CODEWITHSUNDEEP
c++for-loopbreak
Peter
Peter

Reputation: 508

C++ Nested for loop and break

I am new to computer science and had a quick question. I was trying to make a program that would take two integer inputs and print out all of the prime numbers in between them.

The problem I am having is that when I use break in the nested for loop. After it finds a prime number, it does not hit the nested for loop on the next go around of the outside for loop. Therefore, when searching for prime numbers between say 8 and 15, it will print out "11 12 13 14 15." It is correct at first but after finding a prime, it states that the rest of the bounded numbers are also primes. 

#include <iostream>
using namespace std;

int main()
{
    // Prime number finder:
    cout << "Enter two numbers and I will find the prime numbers between them.\n\n";

    int num1, num2, i = 2;
    bool valid;

    cout << "Enter the lower limit: ";
    cin >> num1;
    cout << endl << "Enter the higher limit: ";
    cin >> num2;
    if (num2 <= num1)
    {
        cout << "Enter a number that is larger than the lower limit./n";
    }

    if (num1 <= 1)
    {
        cout << "1 2 ";
        num1 = 3;
    }
    else if (num1 == 2)
    {
        cout << "2 ";
        num1 = 3;
    }

    for (num1; num1 <= num2; num1++)
    {
        valid = true;

        for (i; i < num1; i++)
        {
            if ((num1 % i) == 0)
            {
                valid = false;
                break;
            }
        }

        if (valid == true)
            cout << num1 << " ";
    }

    return 0;
}

Upvotes: 0

Views: 435

Answers (2)

Daniel Daranas
Daniel Daranas

Reputation: 22624

The problem is that you don't reset the value of i. Change your loop to:

for (int i = 2; i < num1; i++)

And delete your earlier definition of i, since it's not needed at that point.

Even better, change the whole check of whether the number is prime to a separate funcion which returns a bool.

This should teach you to write functions with a single responsibility. Your function loops through a bunch of numbers, and for each of them it checks whether it is a prime. This latter part belongs to a separate function. What is interesting is that your bug is highly unlikely to appear if you write the separate functions in the first place.

Upvotes: 3

Deepu
Deepu

Reputation: 7610

In the for loop below i is uninitialized, you have to set i=2, also set the upper bound as sqrt(num1).

for (i=2; i <= sqrt(num1); i++)
{
        if ((num1 % i) == 0)
    {
        valid = false;
        break;
    }
}

To use sqrt() also include the following header file as follows,

#include<cmath.h>

Upvotes: 1

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