Thread: Pointers and Call-by-reference functions

  1. #16
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    Changed "a" to pass by pointer

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <math.h>
    void program_intro(void);
    void input_read(float *a, float b, float c, float x_initial, float x_final, float x_delta);
    int main()
    {
        float a, b, c, x_initial, x_final, x_delta;
        program_intro();
        input_read(&a, b, c, x_initial, x_final, x_delta);
        printf("a = %2.f\n", a);
        printf("b = %2.f\n", b);
        printf("c = %2.f\n", c);
        printf("x = %2.f\n", x_initial);
        printf("x[i] = %2.f\n", x_final);
        printf("delta X = %2.f\n", x_delta);
        return 0;
    }
    void program_intro(void) //Explanation of program to the user
        {
            printf("This program will compute a range of values for\n");
            printf("the second degree polynomial f(x) = ax^2 + bx + c,\n");
            printf("the derivative f'(x) = 2ax + b, \n");
            printf("the integral F(x[i]) = (a/3)x[i]^3 + (b/2)x[i]^2 + cx, and the\n");
            printf("area A = F(x) - F(x[i]).\n\n");
            return;
        }
    void input_read(float *a, float b, float c, float x_initial, float x_final, float x_delta)
        {
            printf("Please input the polynomial coefficients as follows:\n\n");
            printf("Coefficient a: ");
            scanf("%f", a);
            printf("\nCoefficient b: ");
            scanf("%f", &b);
            printf("\nCoefficient c: ");
            scanf("%f", &c);
            printf("\n\nPlease input the range and increment to be tested as follows:\n\n");
            printf("x: ");
            scanf("%f", &x_initial);
            printf("\nx[i]: ");
            scanf("%f", &x_final);
            printf("\ndelta x: ");
            scanf("%f", &x_delta);
        return;
        }
    Edit: Add link Pointers in C - Tutorial - Cprogramming.com
    Please read the "Pointing to Something: Retrieving an Address" first example code till you understand it.


    Tim S.
    Last edited by stahta01; 01-11-2018 at 10:39 AM.
    "...a computer is a stupid machine with the ability to do incredibly smart things, while computer programmers are smart people with the ability to do incredibly stupid things. They are,in short, a perfect match.." Bill Bryson

  2. #17
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    Quote Originally Posted by stahta01 View Post
    In C, you can only return at most a single variable value.

    Therefore this line will not work.
    Code:
    return a, b, c, x_initial, x_final, x_delta;
    This can be fixed by using pointers.

    Tim S.
    Would I modify the prototype and definition to be *a, *b,.. and the declaration and return be &a, &b,...? I'm thinking my printf's need the same &a treatment?

  3. #18
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    Quote Originally Posted by scotdani View Post
    Would I modify the prototype and definition to be *a, *b,.. and the declaration and return be &a, &b,...? I'm thinking my printf's need the same &a treatment?
    I did the changes for parameter "a" and posted that changed code.
    READ THE LINK I POSTED! Then ask an question that makes some sense.

    Edit: Link to the post that you seemed to have ignored Pointers and Call-by-reference functions

    Tim S.
    "...a computer is a stupid machine with the ability to do incredibly smart things, while computer programmers are smart people with the ability to do incredibly stupid things. They are,in short, a perfect match.." Bill Bryson

  4. #19
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    Quote Originally Posted by stahta01 View Post
    I did the changes for parameter "a" and posted that changed code.
    READ THE LINK I POSTED! Then ask an question that makes some sense.

    Edit: Link to the post that you seemed to have ignored Pointers and Call-by-reference functions

    Tim S.
    Look, I'm not interested in being belittled for my novice experience with C, and it was a timing issue that I did not immediately see your edited code.

    That being said, thank you for demonstrating what the syntax looks like for pointers in this example. This is how I am learning the ways of C, something reading cannot match.

    A professor told me this once: "If you don't understand what's on page two, you're not going to understand page 8."

  5. #20
    misoturbutc Hodor's Avatar
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    C doesn't have pass by reference, it only has pass by value (a pointer is a value) so the declaration would be as Tim provided and to call it use &var (which is a pointer to var so it will match what the function wants as an argument/s)

  6. #21
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    Quote Originally Posted by Hodor View Post
    C doesn't have pass by reference, it only has pass by value (a pointer is a value) so the declaration would be as Tim provided and to call it use &var (which is a pointer to var so it will match what the function wants as an argument/s)
    Ok I'm starting to get it now. When using pointers in main, say if I took this function and place it in main, how different or similar would it look?

  7. #22
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    I'm not sure what you mean by "if I took this function and place it in main". C doesn't really support functions inside functions. Did you mean call this function from main?

    Perhaps go back and check out post #2. That shows a clear example of the difference between passing in a thing and a pointer-to-a-thing, including how to call it from main.

    You're doing something similar, but with several parameters, and they're type double instead of int.

  8. #23
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    Quote Originally Posted by anduril462 View Post
    I'm not sure what you mean by "if I took this function and place it in main". C doesn't really support functions inside functions. Did you mean call this function from main?

    Perhaps go back and check out post #2. That shows a clear example of the difference between passing in a thing and a pointer-to-a-thing, including how to call it from main.

    You're doing something similar, but with several parameters, and they're type double instead of int.
    What I was thinking was how to pass pointers in main only compared to between main and a function. One of the assignment constraints is to scan for the variables in main then use a function for each calculation using pointers to these variables. I have trouble visualizing it unless I write it myself since I'm not a programmer.

    Which brings me to the next step in the assignment. The functions mentioned above are to use an incremented loop from x[I] to x[f] at increment delta[x]. Assuming this is an array, say x_range[N], this needs to be a pointer as well for the functions to run through the stored values; would it be smart to make an array for storing each calculated value too? I'm a little rusty on for loop structure with arrays; below is an outline of what I think it might look like, but I'm very uncertain.

    Code:
    #define NMAX 100
    double base_poly_func(*x_range[N];)
    //
    //Previous code with scans
    //
    double x_range[NMAX];
    double base_poly_results[N];
    
    for (i=0; N<NMAX; N+x_delta) //loop for getting all values in data set
    {
    x_range[N] += x_initial + x_delta;
    }
    
    for (i=0; N<NMAX; N++)
    {
    base_poly_results[N] += base_poly_func(&x_range[N]);
    }
    
    //
    //more code to follow
    //
    double base_poly_func(*x_range[N])
    {
    double base_poly_eq;
    base_poly_eq = x;  //actual equation goes here
    return;
    }

  9. #24
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    For, While and Do While Loops in C - Cprogramming.com
    Functions in C - Cprogramming.com
    Quote from Arrays in C - Cprogramming.com
    The fact that arrays can act just like pointers can cause a great deal of confusion.
    Tim S.
    Last edited by stahta01; 01-13-2018 at 05:55 PM.
    "...a computer is a stupid machine with the ability to do incredibly smart things, while computer programmers are smart people with the ability to do incredibly stupid things. They are,in short, a perfect match.." Bill Bryson

  10. #25
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    Modified code from Arrays in C - Cprogramming.com
    to show the normal way to pass an array to an function

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    
    int CheckForA(char * array, int size);
    
    #define ARRAY_SIZE 10
    
    int main()
    {
        char astring[ARRAY_SIZE] = {0};
        
        /* Using scanf isn't really the best way to do this; we'll talk about that
           in the next tutorial, on strings */
        scanf( "%s", astring );
    
        if (CheckForA(astring, ARRAY_SIZE))
        {
            printf( "You entered an a!\n" );
        }
        return 0;
    }
    
    int CheckForA(char * array, int size)
    {
        int i = 0;
    
        for ( i = 0; i < size; ++i )
        {
            if ( array[i] == 'a' )
            {
                return 1; /* true */
            }
        }
        return 0; /* false */
    }
    Tim S.
    Last edited by stahta01; 01-13-2018 at 06:11 PM.
    "...a computer is a stupid machine with the ability to do incredibly smart things, while computer programmers are smart people with the ability to do incredibly stupid things. They are,in short, a perfect match.." Bill Bryson

  11. #26
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    Quote Originally Posted by scotdani View Post
    I have trouble visualizing it unless I write it myself since I'm not a programmer.
    I suggest you avoid taking classes the requires programming; since, you do *not* seem to want to become a programmer!

    Tim S.
    "...a computer is a stupid machine with the ability to do incredibly smart things, while computer programmers are smart people with the ability to do incredibly stupid things. They are,in short, a perfect match.." Bill Bryson

  12. #27
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    Quote Originally Posted by stahta01 View Post
    I suggest you avoid taking classes the requires programming; since, you do *not* seem to want to become a programmer!

    Tim S.
    I certainly agree if I had the choice in the curriculum. I'm just looking for help to get a good grade and attempt to understand it. I hold no shame in holding someones hand through the programming portions.

  13. #28
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    FYI: Your code in post #23 was filled with errors; I posted links and example code to help you fix your code in the two posts after it.
    Pointers and Call-by-reference functions

    I have no plans to fix your bad code; it is so bad that it is not likely anyone will fix it for you.
    Link to Homework Policy: https://cboard.cprogramming.com/announcement.php?a=39

    Tim S.
    "...a computer is a stupid machine with the ability to do incredibly smart things, while computer programmers are smart people with the ability to do incredibly stupid things. They are,in short, a perfect match.." Bill Bryson

  14. #29
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    Quote Originally Posted by stahta01 View Post
    it is so bad that it is not likely anyone will fix it for you.
    Thank you. I will spend my time elsewhere.

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