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 ?

@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 — Happy Green Kid Naps
– Happy Green Kid Naps, Commented Apr 8, 2016 at 22:18
...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); — lifebalance
– lifebalance, Commented Apr 9, 2016 at 4:41

Jarod42 · Accepted Answer · 2016-04-08 22:09:53Z

7

You forget to initialize sum to 0.

int sum = 0;

answered Apr 8, 2016 at 22:09

Jarod42

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Comments

Art Solano · Accepted Answer · 2016-04-08 22:22:40Z

1

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;
}

answered Apr 8, 2016 at 22:22

Art Solano

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3 Comments

Jarod42 Over a year ago

Even better, initialize them at declaration, and reduce scope of the variable.

Art Solano Over a year ago

@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 Over a year ago

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.

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