Thread: dynamic allocation and qsort

  1. #1
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    dynamic allocation and qsort

    Hello
    I don't know why this code doesn't work. I tried everything. Can you help me, please? *puppy face*

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #define COMPARE(a, b) (((a) > (b)) - ((a) < (b)))
    
    
    int compare (const void *a, const void *b)
    {
      float fa = *(const float*) a;
      float fb = *(const float*) b;
      return (fa > fb) - (fa < fb);
    }
    
    
    int main()
    {
            float *v, price;
            int i=0, n=32;
            v=malloc(n*sizeof(float));
            while(scanf("%f",&price)!='*')
            {
                if(++i==n)
                {
                    n+=32;
                    v=realloc(v,n*sizeof(float));
                }
                v[i]=price;
            }
            n=i;
            qsort(v, 10, sizeof(float), compare);
            for(i=0;i<n;i++)
                printf("%f",v[i]);
            free(v);
        return 0;
    }

  2. #2
    and the hat of int overfl Salem's Avatar
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    1. Look up what scanf actually returns.
    2. Pass n rather than 10 to qsort.
    3. Add some newlines in your printfs.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  3. #3
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    1. I changed * in EOF
    2. okay
    3. okay

    i did it but it still do nothing

  4. #4
    Registered User rstanley's Avatar
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    Quote Originally Posted by neko369 View Post
    1. I changed * in EOF
    2. okay
    3. okay

    i did it but it still do nothing
    Please repost your code so we an see the actual changes.

  5. #5
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    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #define COMPARE(a, b) (((a) > (b)) - ((a) < (b)))
    
    
    int compare (const void *a, const void *b)
    {
      float fa = *(const float*) a;
      float fb = *(const float*) b;
      return (fa > fb) - (fa < fb);
    }
    
    
    int main()
    {
            float *v, price;
            int i=0, n=32;
            v=malloc(n*sizeof(float));
            while(scanf("%f",&price)!=EOF)
            {
                if(++i==n)
                {
                    n+=32;
                    v=realloc(v,n*sizeof(float));
                }
                v[i]=price;
            }
            n=i;
            qsort(v, n, sizeof(float), compare);
            for(i=0;i<n;i++)
                printf("%f\n",v[i]);
            free(v);
        return 0;
    }

  6. #6
    misoturbutc Hodor's Avatar
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    You're not storing the price where it should be stored (because you're incrementing 'i' before using it as the index for the array)

  7. #7
    Registered User rstanley's Avatar
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    scanf() should be tested against 0, not EOF.

    Don't increment i until AFTER:
    Code:
    v[i]=price;
    i++;
    In addition, please tell the user how to use the program, and how to end the input. Keyboard input or redirection from a file?

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