Reads an integer and print all perfect number smaller than given integer

Question

I can find perfect numbers but I can't print all perfect numbers smaller than a given integer.

int main() {

    cout << "Enter a number ";
    int number{};
    cin >> number;
    int sum{};

    int j{};
    for (int i = 1; i < number ; i++) {
        j = number % i;
        if (j == 0)
            sum = sum + i;
    }
    if (sum == number){
        cout << sum;

I expect the output 6 28 496 for input=500

Welcome to Stack Overflow! It sounds like you may need to learn how to use a debugger to step through your code. With a good debugger, you can execute your program line by line and see where it is deviating from what you expect. This is an essential tool if you are going to do any programming. Further reading: [How to debug small programs](http://ericlippert.com/2014/03/05/how-to-debug-small-programs/) and [Debugging Guide](http://idownvotedbecau.se/nodebugging/) — NathanOliver, Oct 22 '19 at 20:45
@preciousbetine: https://en.wikipedia.org/wiki/Perfect_number — Fred Larson, Oct 22 '19 at 21:00
I'll give you a hint. Right now, your code is checking if the number entered is a perfect number. What would you have to change to determine which numbers between 1 and the entered number are perfect numbers? — Fred Larson, Oct 22 '19 at 21:08
Create a function with your code and use the function to check if a number is a perfect number. Iterate in a loop from 1 to input and check each number with this function. Print the number of it's a perfect number. — Thomas Sablik, Oct 22 '19 at 21:24
No, it's not necessary. It helps making it simpler but if you haven't learned functions yet it won't be simpler. Create the nested loops. What is your current problem? — Thomas Sablik, Oct 22 '19 at 21:34

score 1 · Answer 1 · answered Oct 22 '19 at 21:36

Seems like you've mushed your "is it perfect" logic with your "all numbers under the given limit" logic together to where you aren't getting desired results.

I'd take your test and put it in a function.

bool isPerfect(int number) {
    int j{}, sum{};
    for (int i=1; i<number; i++) {
        j = number % i;
        if (j == 0) sum = sum + i;
    }
    return (sum == number);
}

Then call that function repeatedly with all integers smaller than the given input:

int main()
{
    std::cout << "Enter a number ";
    int number{};
    std::cin >> number;
    for (int n = 1; n <= number; n++) {
        if (isPerfect(n))
            std::cout << n << " ";
    }
}

Course you could still do it all in your main() function, but it's a lot clearer when you break it up.

Reads an integer and print all perfect number smaller than given integer

1 Answers1