How to fill an array with integers and display it?

Question

I started to get really good at programming but then I reached a part in this course that I always hate: Arrays. I never completely understood arrays in C++ and it's very confusing to me. I have a very simple program and I just need a little bit of help on what I'm doing wrong.

Here is my code so far:

#include <stdio.h>
#include <stdlib.h>

main() {

    int num[50];
    int i;

        for (i = 0; i < 50; i++) {
            printf("Enter a number (-999 to quit)\n ");
            scanf("%i", &num[i]);
            if (num == -999) {
                printf("you chose to quit\n ");
            }
        }
        printf("The numbers you entered are %i \n", num);
        system("pause");
    }

My questions are:

Why isn't -999 working properly? In a previous program I just used while (num != -999) and it worked great but it doesn't seem to be working in this case either. Why isn't the array printed out properly?

Please let me know what I'm doing wrong.

Answer 1

if (num == -999)

This compares the base address of num to the value -999, which is interpreted to be an address. Not the intended behavior. Check against num[i] not num.

printf("The numbers you entered are %i \n", num);

The %i specifier is used for a single int, but num is an array of them. Change to:

printf("The numbers you entered are:\n");
for (int i = 0; i < n; ++i) {
    if (num[i] == -999)
        break;
    printf("%i\n", num[i]);
}

where n is the number of elements in num (which may be less than 50).

A suggestion: since this is C++ code, it's a good idea to use cout and C++ libraries instead of printf and C libraries.

Answer 2

Since you still claim (or are being told) that what you have written is C++, here an example of what it would look like in C++:

#include <iostream>
#include <vector>
#include <iterator>
#include <algorithm>

int main()
{
    std::cout << "Gimme Numbers! Just enter something thats not a number to end.\n";
    std::vector<int> numbers{ std::istream_iterator<int>{ std::cin },
                              std::istream_iterator<int>{} };

    std::cout << "\nYa << " << numbers.size() << " Numbas:\n";
    std::copy(std::begin(numbers), std::end(numbers),
              std::ostream_iterator<int>{ std::cout, "\n" });
}

Sample Output:

Gimme Numbers! Just enter something thats not a number to end.
15
45
97
8545
4654
5454
4564
54654
end

Ya 8 Numbas:
15
45
97
8545
4654
5454
4564
54654

---

Now the same with persons instead of int. Notice how main() is unchanged short of the strings for output and the type person instead of int:

#include <iostream>
#include <vector>
#include <iterator>
#include <algorithm>
#include <string>

struct person {
    std::string name;
    int age;
};

std::istream& operator>>(std::istream &is, person &p)
{
    std::string name;
    if (!(is >> name) || name == ".") {
        is.setstate(std::ios::failbit);
        return is;
    }
    int age;
    if (is >> age)
        p = { name, age };
    return is;
}

std::ostream& operator<<(std::ostream &os, person const &p)
{
    return os << p.name << ", age " << p.age;
}

int main()
{
    std::cout << "Gimme names and their ages! End with \".\"\n";
    std::vector<person> persons{ std::istream_iterator<person>{ std::cin },
                                 std::istream_iterator<person>{} };

    std::cout << "\nYa " << persons.size() << " buddies:\n";
    std::copy(std::begin(persons), std::end(persons),
              std::ostream_iterator<person>{ std::cout, "\n" });
}

Sample Output:

Gimme names and their ages! End with "."
Monica 45
Carl 35
Lydia 23
Alex 89
.

Ya 4 buddies:
Monica, age 45
Carl, age 35
Lydia, age 23
Alex, age 89

Answer 3

To make this work, you have to provide a break statement to exit your for loop.

You also need to read the number into a slot of the array, and then check the slot, not the entire array, to see if the new slot holds the -999

#include <stdio.h>
#include <stdlib.h>

int main()
{

    int num[50];
    int i;

    for (i = 0; i < 50; i++) {
        printf("Enter a number (-999 to quit)\n ");
        // Scan the input into an integer slot in your array
        scanf("%i", &num[i]);

        // Check the slot to see if you an exit condition
        if (num[i] == -999) {
            printf("you chose to quit\n ");
            break;  // you have to exit the for loop by 
            // issuing a 'break;' after you get the -999

        }
    }

    int numbers_read = i;

    // print out the array -- Loop through all of the numbers
    // using a for loop and an index, up to the number that were read in:
    printf("The numbers you entered are: \n");
    for(int j= 0; j < numbers_read; j++) {
        printf("%i \n", num[j]);

    }


    system("pause");
}