C: routine variations for variables

This page presents some various ways in which the C programming language passes parameters.

Call by value: pass a copy, original cannot be changed
Call by reference: pass a reference (pointer), original can be changed

Call my macro text replacement, or call by text, is where the preprocessor (part of define) replaces calls with (evaluated) text.
Call by name (not used very much) is like call by text except that calls are not evaluated (and may have some scope code).

The following examples are some of the ways in which two integers can be added. In doing so, various methods are used involving variables and arrays.

In the examples, the actual parameters are not changed, but when using call by reference, they could be.

The variables x1, y2, and z3are used as local variables in function main.

// local variables
int x1;
int y2;
int z3;

Here variables x1 and y2 are set to values using assignment statements (for example purposes).

// local assigment
x1 = 2;
y2 = 3;

The value of z3 is to be set to the sum of x1 and y2. There are many ways to do this. One way to do this is in-line coding.

// explicit in-line coding
z3 = x1 + y2;
printf("The sum of %d and %d is %d (in-line code)\n",x1,y2,z3);

Call by value passes a copy of the values to the function. Here is the function.

// add function using call by value

int add1(int a1, int b2) {
	return a1 + b2;
	}

Here is the corresponding call in main.

// call by value
z3 = add1(x1,y2);
printf("The sum of %d and %d is %d (call by value)\n",x1,y2,z3);

Call by reference passes a reference (pointer) of the values to the function. Here is the function.

// add function using call by reference

int add2(int *a1, int *b2) {
	return *a1 + *b2;
	}

Here is the corresponding call in main.

// call by reference
z3 = add2(&x1,&y2);
printf("The sum of %d and %d is %d (call by reference)\n",x1,y2,z3);

It is often necessary to change a function f (as an expression) to or from a procedure p (as a statement). Here is the general example form (for integers).

z3 = f(x1, y2); // function call form
p(x1, y2, &z3); // procedure call form

The function f or procedure p must be written accordingly. Here is the general example form (for integers), written on one line to show the differences.

// function definition form
int f(int a1, int b2) { int c3 = a1 + b2; return c3; }

// procedure definition form
void p(int a1, int b2, int *c3) { *c3 = a1 + b2; }

Here is the C code.

#include <stdio.h>

// function definition form
int f(int a1, int b2) { int c3 = a1 + b2; return c3; }
// procedure definition form
void p(int a1, int b2, int *c3) { *c3 = a1 + b2; }

int main() {
	int x1 = 2;
	int y2 = 3;
	int z3;

	z3 = f(x1, y2); // function call form
	p(x1, y2, &z3); // procedure call form

	return 0;
	}

Returning to the original problem, using call by value, the function add2 can be converted to a procedure as follows.

// add procedure using call by value and reference

void add3(int a1, int b2, int *c3) {
	*c3 = a1 + b2;
	}

The call needs to be changed appropriately to the following.

add3(x1,y2,&z3);
printf("The sum of %d and %d is %d (procedure call)\n",x1,y2,z3);

By convention (and in most cases) all formal parameters into the function are at the beginning of the list and all formal parameters changed (by reference) are at the end of the parameter list.

Here is the entire program.

Here is the C code.

#include <stdio.h>

// add function using call by value

int add1(int a1, int b2) {
	return a1 + b2;
	}

// add function using call by reference

int add2(int *a1, int *b2) {
	return *a1 + *b2;
	}

// add procedure using call by value and reference

void add3(int a1, int b2, int *c3) {
	*c3 = a1 + b2;
	}

int main() {
	// local variables
	int x1;
	int y2;
	int z3;

	// local assigment
	x1 = 2;
	y2 = 3;

	// explicit in-line coding
	z3 = x1 + y2;
	printf("The sum of %d and %d is %d (in-line code)\n",x1,y2,z3);

	// call by value
	z3 = add1(x1,y2);
	printf("The sum of %d and %d is %d (call by value)\n",x1,y2,z3);

	// call by reference
	z3 = add2(&x1,&y2);
	printf("The sum of %d and %d is %d (call by reference)\n",x1,y2,z3);

	add3(x1,y2,&z3);
	printf("The sum of %d and %d is %d (procedure call)\n",x1,y2,z3);
	return 0;
	}

Here is the output of the C code.

The sum of 2 and 3 is 5 (in-line code)
The sum of 2 and 3 is 5 (call by value)
The sum of 2 and 3 is 5 (call by reference)
The sum of 2 and 3 is 5 (procedure call)