CSEG1001 Unit 4 Functions and Pointers

FUNCTIONS & POINTERS
Instructor
Dhiviya Rose J , AP-Sr. Scale | SoCSE

WAProgram ????? output
HELLO I AM A FUNCTION NAMED HELLO
WELCOME I AM A FUNCTION NAMED WELCOME
WELCOME I AM A FUNCTION NAMED WELCOME

Need/Advantages of function
• Modular Programming
• Makes C program readable
• Enhance application software process
• Program development becomes easy
• Reusability
• Module once created can be used any no of time

Definition – Function
• encapsulates a subset of a program
• To hide details
• To be invoked from multiple places
• To share with others
• A fragment of code
• accepts zero or more argument values
• produces a result value, and has zero or more side effects.

Functions
• Types of Functions in C
• Function definition
• Function prototypes & Header files
• Pre- and post-conditions
• Scope and storage class
• Implementation of functions
• Recursive functions

Arguments???? -> input to functions

CSEG1001 Unit 4 Functions and Pointers

Common Predefined Functions in C
#include <math.h>
• sin(x) // radians
• cos(x) // radians
• tan(x) // radians
• atan(x)
• atan2(y,x)
• exp(x) // ex
• log(x) // loge x
• log10(x) // log10 x
• sqrt(x) // x  0
• pow(x, y) // xy
• ...
#include <stdio.h>
• printf()
• fprintf()
• scanf()
• sscanf()
• ...
#include <string.h>
• strcpy()
• strcat()
• strcmp()
• strlen()
• ...

Function Definition
resultType functionName(type1 arg1, type2 arg2, …)
{
…
body
…
}
• If no result, resultType should be void
• If no parameters, use void between ()

Definitions
• Argument:–
• an expression passed when a function is called
• int a;
• a=pow(2,3);
• 2 and 3 are the arguments
Function call of pow()
• Returns type = int
• Argument = 2 integers

Examples of function definition
• int pow(int a,int b)
{
int ans,i;
ans=1;
for(i=1;i<=b;i++)
ans=ans*a;
}
// pow function

Function Prototypes
• a function must be used separate from where it is defined
–
• before its definition in the same C program
• Therefore, we need some way to declare a function
separate from defining its body.
• Called a Function Prototype

Function Prototypes (continued)
• Definition:– a Function Prototype in C is a language
construct of the form:–
return-type function-name (parameter declarations) ;

Pointers
• A pointer is a reference to another variable in a program
• Stores the memory location

Pointer Variable Definition
• Basic syntax:
datatype *ptrName
• Can declare pointers to any data type
• Examples:
int *P; /* P is var that can point to an int var */
float *Q; /* Q is a float pointer */
char *R; /* R is a char pointer */
• Complex example:
int *AP[5]; /* AP is an array of 5 pointers to ints *

Pointer Variable Initialization - Address Operator
• The address (&) operator points the memory location of
the variable in memory
Syntax: &variablename
Examples:
int a=10;
int *b;
b=&a;
Pointer variable ‘b’
holding the
address of variable
‘a’

Dereferencing Pointers - Indirection (*) Operator
• Is to get the value of the variable it refers to
• Syntax: *PointerVariable
Example:
int age = 19;
int *P = &V;
printf(“%d”,*P);
/* Prints 19 */

Go through …..
* and & are inverses
Cancel each other out
*&myVar == myVar
and
&*yPtr == yPtr

Knowledge Check
Assume address of A as 2002 and B
as 4005
int A = 3;
int B = 4;
int *P = &A;
int *Q = &B;
printf(“%d %dn”,A,B);
printf(“%d %dn”,*P,*Q);
printf(“%p %pn”,P,Q);

Pointers to Pointers
• A pointer can also be made to point to a pointer variable
Example:
int V = 101;
int *P = &V; /* P points to int V */
int **Q = &P; /* Q points to int pointer P */
printf(“%d %d %dn”,V,*P,**Q); /* prints 101 3 times */

Pointer Arithmetic
• Increment/decrement pointer (++ or --)
• Add an integer to/from a pointer( + or += )
• Pointers may be subtracted from each other (- or -=)
• Pointer arithmetic is performed on an array

Pointer Arithmetic on Arrays
• 5 element int array on a machine using 4 byte ints
• vPtr points to first element v[ 0 ], which is at location 3000
• vPtr = 3000
• vPtr += 2; sets vPtr to 3008
• vPtr points to v[ 2 ]
pointer variable
vPtr
v[0] v[1] v[2] v[4]v[3]
3000 3004 3008 3012 3016
location

Characters and Strings with Pointers
• String assignment
• Character array:
char color[] = "blue";
• Creates 5 element char array, color, (last element is '0')
• variable of type char *
char *colorPtr = "blue";
• Creates a pointer to string “blue”, colorPtr, and stores it somewhere in
memory

Arrays of Pointers
• Arrays can contain pointers
• Commonly used to store an array of strings
char *suit[ 4 ] = {"Hearts", "Diamonds",
"Clubs", "Spades" };
• Each element of suit is a pointer to a char * (a
string)
suit[3]
suit[2]
suit[1]
suit[0] ’H’ ’e’ ’a’ ’r’ ’t’ ’s’ ’0’
’D’ ’i’ ’a’ ’m’ ’o’ ’n’ ’d’ ’s’ ’0’
’C’ ’l’ ’u’ ’b’ ’s’ ’0’
’S’ ’p’ ’a’ ’d’ ’e’ ’s’ ’0’

Knowledge Check
#include<stdio.h>
int main()
{
int a = 10;
void *p = &a;
int *ptr = p;
printf("%u",*ptr);
return 0;
}
(A) 10
(B) Address
(C) 2
(D) Compilation error
(E) None of above

Knowledge Check
(A) Address, Address, 3
(B) Address, 3, 3
(C) 3, 3, 3
(D) Compilation error
(E) None of above
#include<stdio.h>
int main(){
int i = 3;
int *j;
int **k;
j=&i;
k=&j;
printf("%u %u %d ",k,*k,**k);
return 0;
}

Knowledge Check
#include<stdio.h>
int main()
{
int *ptr, b;
b = sizeof(ptr);
printf("%d" , b);
return 0;
}

CSEG1001 Unit 4 Functions and Pointers

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CSEG1001 Unit 4 Functions and Pointers