pointers

ME 172Introduction to C Programming

Language

Cyrus Ashok Arupratan AtisLecturer, Dept. of ME, BUET

Mohd. Aminul HoqueLecturer, Dept. of ME, BUETSourav SahaLecturer, Dept. of ME, BUET

04/22/23 ME 172 : C Programming Sessional 1

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C Programming language

ME-172

CLASS-8DATE: April 22, 2023

04/22/23 ME 172 : C Programming Sessional

Topic…………

Pointer


• A pointer is a programming language object, whose value refers to (or "points to") another value stored elsewhere in the computer memory using its address. A pointer references a location in memory, and obtaining the value stored at that location is known as dereferencing the pointer.

• A pointer is a variable which contains the address of another variable.


POINTERS

Organization of Memory

1 2 3 4

0x2143

i:

p =&i

address INTEGR TYPE DATA

INTEGR TYPE POINTER

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Syntax:variable type *variable_name;

This syntax actually mimics the way how an ordinary variable is declared.

Declaration


POINTERS

For what type of data will the pointer be used?

The target variable

USE:variable_name = &some_variable;

* and & are inverses and cancel each other out. That means *&y is equivalent to y.

Dereferencing variable

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Implementation example:Declaration


POINTERS

int x =1, y =2, z[10]; //two integers and one array declaration

int *ip; // ip is an integer type pointer

ip = &x; //ip points to the memory of x

y =*ip; // y is now 1

*ip = 0; //x is now 0

ip = &z[0]; // ip now points to z[0]

Similarly one can declare:

char *x;double *y;atof(char *); etc….

Simple examplesint x,*y;x=1;y=&x;printf("x = %d\n",x);printf("&x = %x\n",&x);printf("y = %x\n",y);printf("*y = %d\n",*y);


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Format specifier for printing Hexadecimal values

Output

Simple examples

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int x,*y;x=1;y=&x;printf("x = %d\n",x);printf("&x = %x\n",&x);printf("y = %x\n",y);printf("*y = %d\n",*y);*y = 9;printf("after *y = 9 operation x = %d\n",x);


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Output


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Pointers and Function Arguments

Ordinarily in C programming language arguments are passed to function by value. So, there is no direct way to alter the actual variable with the help of a function. A function merely can alter the copies of actual variables of the caller function.

However, by passing pointers to the variables of interest one can easily modify the actual variables through function.

Return type function_name(pointer type *a);Calling method


POINTERSPointers and Function Arguments

#include <stdio.h>void sample(int *b);void main(){int x,*a;x = 5;a =&x;printf("Before calling function x = %d\n",x);sample(a);printf("\nAfter calling function x = %d",x);}void sample(int *b){ *b = 6;}

Method of declaration


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Class Performance 1!

Using pointer and a separate function write a program to swap two variables. (e.g. if two variables are a=5 and b=10, swap their

values to a=10 and b=5).


POINTERS#include <stdio.h>void swap(int *a, int *b);void main(){int x,y,*p,*q;x = 5;y = 10;p = &x;q = &y;printf("Before swapping x = %d\ty = %d\n",x,y);swap(p,q);printf("\nAfter swapping x = %d\ty = %d\n",x,y);}void swap(int *a, int *b){ int temp; temp = *a; *a = *b; *b = temp;}

• In C, there is a strong relationship between pointers and arrays.• Any operation that can be done by array subscripting can also be done by using

pointer and using pointers actually lessens the runtime.


POINTERS and ARRAYS

Before going into the discussion, lets recall what we learned about arrays.

1 2 3 4

ARRAY (say, int arr[4])

arr[0] arr[1] arr[2] arr[3]

This array is stored in memory with reference to its name. The name points to the first element.

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A: A[0] A[1] A[9]

&A[0] &A[1] &A[9]

The declaration int A[10];

defines an array A of size 10, that is, a block of 10

consecutive objects namedA[0],…,A[9] in memory.

pa pa+ 1 pa+ 9

*pa *(pa+1) *(pa+9)

Now the assignment x = *pa, will actually copy the element of A[0] to the variable x. if pa is incremented as *(pa+1) it will point to the element inside A[1]. In similar manner, *(pa+i) points to the i-th element in an array.

Remember that pa + 1 points to the next object, not the next byte.


POINTERS and ARRAYS

Now assume we declare an array int A[10].

Say, pa is an integer type pointer.int *pa;

Now if we declare,pa = &A[0];

pa would actually point to the firstelement in the array.

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We know that the name of an array is a synonym for the location of the initial element. Therefore,the assignment pa = &A[0] and pa = A are identical. After this declaration if you increment the

pointer as before the result will be similar.


POINTERS and ARRAYS

If you are clever enough you have already guessed A[i] and *(A+i) point to the same element.Similarly pa[i] is identical to [pa +i].

BUT there is a difference between a pointer and an array. The pointer is actually a variable. Hence, the commands pa = a or pa++ are valid commands. However, A++ is not a valid command!


POINTERS and ARRAYS

#include <stdio.h>void main(){int i,y[3],*p,*q;for(i=0;i<3;i++) scanf("%d",&y[i]);for(i=0;i<3;i++) printf("%d\n",y[i]);p = &y[0];printf("***************\n");for(i=0;i<3;i++) {printf("%d\n",*(p+i));}q = y;printf("***************\n");for(i=0;i<3;i++) {printf("%d\n",*(q+i));}}

Example


CLASS PERFORMANCE 2!!!

POINTERS and ARRAYS

Write a program using pointer that counts the number of characters in a string. This can be done using strlen() function. DO NOT USE IT!!


POINTERS and ARRAYS

#include <stdio.h>int strcnt(char *a);void main(){int i,*p;char s[12];gets(s);p = s;i = strcnt(p);printf("\nNumber of characters in input string is \t= %d",i);}int strcnt(char *a){ int count=0; for(count=0;*a!='\0';a++) count++; return count;}

Solution


POINTERS: Dynamic Memory Allocation

From the discussion of array and pointer it is quite clear that an array can be defined as a pointer. We have used it in case of string input in previous lecture without going into details.

So it means we can use int *x instead of int x[10]; In case of arrays, if we declared an array with specified size, the compiler would allocate

a fixed memory for the array (we have seen it). But for pointer variables representing arrays, the compiler doesn’t allocate a memory straightaway since there is no mention of size. Hence, we need to manually allocate some contiguous memory locations.

There are quite a few functions that we can use for this purpose, alloc(), calloc(), realloc() and malloc() are examples. To empty the memory location we use afree() and free(). Only malloc() is discussed here.


POINTERS: Dynamic Memory Allocationvariable name = (type *) malloc(number of blocks*sizeof(int));

If type is integer and number of blocks = 2 then,

1 byte

4 bytes

Same would happen for any other type of variables. Target and pointer must have same type.



#include <stdio.h>void main(){int i,n,*p;n = 5;p =(int*) malloc(n*sizeof(int));for(i=0;i<n;i++) scanf("%d",p+i);for(i=0;i<n;i++)printf("\n%d",*(p+i));}



CLASS PERFORMANCE 3!!

USING DYNAMIC MEMORY ALLOCATION DECLARE AN INTEGER ARRAY AS POINTER TO TAKE 5 INTEGERS AS INPUTS. FIND OUT THEIR AVERAGE IN A SINGLE ELEMENT FLOAT TYPE

ARRAY DECLARED AS POINTER.


POINTERS: Dynamic Memory Allocation#include <stdio.h>#include<math.h>void main(){int i,n,*p;float sum=0.0,avg,*q;n = 5;p =(int*) malloc(n*sizeof(int));for(i=0;i<n;i++) scanf("%d",p+i);for(i=0;i<n;i++)sum+=*(p+i);avg = sum/n;q = (float *) malloc(1*sizeof(float));printf("\nAverage: \t = %f",avg);}


POINTERS

There is a significant difference in strings (a type of array) and pointers. You see, a string is a character type array and the compiler reserves certain memory blocks for those. However, if a string is represented as a pointer, this doesn’t mean that these are fixed to some particular memory location. Like other variables the same pointer can point to some other memory location of choice.

Consider the example of: char amessage[]= “Mechanical” and char *pmessage = “Mechanical”; amessage is fixed to a memory location. But pemssage is not.For the same reason, we had to use strcpy() function instead of just writing string1 = string2.


POINTERS: Arrays of Pointer.

Enough about Arrays AND Pointers!

Lets talk about arrays who are pointers


POINTERS: Arrays of Pointer.Since pointers are variables themselves, these can be stored in arrays just as other variables.At first let us understand the declaration method and how to access to a particular element

in a one-dimension array of pointer.For simplicity we restrict to one-dimensional arrays only.

data type *array[expression]

Name of the array

int*number[4]

LET’S SEE HOW IT WORKS!



int*number[4]

number[0]

number[1]

number[2]

number[3]

If we want to access third element of the fourth array we write the expression as*(number[4]+2)

0 1 2



Sorting list of strings

In order to sort strings we can use character type arrays of pointers effectively. In such

cases, the pointer point to the first character of each string. Hence, we can compare

strings comfortably.

We can use strcmp() function for our aid. If strcmp(string1, string2) gives:

I.Negative value, first string precedes the second.

II.Zero, both strings are same.

III.Positive value, second string precedes the first.


POINTERS: Arrays of Pointer.Illustrative Example: Sort names of three clubs United, Barca, and Real in alphabetical order.

#include <stdio.h>#include <stdlib.h>#include<string.h>void main(){ int i; char *temp,*nam[3] = {“United", “Barca", “Real"}; for(i=0;i<3;i++){if(strcmp(nam[i],nam[i+1])>0) { temp = nam[i]; nam[i] = nam[i+1]; nam[i+1]= temp;} } printf("\n\n\n********\n\n"); for(i=0;i<3;i++) puts(nam[i]);}



CLASS PERFORMANCE 4: Sort names of four clubs United, Barca, Chelsea, and Real in reverse

of alphabetical order.


POINTERS: To Functions….

A very useful feature of pointer is, through it, you can pass one function as argument of another function, or use the function as a variable!!!!!

Let us call the argument function as guest and caller function as host.

For the guest function method of declaration is usual: data type func_name(arg1,arg2);

For the host function method of declaration is usual:

data type host_func_name(data_type (*func_name)(arg1,arg2));


POINTERS: To Functions….Swappingvoid swaph(void (*pf)(int x, int y), int p, int q);void swapg(int a, int b);void main(){ int p,q; printf("Enter x = "); scanf("%d",&p); printf("Enter y = "); scanf("%d",&q); swaph(swapg, p, q); }void swaph(void(*pf)(int x, int y), int p, int q){ (*pf)(p,q); }void swapg(int a, int b){int temp;temp = a;a = b;b = temp;printf("\nAfter swapping x = %d and y = %d\n",a,b);}


POINTERS

ASSIGNMENTS!

1) Write a simple C program using pointer that splits a string. Suppose, you input a string “Mustafizur/Rahman”. The program must find the slash and output as: “First = Mustafizur; Last = Rahman”.

2) Write your own version of strcmp() library function using pointers. That means, the function will compare two strings and find out if these are similar or not. For a change, your function should return character instead of integer. If two strings match, the function should return ‘y’ otherwise ‘n’.


Thank youThe only true wisdom is in knowing; you know nothing.

Socrates

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