While loop problem

[semsemdiver]

the loop is not repeated as i need. this program is to print the primary number starting from the number entered by user,but it print one time only if the entered number is primary number. i don't know the reason. please help.

  #include<iostream>
using namespace std;

int main()
{

  char again;
  do  //this loop is to ask user if he needs to try again after calculation.
  {

		int x;
		cout<<"please enter number\n";
		cin>>x;  //get the number from user
	

		while(x<=2)   //test the number if it is less than 2.
		{
			cout<<"please enter more than 2"<<endl;
			cin>>x;
		}

	 int i;
	 int z;
	 bool isprim=true;
	 z=x;

	while(z>2) //to test descending.
	{

	for(i=x-1;i>1;i--) //to test every number divided by the previous nu. 
	 {

		if(x%i==0)
		{
		  isprim=false;
		  break;
		}
		
	 }

	 if(isprim==true)
	 {
		 cout<<x<<endl;
	 }

	 x=z;
	 x--;
	 z--; 
	}       //while loop should repeat at this point if z>2.
	cout<<"tryagain??\n press 'y' for yes or any key for exit"<<endl;
	cin>> again;

  }while(again=='y');

 return (0);
}

[deleted account xyzzy]

Which "while" loop isn't repeating?

You have 1 "do-while" loop, lines 8 - 53, and two "while" loops, lines 16-20 and lines 27-49. (Your indentation formatting needs a bit of work)

I copy'n'paste the code, compile (no errors) and run, and get the following output:

please enter number
3
3
tryagain??
 press 'y' for yes or any key for exit
y
please enter number
9
tryagain??
 press 'y' for yes or any key for exit
y
please enter number
7
7
tryagain??
 press 'y' for yes or any key for exit
n

Your program's output could be a bit more user friendly, but the code works for me. No problems as you describe.

[semsemdiver]

Hi Furry Guy

Thanks for reply.

line number 27 this while loop with for loop should test the primary number and print it

thanks
semsemdiver

[deleted account xyzzy]

Printing out the entered number again if it is prime does happen. Look at my test run. 3 and 7 did print out, while 9 didn't.

Changing line 43 to something like:
std::cout << x << " is a prime number.\n";
would help make the output more visible and user-friendly.

#include<iostream>

int main()
{
   char again { };

   do  //this loop is to ask user if he needs to try again after calculation.
   {
      int x { };
      std::cout << "please enter number: ";
      std::cin >> x;  //get the number from user

      while (x <= 2)   //test the number if it is less than 2.
      {
         std::cout << "please enter more than 2\n";
         std::cin >> x;
      }

      int z { x };
      bool isprim { true };

      while (z > 2) //to test descending.
      {
         for (int i = x - 1; i > 1; i--) //to test every number divided by the previous nu.
         {
            if (x % i == 0)
            {
               isprim = false;
               break;
            }
         }

         if (isprim == true)
         {
            std::cout << x << " is a prime number.\n";

            // if the number is prime, why continue doing calculations?
            // with or without the break, the output happens only once
            break;
         }

         x = z;
         x--;
         z--;
      }       //while loop should repeat at this point if z>2.

      std::cout << "try again?? press 'y' for yes or any key for exit: ";
      std::cin >> again;
   }
   while (again == 'y');
}

please enter number: 18
try again?? press 'y' for yes or any key for exit: y
please enter number: 127
127 is a prime number.
try again?? press 'y' for yes or any key for exit: n

[semsemdiver]

Hi Furry Guy

the output shouldn't be 127 only, it should be all primary numbers starting from 127 to 2 this is the idea of this program, for some reason the while loop line 22 doesn't repeat??

Thank you for your help,
semsemdiver

[deleted account xyzzy]

#include <iostream>

int main()
{
   char again { };

   do
   {
      int low { 2 };

      std::cout << "Enter number: ";
      int high { };
      std::cin >> high;
      std::cout << '\n';

      std::cout << "Prime numbers in reverse order are:\n";

      // go from high to low
      for (int loop { high }; loop >= low; loop--)
      {
         // perform a "standard" test for "is this number prime?"
         bool isPrime { true };

         for (int i { 2 }; i <= loop / 2; i++)
         {
            if (loop % i == 0)
            {
               isPrime = false;
               break;
            }
         }

         if (isPrime) { std::cout << loop << ' '; }
      }
      std::cout << "\n\n";

      std::cout << "Do you want another number to check if prime? (y or n): ";
      std::cin >> again;
   }
   while (again == 'y' || again == 'Y');
}

Enter number: 127

Prime numbers in reverse order are:
127 113 109 107 103 101 97 89 83 79 73 71 67 61 59 53 47 43 41 37 31 29 23 19 17 13 11 7 5 3 2

Do you want another number to check if prime? (y or n): y
Enter number: 20

Prime numbers in reverse order are:
19 17 13 11 7 5 3 2

Do you want another number to check if prime? (y or n): b

Ooops! I forgot the check if the user wants to enter another number! Added!

If you want low to high primes, change line 14 to:
for (int loop { low }; loop <= high; loop++)

[semsemdiver]

hi

Furry Guy

Thanks for your help.

but i want to know why doesn't my code work? as you see know errors and logically i think it is correct, so what is the problem?

anyway thank you

semsemdiver

[deleted account xyzzy]

If you want/need while or do/while loops:

#include <iostream>

int main()
{
   char again { };

   do
   {
      int low { 2 };

      std::cout << "Enter number: ";
      int high { };
      std::cin >> high;
      std::cout << '\n';

      std::cout << "Prime numbers in reverse order are:\n";

      // go from high to low
      int loop { high };

      do
      {
         bool isPrime { true };

         int i { 2 };

         while (i <= loop / 2)
         {
            if (loop % i == 0)
            {
               isPrime = false;
               break;
            }
            i++;
         }
         if (isPrime) { std::cout << loop << ' '; }

         loop--;
      }
      while (loop >= low);

      std::cout << "\n\n";

      std::cout << "Do you want another number to check if prime? (y or n): ";
      std::cin >> again;
   }
   while (again == 'y' || again == 'Y');
}

[deleted account xyzzy]

logically i think it is correct

Compare your code to mine with the while/do-while loops. You are overthinking.

As was I originally when looking at your code.

I broke the problem down into easy to achieve steps:

1. what's the code for finding a prime number. The code I could remember off-hand used a for loop.

2. Ask the user for the high number. The low number is already set (2), but having it as a variable makes it a quick change if you want a different low number from the user.

3. Loop from high to low. FOR LOOP!

4. slap the code to find a single prime number into the for loop and BINGO!

5. Change the for loops into while/do-while loops.

Assignment done! After adding the code to ask the user if they want to enter another number.

[semsemdiver]

Thank you

[semsemdiver]

Furry Guy

finally i found the reason, i forgot to turn the bool variable to true again before entering the for loop. now my code works :)

thanks furry and please talking to you.