Calculating lowest subtotal of array in C - arrays

Question

I want to search for the lowest subtotal of k values in an array z with n elements, filled by user input. This works fine with k==1 but I do not understand why my code does not work for k>1.
My program outputs the wrong index and subtotal.
The function is being called from main, after filling the array with n elements of int values in a loop until it's filled, asking for the desired length of k.
Please help me understand what I'm doing wrong
#include <stdio.h>
void lowest_subtotal(int z[], int n, int k){
int t[]={0};
int i = 0, position = 0, tempmin;
if(k==1){ //working 100%
while(i<n){
if(z[i]<tempmin){
tempmin=z[i];
position=i;
}
i++;
}
printf("Lowest subtotal on Index %d. (sum: %d.)", position, tempmin);
}
else{
//calculate subtotals
while(i<=n){
for(int j=0; j<k; j++){
if(z[i+j]<0){
t[i] -= z[j];
}
else{
t[i] += z[j];
}
}
i++;
}
//search for lowest subtotal
i = 0;
while(i<n-k+1){
if(t[i] < tempmin){
tempmin = t[i];
position=i;
}
i++;
}
//wrong result of index and sum?
printf("Lowest subtotal on Index %d. (sum: %d.)",position-k-1,tempmin);
}
}
int main(){
int n,k; //length of vector, length of subtotal
printf("n: ");
scanf("%d",&n);
int z[99] = { 0 }; //initialize array
printf("input elements:\n");
for(int i=0; i<n; i++){
scanf("%d",&z[i]);
}
printf("length of subtotal?: ");
scanf("%d",&k);
lowest_subtotal(z,n,k);
}

Answer 1

It will be much easier for you if you split it into separate tasks (functions.
long long subtotal(int *arr, size_t size, size_t start, size_t k)
{
long long result = 0;
for(size_t index = 0; index < k; index ++)
{
result += arr[start + index];
}
return result;
}
long long smallestSubtotal(int *arr, size_t size, size_t k, size_t *startpos)
{
long long result = LLONG_MAX;
for(size_t start = 0; start < size - k; start++)
{
long long sum = subtotal(arr, size, start, k);
if(result > sum)
{
if(startpos) *startpos = start;
result = sum;
}
}
return result;
}
void lowest_subtotal(int z[], size_t n, size_t k)
{
size_t index;
long long subt;
subt = smallestSubtotal(z, n, k, &index);
printf("The smallest subtotal of length %zu is %lld at index %zu\n", k, subt, index);
}
int main(void)
{
int arr[SIZE];
srand(time(NULL));
for(size_t index = 0; index < SIZE; index++)
{
arr[index] = rand();
}
lowest_subtotal(arr, SIZE, 15);
}

Answer 2

As I understand from the OP code, the task is to find the starting index of k consecutive numbers in an array of n numbers such that the sum of the k numbers is minimal.
The case of k=1 need not be a special case. Besides, the idea of updating the sum of a "window" of k elements is good, but there are several errors in the implementation. A working version of lowest_subtotal() could be as follows:
#include<limits.h>
int lowest_subtotal(int z[], int n, int k)
{
if(k > n || k < 1)
{
return -1; // Error, k must be in the range from 1 to n
}
// Calculate sum in window at index 0
int sum = 0;
for(int i = 0; i < k; i++)
{
sum += z[i];
}
// Slide window and update sum
int index = 0;
int min_sum = sum;
for(int i = 1; i < n - k; i++)
{
sum += z[i+k-1] - z[i-1]; // Add incoming value and subtract the leaving value
if(sum < min_sum)
{
min_sum = sum;
index = i;
}
}
printf("Lowest subtotal at index %d (sum=%d)\n", index, min_sum);
return index;
}