Homework help for a beginner in C programming - c

Question

I have to separate a program I created into 1 main function and 3 user defined functions.
The instructions for my work is as follows:
// gets an integer from the user and returns it // make 3 calls to
this function: // get the length of the rectangle from the user and
return it to main // get the width of the rectangle from the user and
return it to main // get the radius of the circle from the user and
return it to main int GetNum(void);
// takes two arguments, the length and width of the rectangle and
returns the area int CalculateAreaR(int length, int width);
// takes one argument, the radius of the circle and returns the area
double CalculateAreaC(int radius);
I am pretty stuck though. I've written the functions but can't correctly call them to the main function. I know it may be simple but there is just something im not seeing right. the code I have written is as follows:
#include <stdio.h>
#include <math.h>
#define PI 3.14
int GetNum(void)
{
int length;
int width;
int radius;
printf( " Please enter the length of a rectangle \n");
scanf(" %d", &length);
printf(" Please enter the width of a rectangle \n");
scanf(" %d", &width);
printf(" Please enter the radius of a circle \n");
scanf(" %d", &radius);
return length, width, radius;
}
int CalculateAreaR(int length, int width)
{
return length*width;
}
double CalculateAreaC(int radius)
{
return PI*radius*radius;
}
int main(void)
{
int length;
int width;
int radius;
int areaR;
double areaC;
GetNum();
printf("\nThe area of the rectangle is %d\n", CalculateAreaR);
printf("\nThe length is %d, the width is, %d and thus the area of the rectangle is %d\n\n", length, width, areaR);
areaC = CalculateAreaC();
printf("\nThe area of the circle is %.3f\n", CalculateAreaC);
printf("\n\n The radius of the circle is %d and the area of the circle is %.3f\n\n", radius, areaC);
return 0;
}
Can anyone please help me? I'd be very thankful. Im trying my best to learn.

Answer 1

In C, a function can only return a single value via the return statement.
For a simple program, you could make your GetNum() function modify global variables.
// variables placed outside any function have global scope
int length;
int width;
int radius;
int GetNum(void)
{
printf( " Please enter the length of a rectangle \n");
scanf(" %d", &length);
printf(" Please enter the width of a rectangle \n");
scanf(" %d", &width);
printf(" Please enter the radius of a circle \n");
scanf(" %d", &radius);
return 0;
}
This shows declaring the variables at global scope, then using them in the function.
A more advanced, but usually better, way to return multiple values is for the caller to pass a pointer to a variable, and the function to use the pointer to set the variable. #bash0r showed this technique in his/her answer.
Now, to call a function, you must always put parentheses after the function name. Always always always. If you put the name without the parentheses, you are not calling the function; you are just referring to the function (you are referencing the address of the function). You have a couple of places where you wanted to call functions but you didn't put the parentheses.
Some functions take arguments. When you call a function that takes arguments, you need to pass the arguments in. Here is an example of a function that multiplies a number by a factor and then adds a term.
float adjust_num(float x, float factor, float term)
{
return x * factor + term;
}
// example of calling the above:
float adjusted;
adjusted = adjust_num(input_value, scale_factor, 0.0f);
In the example, we pass in input_value and scale_factor. We don't have a constant or variable with a term to add, so we just pass in a literal 0.0 value. So for this example, we are passing all the required arguments. Then the function returns a value, and we collect that output value in the variable adjusted.
If you do try the global variables as I suggested, you will need to delete the lines that declare the variables inside the main() function. Otherwise you will declare two sets of variables, the ones private inside of main() and the other, global ones. The ones in main() will sort of hide the global ones; we call that "shadowing", as in "the local variables inside of main() are shadowing the global variables."
Good luck.

Answer 2

int GetNum(void)
{
...
return length, width, radius;
}
You can't do that, C functions can only return 1 return value. You can basically follow three approaches to solve this problem:
store length, width, radius in variables that are outside of the function (ie, global)
package the elements to be returned in a struct ure
pass pointers to variables containing length, width, radius to that function
Working with global variables is the easiest of these three, and probably sufficient in this case, but in real programs we tend to avoid global variables.
printf("\nThe area of the rectangle is %d\n", CalculateAreaR);
You can call a function inside a call to another function (in this case printf) you have to call the function, what you're doing here is giving a pointer to the function to printf. Also, if you declare a function as taking parameters, you have to give these parameters to the function, like this:
printf("\nThe area of the rectangle is %d\n", CalculateAreaR(length,width));
A comparable issue can be found here:
areaC = CalculateAreaC();
printf("\nThe area of the circle is %.3f\n", CalculateAreaC);
Same remarks as above (parameters should be given), however here you already store the result in areaC so you can just say
printf("\nThe area of the circle is %.3f\n", areaC);

Answer 3

You can't return 3 values at the same time. If you need more than 1 return value you must do it like this:
int x = 0, y = 0, z = 0;
myFunction( &x, &y, &z );
Where myFunction must be declared like this:
void myFunction( int *x, int *y, int *z ) { }
You can read from/write to x, y and y via the * operator. You can use it like this:
*x = 10; // write 10 to the variable behind x.
*y = *x; // write to variable behind y and read from variable behind x.
int *x is called a pointer variable/parameter. It stores an address instead of a value. With pointers you can 'point' to another variable and modify it's value from somewhere else (for example in a function which you call). The size of a pointer depends on the operating system (32-bit systems have 4 byte addresses and 64-bit systems have 8 byte adresses).

Answer 4

Inside of the main function, you should receive the input from your input function: getNum();
I usually like to encapsulate input data into separate functions so I specifically know that this function only does one thing. The whole point of declaring length, width, and radius inside the main is to actually be able use those values when I need them. The simplest approach can be done like so:
Function:
int get_length()
{
int length = 0;
printf("Enter length of rectangle: ");
scanf("%d", &length);
return length;
}
Main Function:
int main()
{
int length = 0;
length = get_length();
// collect additional input...
return 0;
}