Thread: A Complete Beginner With a Big Probelm

so i have to make a program that displays only prime numbers that are palindromes, im supplied with this code

Code:

#include <stdio.h> 
#include <stdlib.h>
#include <errno.h> 
#include <string.h>
                   
 FILE *outputFile; 
 char buffer[81];  
 int bytesWritten; 
                   
main()             
{                  
 int isPrime(int); 

 int i,j;                                                
                                                         
                                                         
/*************************************/                  
/*     Open file to write output     */                  
/*************************************/                  
outputFile = fopen("DD:OUTPUT", "w");                    
if (outputFile == NULL)                                  
{                                                        
 printf("open error:   %d/%s\n", errno, strerror(errno));
 exit(99);                                               
}                                                        
                                                         
/*************************************/                  
/*     Run program                   */                  
/*************************************/                  
                                                         
 for (i=1; i<15000; i++)                                 
 
  {                                                     
    if (isPrime(i)==1)                                  
    {                                                   
     bytesWritten = sprintf(buffer,"%d is prime!!\n",i);
     fwrite(buffer, 1, bytesWritten, outputFile);       
    }                                                   
  }                                                     
                                                        
 /*************************************/                
 /*     Close output file             */                
 /*************************************/                
 fclose(outputFile);                                    
                                                        
  return 0;                                             
}                                                       
                                                        
int isPrime (int myInt)                                 
{                                                       

 int loop;                               
                                         
 for (loop = 2; loop < myInt/2+1; loop++)
 {                                       
   if (myInt%loop==0)                    
     return 0;                           
 }                                       
   return 1;                             
}

now my problem is i have literally never used C before and the reference material supplied is about 1000 pages long and i have no clue where to begin looking, when the program runs this only displays prime numbers i need to modify the code so that it it to only display prime numbers that are palindromes ANY help at all would be appreciated.

Code:

 
int is_palindrome(unsigned long orig)
{
  unsigned long reversed = 0, n = orig;

  while (n > 0)
  {
    reversed = reversed * 10 + n % 10;
    n /= 10;
  }

  return orig == reversed;
}

okay well this lists all the palindromes now how do i go about combining them so it lists only the palindromic prime numbers

So you want to check if a number is prime and it is a palindrome. How might you put that into code?

This looks like a likely spot doesn't it?

Code:

/*************************************/                  
/*     Run program                   */                  
/*************************************/                  
                                                         
 for (i=1; i<15000; i++)                                 
 
  {                                                     
    if (isPrime(i)==1)                                  
    {                                                   
      < cough, cough >
     bytesWritten = sprintf(buffer,"%d is prime!!\n",i);
     fwrite(buffer, 1, bytesWritten, outputFile);       
    }                                                   
  }

Originally Posted by anduril462

So you want to check if a number is prime and it is a palindrome. How might you put that into code?

I'm not sure that's exactly my problem

I will try this when I get home thank you

Originally Posted by Adak

This looks like a likely spot doesn't it?

Code:

/*************************************/                  
/*     Run program                   */                  
/*************************************/                  
                                                         
 for (i=1; i<15000; i++)                                 
 
  {                                                     
    if (isPrime(i)==1)                                  
    {                                                   
      < cough, cough >
     bytesWritten = sprintf(buffer,"%d is prime!!\n",i);
     fwrite(buffer, 1, bytesWritten, outputFile);       
    }                                                   
  }

Originally Posted by ayoonisito

I'm not sure that's exactly my problem

I am. That's exactly what your problem description said in your first post:

Originally Posted by ayoonisito

so i have to make a program that displays only prime numbers that are palindromes

It would be equivalent to Adak's solution, but expressed in one if statement with a compound condition, instead of two nested if statements. The hard part of determining whether something is prime and whether something is a palindrome is done. You just have to combine those two conditions.

i get that i have to combine the two conditions but how exactly do i do that i put the palindrome condition where Adak said to put it but it just returns 0 when i do

Originally Posted by anduril462

I am. That's exactly what your problem description said in your first post:

It would be equivalent to Adak's solution, but expressed in one if statement with a compound condition, instead of two nested if statements. The hard part of determining whether something is prime and whether something is a palindrome is done. You just have to combine those two conditions.

Post your updated code so we can see what you did, and then we can tell you why it doesn't work and how to fix it.

heres what i tried it doenst return anything just 0. i

Code:

#include <stdio.h> 
#include <stdlib.h>
#include <errno.h> 
#include <string.h>
                    
 FILE *outputFile; 
 char buffer[81];  
 int bytesWritten; 
                    
main()             
{                  
 int isPrime(int); 
 
 int i,j;                                                
                                                          
                                                          
/*************************************/
/*     Open file to write output     */
/*************************************/
outputFile = fopen("DD:OUTPUT", "w");                    
if (outputFile == NULL)                                  
{                                                        
 printf("open error:   %d/%s\n", errno, strerror(errno));
 exit(99);                                               
}                                                        
                                                          
/*************************************/
/*     Run program                   */
/*************************************/
                                                          
 for (i=1; i<15000; i++)                                 
  
  {                                                     
    if (isPrime(i)==1)                                  
    {
      
int is_palindrome(unsigned long orig)
{
  unsigned long reversed = 0, n = orig;
 
  while (n > 0)
  {
    reversed = reversed * 10 + n % 10;
    n /= 10;
  }
 
  return orig == reversed;
}

                                             
     bytesWritten = sprintf(buffer,"%d is a palindromic prime!!\n",i);

     fwrite(buffer, 1, bytesWritten, outputFile);       
    }                                                   
  }                                                     
                                                         
 /*************************************/
 /*     Close output file             */
 /*************************************/
 fclose(outputFile);                                    
                                                         
  return 0;                                             
}                                                       
                                                         
int isPrime (int myInt)                                 
{                                                       
 
 int loop;                               
                                          
 for (loop = 2; loop < myInt/2+1; loop++)
 {                                       
   if (myInt%loop==0)                    
     return 0;                           
 }                                       
   return 1;                             
}

You can't put functions inside other function in C. Adak did not intend for you to put the whole function there, just to call the function at that point.

what exactly do you mean by call? im sorry for these incredibly basic quesitons but like i said im a complete beginner.

Originally Posted by anduril462

You can't put functions inside other function in C. Adak did not intend for you to put the whole function there, just to call the function at that point.

Originally Posted by ayoonisito

what exactly do you mean by call? im sorry for these incredibly basic quesitons but like i said im a complete beginner.

Then get out your class notes, crack open your text book and start reading. Work some examples. Look at some tutorials online. We have one here, and Google will turn up lots more. Use the code you were given as an example of what to do.