Functions in C - C Programming - Lecture Slides, Slides of Computer Science

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Functions in C

Function Terminology

• Identifier scope
• Function declaration, definition, and use
• Parameters and arguments
• Parameter order, number, and type
• Function prototype
• Input parameter, output parameter
• Pass by value, pass by reference

Identifier Scope (i.e., visibility)

int a = 1;

int findMax (int x, int y);

int main (void) { const int j = 10; int k; k = findMax (a, j); return (0); } // End main

int findMax (int x, int y) { int z; if (x < y) z = y; else z = x; a = z; return (z); } // End findMax

Global scope

for variable a

Local scope for j, k

Local scope for x, y, z

Reading from a global variable

Writing to a global variable

• Declaring a function tells the compiler the function exists. The parameters tell the order, number, and type of arguments the function expects and the type of the value the function returns. The identifier for each parameter is optional but recommended. A function declaration is also called a function prototype float minValue (float x, float y);
• Defining a function tells the compiler what the function does

float minValue (float x, float y) { if (x < y) return x; else return y; } // End minValue

• Using (calling) a function tells the compiler to switch control to this function and pass the argument values to the function c = minValue(a,b);

Syntax for Function Declaration,

Definition, and Use

Coding Standards for Functions

• Function names follow the same naming standard

as variables

• Explicitly declare all programmer-defined

functions before defining or using them. This is

done by placing all function prototypes just above

the definition of the function main

• If the function main is defined in a source code

file, it's definition should occur before the

definition of any other functions in the file

Location of Function Parts in a File

// Include files #include <stdio.h>

// Global constants and types

// Function prototypes

int main (void) { // Function calls

return (0); } // End main

// Function definitions

Output Parameter Example

#include <stdio.h>

int swap ( **int *** x, **int *** y);

int main (void)

{

int a = 27;

int b = 56;

// swap(a, b); // Wrong

swap ( & a, & b); // Right

return(0);

} // End main

"int *" is a pointer type, so the type of x or y is a pointer, which is a memory address

The * here is NOT an operator in this case. It is a decoration that goes with the type

The & is the "address of" operator. It returns the memory address of the variable

This is read as "the address of a"

Output Parameter Example

(continued)

int swap ( **int *** x, **int *** y)

{

int temp;

temp = ***** x;

***** x = ***** y;

***** y = temp;

return(0);

} // End swap

"int *" is a pointer type, so the type of x or y is a pointer, which is a memory address

The * here is NOT an operator in this case. It is a decoration on the variable name

This * here IS an operator. It is called the indirection operator.

In the location named temp, store the value pointed to by x In the location pointed to by x, store the value pointed to by y In the location pointed to by y, store the value in temp Docsity.com

Interpreting the * Operator

• ***** is the indirection operator (among other uses in C)
• When applied to a variable on the right side of an assignment statement, the ***** operator returns the value located in the "mailbox" of the memory address contained in the variable. temp = *x;
• When applied to a variable on the left side of an assignment statement, the ***** operator stores the value on the right side of the assignment statement into the "mailbox" of the memory address contained in the variable on the left side. *y = temp;
• If the ***** indirection operator occurs on both sides of the assignment statement, it means the following for the example below. The value pointed to by y is stored in the memory location pointed to by x *x = *y;

Interpreting the * Operator (continued)

• In the parameter list of a function, an output parameter is

created by declaring the parameter as a pointer to the value that the function will "return" int swap ( **int *** x, **int *** y);

• This is the second step the programmer needs to do to

implement "pass by reference"

printf and scanf Functions

• The printf and scanf functions are declared in the source code file named stdio.h that is located in the standard …\include folder location. Their prototypes are inserted into a program's source code file by using #include <stdio.h>
• The printf and scanf functions are defined in the object code library file named libc.a that is located in the standard …\lib folder location
• The printf and scanf functions are used by placing them in a program's source code file printf("The value is %d", accountValue); scanf("%f", &salesAmount);