Functions and Pointers

Functions and Pointers Lecture 29

Summary of Previous Lecture • Algorithms • Functions • Using functions in top-down design • Functions in C Language • User defined Functions • Parameters • Return values

Topics • Functions • Prototypes • Variable 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

Example: isNegative.c #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; } int isNegative ( int n ) { int result; if ( n<0 ) { result=1; } else { result = 0; } return result; }

Function Prototype Example: isNegative.c #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; } int isNegative ( int n ) { int result; if ( n<0 ) { result=1; } else { result = 0; } return result; }

Example: isNegative.c #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; } int isNegative ( int n ) { int result; if ( n<0 ) { result=1; } else { result = 0; } return result; } Function Definition

Example: isNegative.c #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; } int isNegative ( int n ) { int result; if ( n<0 ) { result=1; } else { result = 0; } return result; } Function Call (Must be after prototype, but can be before definition)

Example: isNegative.c #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; } int isNegative ( int n ) { int result; if ( n<0 ) { result=1; } else { result = 0; } return result; } Header files (filename.h) contain function prototypes and global variable declarations

Example: isNegative.c #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; } int isNegative ( int n ) { int result; if ( n<0 ) { result=1; } else { result = 0; } return result; } 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"

Example: isNegative.c #include <stdio.h> #include "myFunctions.h" #include "myFunctions.c" int main (void) { int number; printf ("Enter an integer: "); scanf ("%d",&number); if (isNegative(number)) { printf("Negative\n"); } else { printf("Positive\n"); } 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.h Example: myFunctions.c int isNegative ( int n ) { int result; if ( n<0 ) { result=1; } else { result = 0; } return result; } 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

Example: isNegative.c #include <stdio.h> int isNegative ( int n ) { int result; if (number<0) { result=1; } else { result = 0; } return result; } 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 #include <stdio.h> int isNegative ( int n ) { int result; if (number<0) { result=1; } else { result = 0; } return result; } int main (void) { int number; printf ("Enter an integer: "); scanf ("%d",&number); if (isNegative(number)) { printf("Negative\n"); } else { printf("Positive\n"); } return 0; } ERROR! Number is local to the main function, not accessible here

Example: isNegative.c #include <stdio.h> int isNegative ( int n ) { int result; if ( n<0 ) { result=1; } else { result = 0; } return result; } int main (void) { int number; printf ("Enter an integer: "); scanf ("%d",&number); if (isNegative(number)) { printf("Negative\n"); } else { printf("Positive\n"); } return 0; } Use the parameter n which is local to the function isNegative()

Example: isNegative.c #include <stdio.h> int isNegative ( int n ) { int result; if ( n<0 ) { result=1; } else { result = 0; } return result; } int main (void) { int number; printf ("Enter an integer: "); scanf ("%d",&number); if (isNegative(number)) { printf("Negative\n"); } else { printf("Positive\n"); } return 0; } n is the formal parameter number is the actual parameter

Example: isNegative.c #include <stdio.h> int isNegative ( int n ) { int result; if ( n<0 ) { result=1; } else { result = 0; } return result; } int main (void) { int number; printf ("Enter an integer: "); scanf ("%d",&number); if (isNegative(number)) { printf("Negative\n"); } else { printf("Positive\n"); } return 0; } result & n: local to isNegative() number:local to main()

Example: isNegativeGlobal.c #include <stdio.h> int number; int isNegative ( void ) { int result; if ( number <0 ) { result=1; } else { result = 0; } return result; } int main (void) { printf ("Enter an integer: "); scanf ("%d",&number); if (isNegative()) { printf("Negative\n"); } else { printf("Positive\n"); } return 0; }

Example: isNegativeGlobal.c #include <stdio.h> int number; int isNegative ( void ) { int result; if ( number <0 ) { result=1; } else { result = 0; } return result; } int main (void) { printf ("Enter an integer: "); scanf ("%d",&number); if (isNegative()) { printf("Negative\n"); } else { printf("Positive\n"); } 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 prototypebefore 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.

ch: ’A’ 0x2000 Memory Address of a Variable char ch = ’A’; The memory address of the variable ch The value of the variable ch

char ch = ’A’; ’A’ 0x2000 &ch yields the value 0x2000 The &Operator • Gives the memory address of an object • Also known as the “address operator”

“conversion specifier” for printing a memory address Example: char ch; printf(“%p”, &ch);

0x3A15 0x2000 chPtr Pointers A variable which can store the memory address of another variable 0x1FFE 0x1FFF 0x2000 0x2001 0x2002 etc ‘B’ ch

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

Example: char* cPtr; cPtr: 0x2004 Can store an address of variables of type char • We say cPtris a pointer to char

c: cPtr: A 0x2000 0x2004 Pointers and the & Operator Example: char c = ’A’; char *cPtr; cPtr = &c; Assigns the address ofc to cPtr 0x2000

We can have pointers to any data type Example: int* numPtr; float* xPtr; • The * can be anywhere between the type and the variable Example: int *numPtr; float * xPtr; Notes on Pointers

You can assign the address of a variable to a “compatible” pointer using the &operator int aNumber; int *numPtr; numPtr = &aNumber; Example: • You can print the address stored in a pointer using the %pconversion specifier Example: printf(“%p”, numPtr); Notes on Pointers ..

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

0x2000 cPtr: c: A NULL 0x2000 0x2004 Pointers and the  Operator Example: char c = ’A’; char *cPtr = NULL; Changes the value of the variable which cPtr points to cPtr = &c; *cPtr = ’B’; B

num: ch: ‘A’ x: Easy Steps to Pointers • Step 1: Declare the variable to be pointed to int num; char ch = ‘A’; float x;

NULL NULL NULL Easy Steps to Pointers … • Step 2: Declare the pointer variable int num; char ch = ‘A’; float x; numPtr: chPtr: int* numPtr = NULL; char *chPtr = NULL; float * xPtr = NULL; xPtr: num: ch: ‘A’ x:

Easy Steps to Pointers .. • Step 3: Assign address of variable to pointer int num; char ch = ‘A’; float x; numPtr: addr of num chPtr: addr of ch int* numPtr = NULL; char *chPtr = NULL; float * xPtr = NULL; xPtr: addr of x numPtr = &num; num: chPtr = &ch; xPtr = &x; ch: ‘A’ x: A pointer’s type has to correspond to the type of the variable it points to

Easy Steps to Pointers .. • Step 4: De-reference the pointers int num; char ch = ‘A’; float x; numPtr: addr of num chPtr: addr of ch int* numPtr = NULL; char *chPtr = NULL; float * xPtr = NULL; xPtr: addr of x numPtr = &num; chPtr = &ch; xPtr = &x; num: 65 ch: ‘A’ *xPtr = 0.25; *numPtr = *chPtr; x: 0.25

Notes on Pointers .. Beware of pointers which are not initialized! int *numPtr; ??? numPtr

Notes on Pointers .. • When declaring a pointer, it is a good idea to always initialize it toNULL (a special pointer constant) int *numPtr = NULL; NULL numPtr

Pointers and Function Parameters • Example: Function to swap the values of two variables swap x: 1 y: 2 x: 2 y: 1

Bad swap #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> 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; } x: 1 y: 2

Bad swap #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; } tmp: a: 1 b: 2 x: 1 y: 2

Functions and Pointers