C++ Program producing false result... What is the cause? - c++

Question

Something is wrong with this code as it modifies an array. All elements are assigned 0. But why ?? Here's the question of program:
Write a function named "subtotal" takes as its arguments the following:
(1) an array of floating point values;
(2) an integer that tells the number of cells in the array.
The function should replace the contents of each cell with the sum of the contents of all the cells in the
original array from the left end to the cell in question.
Thus, for example, if the array passed to the
function looks like this:
[0] [1] [2] [3] [4]
5.8 | 2.6 | 9.1 | 3.4 | 7.0
then when the function returns, the array will have been changed so that it looks like this:
[0] [1] [2] [3] [4]
5.8 | 8.4 | 17.5 | 20.9 | 27.9
because 5.8 + 2.6 = 8.4 and 5.8 + 2.6 + 9.1 = 17.5 and so on. Note that the contents of cell [0] are not
changed. The function should not return a value.
And here is the program which I've made:
#include<iostream>
using namespace std;
void subtotal(float*,int);
int main(void)
{
int n;
cout << "Enter the number of cells: ";
cin >> n;
float* cell;
cell = new float[n];
cout << "Enter the elements: ";
for(int i = 0; i < n; i++)
cin >> *(cell + i);
subtotal(cell, n);
for(int i = 0; i < n; i++)
cout << *(cell + i) << "|";
delete[] cell;
}
void subtotal(float* cell, int n)
{
int i, j;
for(i = 0; i < n; i++)
{
float sum = 0;
for(j = 0; j < i; j++)
{
sum += *(cell + j);
}
*(cell + i) = sum;
}
}

Answer 1

Why using nested loop? You can use only one loop so that the you'll iterate once is enough as long as you want only sum the consecutive elements so change the function subtotal to:
void subtotal(float* cell, int n)
{
for(int i = 0; i < n - 1; i++) // `n - 1` to avoid outband
{
float sum = cell[i] + cell[i + 1];
cell[i + 1] = sum;
}
}
Or if you want to do it using pointer like in your case:
void subtotal(float* cell, int n)
{
for(int i(0); i < n - 1; i++)
{
float sum = *(cell + i) + *(cell + i + 1);
*(cell + i + 1) = sum;
}
}

Answer 2

You should do j <= i in the inner loop. If you don't do that, the first new element in the array will be zero (summing over no elements at all). For the second element, it will only sum over the first, which is 0 again. Etc.
In addition, your code is very non-idiomatic. The normal way to address an array element is cell[i], not *(cell+i), although both ways are possible for plain arrays.

Answer 3

This is how your code should be:
#include<iostream>
using namespace std;
void subtotal(float*,int);
int main(void)
{
int n = 5;
float *cell=new float[n];
cell[0] = 5.8f;
cell[1] = 2.6f;
cell[2] = 9.1f;
cell[3] = 3.4f;
cell[4] = 7.0f;
subtotal(cell,n);
for(int i=0;i<n;i++)
cout<<*(cell+i)<<"|";
delete []cell;
}
void subtotal(float*cell,int n)
{
float * temp = new float [n];
int i,j;
for(i=0;i<n;i++)
{
float sum=0;
for(j=0;j<=i;j++) // Change here
{
sum+=*(cell+j);
}
*(temp+i)=sum; // Change here
}
for (int i =0; i < n; ++i) {
cell[i] = temp[i]; // Copy the values back
}
delete [] temp;
}
There are two problems with your subtotal function.
First, you should do j <= i in the inner loop.
Next, you should store the summation in a temporary array, else you are going to make changes to the original input after each subset sum and the output you will get will not be the desired one. You would have got output like this :
5.8|8.4|23.3|40.9|85.4|
However, with the changes I have proposed, you should get the desired output:
5.8|8.4|17.5|20.9|27.9|
corrected-code-here