Thread: Prime factorization of a number

  1. #1
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    Prime factorization of a number

    So I am writing a code where the program will keep asking for a number and calculate its prime factorization unless the user enters 1 or less. What am I doing wrong?
    Code:
    #include<stdio.h>
    int main()
    {
      int x, i;
    
    
      while (x > 1) {
        printf("Enter an integer:  ");
        scanf("%d", &x);
        printf("The prime factorization of %d is ", x);
    
    
        while (x % 2 == 0) {
          printf("2 ");
          x = x / 2;
        }
        for (i = 3; i < 1009; i = i + 2) {
          while (x % i == 0) {
            printf("%d ", i);
            x = x / i;
          }
        }
    
      }
    
    
      return 0;
    }
    Last edited by Salem; 11-14-2020 at 12:06 AM. Reason: Removed crayola - use copy/paste as TEXT ONLY!!!!!!!!!!!!!!!!!!!!

  2. #2
    C++ Witch laserlight's Avatar
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    How does it not work?
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

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    It terminates after computing for the first input.

  4. #4
    C++ Witch laserlight's Avatar
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    Notice that your loop condition involves x, but you modify x after reading it. You need to check what the user entered immediately after reading it.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  5. #5
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    How do I fix that error?

  6. #6
    C++ Witch laserlight's Avatar
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    One way that doesn't require so much changes would be to change your outer while loop condition to 1, i.e., true. This makes your loop an infinite loop, but then you add a check for the user input right after reading it, and if it is 1 or less, break from the loop.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  7. #7
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    What would that loop look like?

  8. #8
    C++ Witch laserlight's Avatar
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    Um, I just told you.

    Let's put it another way: why does your program "terminates after computing for the first input"?
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  9. #9
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    Not sure if you have learned about functions yet, but they would make your task easier:
    Code:
    #include <stdio.h>
    #include <stdbool.h>
    
    
    void print_prime_factors(int num)
    {
      printf("The prime factors of %d are ", num);
      // your code here
    }
    
    
    int main()
    {
      int num;
      while(true)
      {
        printf("Enter an integer:  ");
        scanf("%d", &num);
        if (num < 2)
          break;
          
        print_prime_factors(num);
      }
        
      return 0;
    }

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