Simple Array Sum in c++

Question

I'm beginner in C++, and I've got a question about a simple sum code in c++.

Here is my code :

#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
int main()
{
 int n;
 int sum;
 int arr_i = 0;
 cin >> n;
 vector<int> arr(n);
 while (arr_i != n)
 {
 cin >> arr[arr_i];
 sum += arr[arr_i];
 //cout << sum << endl;
 if (arr_i == n - 1)
 cout << sum;
 arr_i++;
 }
 return 0;
}

The output doesn't print the correct answer without "cout << sum" before the if condition.

How can I solve this problem ?

• Thomas Matthews: You don't need to include cmath.h for a simple sum.
• Happy Green Kid Naps: @ThomasMatthews - He doesn't need to put them in a vector either. All he needs is iostream. He is beginning C++. Nit: It is either math.h or cmath, not cmath.h
• lifebalance: ...if you happen to be on C++14, a one-line equivalent of the while loop would be cout << accumulate(arr.begin(), arr.end(), 0);

Answer 1

You forget to initialize sum to 0.

int sum = 0;

Answer 2

As the previous post mentioned, sum was not initialized to 0. In terms of good practices and styles its a good idea to initialize any variables that are modified within a loop right before the loop body so that someone reading the code can easily grasp the context of your variables.

int main()
{
 int n;
 int sum;
 int arr_i;
 cin >> n;
 vector<int> arr(n);
 sum = 0;
 arr_i = 0;
 while (arr_i != n)
 {
 cin >> arr[arr_i];
 sum += arr[arr_i];
 //cout << sum << endl;
 if (arr_i == n - 1)
 cout << sum;
 arr_i++;
 }
 return 0;
}

or as I prefer a "for" loop...

int main()
{
 int n;
 int sum;
 int arr_i;
 cin >> n;
 vector<int> arr(n);
 for (sum = 0, arr_i = 0; arr_i != n; arr_i++)
 {
 cin >> arr[arr_i];
 sum += arr[arr_i];
 //cout << sum << endl;
 if (arr_i == n - 1)
 cout << sum;
 }
 return 0;
}

• Jarod42: Even better, initialize them at declaration, and reduce scope of the variable.
• Art Solano: @Jarod42 Good point. Yes, i didn't want to deviate too much from his original post, but definitely declaring and initializing the variable in a block of code where it is only used is even better. So a "for (int sum = 0; int array_i = 0; arr_i != n; arr_i++)" would be best in this case.
• user2277872: A for loop would actually be the preferred method as you know how many times the loop is iterating because they are having the user enter "n" times of iterations. Even though a while loop is justifiable, a for loop seems more proper.