Troubleshooting for loop - problems with sum - c

Question

Im taking a C programming beginning course, and have a problem with a for loop.
I want to make two double arrays. One that takes input from user, and one that sums the inputs.
I then want to output the two arrays. One that shows the input, and one that shows the sums og the input in every cell.
The problem comes when I try to show the sum for every "cell" in the array.
The output just becomes the same as the input.
I can solve it with:
// print all the numbers in the second array
for (i = 0; i < n; i++) {
sum = sum + a[i];
printf("%lf ", sum);
but then the assignment wouldn't be solved. Hope you can educate me.
#include <stdio.h> #include <stdlib.h>
int main(void) {
int n; //numbers of cells
int i; //The numbers in the cells
printf("How many cells in the array would you like? \n");
scanf("%d", &n);
double a[n], sum=0; // declare an array and a loop variable.
double b[n], sumo=0;
printf("Enter the numbers:\n");
for (i = 0; i < n; i++) { // read each number from the user
scanf("%lf", &a[i]);
}
printf("The results, my lord:\n");
// print all the numbers in the first array
for (i = 0; i < n; i++) {
printf("%lf ", a[i]);
}
printf("\n"); //THIS IS WHERE THE PROBLEM STARTS
// print all the numbers in the second array
for (i = 0; i < n; i++) {
b[i] = b[i] + a[i];
printf("%lf ", b[i]);
}
return 0;
}

Answer 1

For starters the array b is not initialized
double b[n], sumo=0;
So this statement
b[i] = b[i] + a[i];
invokes undefined behavior.
It seems what you need is the following for loop
for (i = 0; i < n; i++) {
b[i] = a[i];
if ( i != 0 ) b[i] += a[i-1];
printf("%lf ", b[i]);
}

Answer 2

You can calculate the sum (aka the b array) when you read the user data:
for (i = 0; i < n; i++)
{
scanf("%lf", &a[i]);
if (i == 0)
{
b[i] = a[i];
}
else
{
b[i] = b[i-1] + a[i];
}
}
or do it like:
scanf("%lf", &a[0]);
b[0] = a[0];
for (i = 1; i < n; i++)
{
scanf("%lf", &a[i]);
b[i] = b[i-1] + a[i];
}
or do it like:
sum = 0;
for (i = 0; i < n; i++)
{
scanf("%lf", &a[i]);
sum += a[i];
b[i] = sum;
}

Answer 3

The last part is how I solved the problem. I found the issue that. I didn't update the "sum" for ever loop. It adds a new value in b[I] this way. :-)
double a[n], sum=0; // declare an array and a loop variable.
double b[n];
printf("Enter the numbers:\n");
for (i = 0; i < n; i++) { // read each number from the user
scanf("%lf", &a[i]);
}
printf("The results, my lord:\n");
// print all the numbers in the first array
for (i = 0; i < n; i++) {
printf("%.2lf ", a[i]);
}
printf("\n");
// print all the numbers in the second array
for (i = 0; i < n; i++) {
sum = sum + a[i];
b[i] = sum;
printf("%.2lf ", b[i]);