Functions and Pointers

Functions and Pointers

Lecture 29

Summary of Previous Lecture Algorithms

FunctionsUsing functions in top-down design

Functions in C LanguageUser defined FunctionsParametersReturn values

Topics

FunctionsPrototypesVariable Scope

Pointers Introduction to pointers Pointers and function parameters

Summary

Prototyping of Functions

Must declare functions before use (like variables)

Declaration is called a “prototype” Specifies the name, parameters and return

type of the function, but not the code

#include <stdio.h>

int isNegative (int);

int main (void){

int number;

printf ("Enter an integer: ");scanf ("%d",&number);

if (isNegative(number)){

printf("Negative\n");}else{

printf("Positive\n");}

return 0;}

Example: isNegative.c

int isNegative ( int n ){

int result;if ( n<0 ){

result=1;}else {

result = 0;}return result;

}

#include <stdio.h>


int main (void){

int number;





return 0;}



result=1;}else {


}

Example: isNegative.cFunction Prototype

#include <stdio.h>


int main (void){

int number;





return 0;}



result=1;}else {


}

Function Definition


#include <stdio.h>


int main (void){

int number;





return 0;}



result=1;}else {


}

Function Call (Must be after prototype, but can be before definition)


#include <stdio.h>


int main (void){

int number;





return 0;}



result=1;}else {


}

Header files (filename.h) contain function prototypes and global

variable declarations


#include <stdio.h>


int main (void){

int number;





return 0;}



result=1;}else {


}

stdio.h contains function prototypes for printf(), scanf(), and

other I/O functions


Header files You can make your own header files with

prototypes of frequently used functions:#include "myFunctions.h"

Put the functions in a corresponding C file, and include those too:#include "myFunctions.c"

#include <stdio.h>#include "myFunctions.h"#include "myFunctions.c"

int main (void){

int number;





return 0;}


Note:• " " around file name for user-defined files• < > for standard system files

isNegative() is declared in myFunctions.h and defined in myFunctions.c, not in this file!

Example: myFunctions.c



result=1;}else {


}

Example: myFunctions.h

int isNegative ( int );

Scope

Where can you use a variable which is declared in a function?

In that function only Not in a calling function Not in a called function

Scope: Local Variables Formal parameters: only accessible whilst

function executing Variables declared in a function body: only

accessible whilst function executing In fact, this is true of every block in a

program

#include <stdio.h>


int result;if (number<0){

result=1;}else {


}


int main (void){

int number;





return 0;}

#include <stdio.h>


int result;if (number<0){

result=1;}else {


}


int main (void){

int number;





return 0;}

ERROR! Number is local to the main function, not accessible

here

#include <stdio.h>



result=1;}else {


}


int main (void){

int number;





return 0;}

Use the parameter n which is local to the function isNegative()

#include <stdio.h>



result=1;}else {


}


int main (void){

int number;





return 0;}

number is the actual parameter

n is the formal parameter

#include <stdio.h>



result=1;}else {


}


int main (void){

int number;





return 0;}

result & n: local to isNegative()number: local to main()

#include <stdio.h>

int number;

int isNegative ( void ){

int result;if ( number <0 ){

result=1;}else {


}

Example: isNegativeGlobal.c

int main (void){


if (isNegative()){



return 0;}

#include <stdio.h>

int number;

int isNegative ( void ){

int result;if ( number <0 ){

result=1;}else {


}

Example: isNegativeGlobal.c

int main (void){


if (isNegative()){



return 0;}

number is now GLOBAL - declared outside any function,

accessible in all functions (after the declaration)

Scope: Global Variables Global variables are accessible in any

function after their declaration to the end of that source file

They're useful, but risky if any and every function can modify them, it

can be difficult to keep track of their value Better to use local variables and

parameter passing if possible

Scope: Functions Functions are also accessible in any

function after their declaration to the end of that source file

Functions Summary Include function prototype before its use Be careful about the scope of variables

Pointers

Pointers Topics

Introduction to pointers Pointers and function parameters

What is a Pointer in C? As the name suggests, Pointer is to point

towards anything.For example pointer on the road, city etc.

Pointing towards “Road”

Pointers in C C pointers are used to point towards the

contents of any memory location.

char ch = ’A’;

’A’0x2000

ch:

Memory Address of a Variable

The value of the variable ch

The memory address of the

variable ch

The & Operator

• Gives the memory address of an object

• Also known as the “address operator”

&ch yields the value 0x2000

char ch = ’A’;

’A’0x2000

“conversion specifier” for printing a memory address

char ch;

printf(“%p”, &ch);

Example:

Pointers

ch

0x1FFF 0x2000 0x2001 0x20020x1FFEetc

‘B’

0x2000

chPtr

0x3A15

A variable which can store the memory address of another variable

Pointers

A pointer is a variable Contains a memory address Points to a specific data type Pointer variables are usually named varPtr

cPtr:

char* cPtr;Example:

• We say cPtr is a pointer to char

0x2004

Can store an address of variables of type char

Pointers and the & Operator

Example:

A

c:

0x2000

char c = ’A’;

char *cPtr;

cPtr:

0x2004

cPtr = &c;

0x2000

Assigns the address of c to cPtr

Notes on Pointers

int* numPtr;float* xPtr;

Example:

• We can have pointers to any data type

int *numPtr;float * xPtr;

Example:

• The * can be anywhere between the type and the variable

Notes on Pointers ..

• You can print the address stored in a pointer using the %p conversion specifier

printf(“%p”, numPtr);Example:

• You can assign the address of a variable to a “compatible” pointer using the & operator

int aNumber;int *numPtr;

numPtr = &aNumber;

Example:

The * Operator

• Allows pointers to access variables they point to

• Also known as “dereferencing operator”

• Should not be confused with the * in the pointer declaration

A

c:

0x2000

B

Pointers and the Operator

Example: char c = ’A’;

char *cPtr = NULL;

cPtr = &c;

*cPtr = ’B’;

Changes the value of the variable which cPtr

points to

cPtr:

0x2004

NULL0x2000

Easy Steps to Pointers Step 1: Declare the variable to be pointed to

int num;char ch = ‘A’;float x;

num:

‘A’ch:

x:

Easy Steps to Pointers … Step 2: Declare the pointer variable

int* numPtr = NULL;char *chPtr = NULL;float * xPtr = NULL;


numPtr:

chPtr:

xPtr:

num:

‘A’ch:

x:

NULL

NULL

NULL

Easy Steps to Pointers .. Step 3: Assign address of variable to pointer



numPtr = &num;

numPtr: addr of num

addr of chchPtr:

addr of xxPtr:

num:

‘A’ch:

x:A pointer’s type has to correspond to the type of the variable it points to

chPtr = &ch;xPtr = &x;

Easy Steps to Pointers .. Step 4: De-reference the pointers



numPtr = &num;chPtr = &ch;xPtr = &x;

*xPtr = 0.25;*numPtr = *chPtr;

num: 65

‘A’ch:

0.25x:

numPtr: addr of num

addr of chchPtr:

addr of xxPtr:


int *numPtr;Beware of pointers

which are not initialized!

???

numPtr


int *numPtr = NULL;

NULL

numPtr

• When declaring a pointer, it is a good idea to always initialize it to NULL (a special pointer constant)

Pointers and Function Parameters

Example: Function to swap the values of two variables

x: 1

y: 2swap

x: 2

y: 1

#include <stdio.h>

void swap1(int a, int b){ int tmp;

tmp = a; a = b; b = tmp; return;}

int main(){ int x = 1, y = 2;

swap1(x, y); printf(“%d %d\n”, x, y); return 0;}

Bad swap

#include <stdio.h>





1

2

x:

y:

Bad swap

#include <stdio.h>





1

2

x:

y:

1

2

a:

b:

tmp:

Bad swap

#include <stdio.h>





1

2

x:

y:

1

2

a:

b:

1tmp:

Bad swap

#include <stdio.h>





1

2

x:

y:

2

2

a:

b:

1tmp:

Bad swap

#include <stdio.h>





1

2

x:

y:

2

1

a:

b:

1tmp:

Bad swap

#include <stdio.h>





1

2

x:

y:

2

1

a:

b:

1tmp:

Bad swap

#include <stdio.h>

void swap2(int* a, int* b){ int tmp;

tmp = *a; *a = *b; *b = tmp; return;}


swap2(&x, &y); printf(“%d %d\n”, x, y); return 0;}

Good swap

#include <stdio.h>





1

2

x:

y:

Good swap

#include <stdio.h>





1

2

x:

y:

addr of x

addr of y

a:

b:

tmp:

Good swap

#include <stdio.h>





1

2

x:

y:

addr of x

addr of y

a:

b:

1tmp:

Good swap

#include <stdio.h>





2

2

x:

y:

addr of x

addr of y

a:

b:

1tmp:

Good swap

#include <stdio.h>





2

1

x:

y:

addr of x

addr of y

a:

b:

1tmp:

Good swap

#include <stdio.h>





2

1

x:

y:

Good swap

Pointers and Function Arguments

Change the value of an actual parameter variable

scanf exposed!

char ch;

int numx;

float numy;

scanf(“%c %d %f”, &ch, &numx, &numy);

Summary of Pointers Pointers point towards the content of

another memory location.Address of the other memory location is

stored in the pointer variable towards which it is pointing.

Pointers needed to be declared before used.

Never left any pointer un-initialized.

Documents

Functions and Pointers