Returning results from a function using pointers

This is a discussion on Returning results from a function using pointers within the C Programming forums, part of the General Programming Boards category; Hello all, I am needing to calculate the amounts of the values stored in the arrays within a function. The ...

  1. #1
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    Returning results from a function using pointers

    Hello all, I am needing to calculate the amounts of the values stored in the arrays within a function. The calculations within the function however have to be done using pointers. I am having trouble figuring out how to return the value calculated and display it within main. This is what I have so far, any help would be great.
    Code:
    #include <stdio.h>
    double extend (double [], double [], double []);
    
    
    int main ()
    {
        #define max 10
        double price [max] = {10.62, 14.89, 13.21, 16.55, 19.62, 9.47, 6.58, 19.32, 12.15, 3.99};
        double quantity [max] = {4, 9.5,6, 7.35, 9, 15.3, 3, 5.4, 2.9, 4.9};
        double amount [max];
        int i;
        
        
            
        for (i=0; i<max; i++){   
             
        printf ("The amount is %f\n", extend(price, quantity, amount));
    }
    
    
    
    
    system ("PAUSE");
    return 0;
    }
    
    
    double extend (double price[], double quantity[], double amount [])
    {
        int i;
        double *gPtr, *mPtr, *nPtr;
        
        gPtr= price;
        mPtr= quantity;
        nPtr= amount;
        
        
        
        for (i=0; i<max; i++){
            *nPtr++= *mPtr++ * *gPtr++;
            }
            
    return (amount);
    }

  2. #2
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    You want to return a pointer to an array but you have declared extend() to return a double. That doesn't work.

    Your results are stored in "amount" after calling the function, thus the easiest solution is to change the return value to void, call the function once in main and then iterate through the array and print the values.

    Bye, Andreas

  3. #3
    SAMARAS std10093's Avatar
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    Hmm.. If i were you i wouldn't return anything..Why?

    Because the arrays are really passed by pointer...So what changes are going to be made to them are going to be "alive", even after the function terminates.

    Now because you assign the pointers correct to the addresses of the pointers, these local pointers inside your function are going to affect the memory where price,quantity and amount are pointing to..Thus the modifications made inside the function will be preserved.

    So modify the function extend to have as return value a void instead of a double.Do not forget to modify the prototype of the function too
    Of course now because the return value is void, the return amount (line 43) should be removed.
    Also something must be modified in the main function too.Think that and post back if needed

  4. #4
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    Thanks all that worked great I changed the function to a void, declared the function in main and then changed the print function a bit and I got this:
    Code:
    #include <stdio.h>
    void extend (double [], double [], double []);
    
    
    int main ()
    {
        #define max 10
        double price [max] = {10.62, 14.89, 13.21, 16.55, 19.62, 9.47, 6.58, 19.32, 12.15, 3.99};
        double quantity [max] = {4, 9.5,6, 7.35, 9, 15.3, 3, 5.4, 2.9, 4.9};
        double amount [max];
        int i;
        
        extend (price, quantity, amount);
            
        for (i=0; i<max; i++){   
             
        printf ("The amount is %f\n", amount[i]);
    }
    
    
    
    
    system ("PAUSE");
    return 0;
    }
    
    
    void extend (double price[], double quantity[], double amount [])
    {
        int i;
        double *gPtr, *mPtr, *nPtr;
        
        gPtr= price;
        mPtr= quantity;
        nPtr= amount;
        
        
        
        for (i=0; i<max; i++){
            *nPtr++= *mPtr++ * *gPtr++;
            }
    }
    Thanks a ton!!!

  5. #5
    SAMARAS std10093's Avatar
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    You are welcome

    Also a tip.When we use define, we place it before main, so its global.But for some reason you may need it to be not global ( i do not really know what that reason can be).
    But always , it is good to write the defined word with all caps, like this
    Code:
    #define MAX 100
    an all-caps variable immediately states to user of your code, that this variable he is looking to is a constant one!

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