0]=3;
        a[1]=-5;
        a[2]=12;
        a[3]=7;
        a[4]=-4;
        a[5]=-3;
        a[6]=2;
        a[7]=8;
        a[8]=4;
        a[9]=10;
        q=0;
        r=0;
        s=1;
        for (i=0;i<=n-1;i++){
                q=i*a[i];
                r=i+a[i];
                s=s+q*r;
        }
        printf("%d %d %d",q,r,s);
        return 0;
}

Upon entering 5 as n, Pascal outputs -20 1 871 while C outputs -16 0 567. Where's the error? Background for this question: I'm doing a problem solving competition at my school, next challenge is close, some questions require programming, I've done the first 2 challenges with Bash and Pascal, now I'm learning C so I'd like to do them in C. But I MUST avoid wrong results, since they mean lost points. This is a training on the competition's site, to convert pseudocode into code.

Answer 1

The values

q = i * a[i];
r = i + a[i];

depend on the values of i inside the loop, which are different between one-based and zero-based indexing loops. You need to do arithmetic with i+1 inside the C loop:

for (i = 0; i < n; i++) {
    int i1 = i + 1;
    q = i1 * a[i];
    r = i1 * a[i];
    s += q * r;
}