Pointers and Call-by-reference functions

**stahta01** · 01-11-2018

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.

**scotdani** · 01-11-2018

Originally Posted by stahta01

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?

**stahta01** · 01-11-2018

Originally Posted by scotdani

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.

**scotdani** · 01-11-2018

Originally Posted by stahta01

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."

**Hodor** · 01-12-2018

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)

**scotdani** · 01-12-2018

Originally Posted by Hodor

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?

**anduril462** · 01-12-2018

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.

**scotdani** · 01-13-2018

Originally Posted by anduril462

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;
}

**stahta01** · 01-13-2018

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.

**stahta01** · 01-13-2018

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.

**stahta01** · 01-14-2018

Originally Posted by scotdani

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.

**scotdani** · 01-14-2018

Originally Posted by stahta01

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.

**stahta01** · 01-14-2018

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.

**scotdani** · 01-14-2018

Originally Posted by stahta01

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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