C++ How to add numbers in an array within a given range?

Question

I'm working on an assignment for class and am having a bit of a hard time putting it together. I had just started learning arrays and am not sure exactly how to get user input in the array.

Here is the assignment prompt: Create a program that inputs up to 100 integers (space separated!) and outputs their sum. For example:

1 2 3 4 5 6 7 8 9 10 55

This is what I have so far (edit because I forgot to change comments):

#include <iostream>
using namespace std;

int addNum(int n);

int main() {
   int n;

   // prompt user to input numbers
   cout << "Please enter in values to add together: ";
   cin >> n;

   cout << addNum(n);

   // pause and exit
   getchar();
   getchar();
   return 0;
}

// function
int addNum(int n) {
   int arr[99] = {};
   int sum = 0;

   for (int i = 0; i < n; i++) {
      sum = sum + arr[i];
   }
   return sum;
}

If your array needs to store up to 100 elements why do you set the size to 99 (`int arr[99] = {};`)? — NathanOliver, Aug 03 '17 at 16:51
`// declaration of a new array` says your comment, but you aren't declaring any array there at all. Also where is the user supposed to input anything into the array? — UnholySheep, Aug 03 '17 at 16:51
I thought that since the arrays start at 0 then going to 99 would make it 100 elements? — J.Chu, Aug 03 '17 at 16:52
The number in the brackets when you declare the arrays is the size, not the last valid index. Sounds like you could use a [good C++ book](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) — NathanOliver, Aug 03 '17 at 16:53
The `arr[]` is created inside `addNum()` which means it's deleted when the function returns, all previous values are lost. Declare it outside the func. — Ripi2, Aug 03 '17 at 16:55
`for (int i = 0; i < n; i++)` -- This loop is not safe in that you do not check if `n` exceeds the highest valid index of the array. — PaulMcKenzie, Aug 03 '17 at 17:00

score 4 · Answer 1 · answered Aug 03 '17 at 16:57

Since this is a learning exercise, I wouldn't correct your code, but explain what you've missed so far:

The assignment asks to read integers until there's no more input; your code prompts the user for the count upfront, which should be removed.
You do not need an array to store the individual numbers, because the assignment asks only for the total. This can be computed on the fly: read a number, add it to sum, and forget the number.
You can read numbers until the end of input with a simple loop that uses >> operator below.

Here is an example that limits the input to 100 numbers, or stops when the input stream ends:

int limit = 0;
int nextNumber;
while ((limit++ != 100) && (cin >> nextNumber)) {
    ... // Process the next number
}

If you are giving your program input from console (as opposed to feeding it a file with numbers) and you need to end your input sequence, press Ctrl+z on Windows or Ctrl+d on UNIX.

score 1 · Answer 2 · answered Aug 03 '17 at 17:22

In order to provide a diverse range of answers, std::accumulate is pretty cool.

http://en.cppreference.com/w/cpp/algorithm/accumulate

int sum = std::accumulate(v.begin(), v.end(), 0);

or in your case

int sum = std::accumulate(arr, arr + 99, 0);

Another functional approach is to use std::reduce introduced in C++17

http://en.cppreference.com/w/cpp/algorithm/reduce

J.Chu · Answer 3 · 2017-08-11T02:32:12.667

Was able to get a working code after talking to the professor about it! Here's the working code:

#include <iostream>
using namespace std;

int main() {
    // variable declarations
    int sum = 0, count = 0;
    int c;

    // array declaration
    int arr[100] = { 0 };

    // prompt user to input numbers & add
    cout << "Please enter in values to add together: ";
    for (int i = 0; i < 100; i++) {
        cin >> arr[i];
        c = cin.peek();
        sum = sum + arr[i];

        // if user presses enter, skip to outputting sum without waiting for 100 values
        if (c == '\n'){
            break;
        }
    }

    // output the sum of input
    cout << sum;

    // pause and exit
    getchar();
    getchar();
    return 0;
}

C++ How to add numbers in an array within a given range?

3 Answers3