Arrays and Pointers, Lecture notes of C programming

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T h e G r o u p o f T h r e e

Arrays and Pointers

Class Notes

Comp 2401

Introduction To Arrays:

In C programming, one of the frequently problem is to handle similar types of data. For example: if the user wants to store marks of 500 students, this can be done by creating 500 variables individually but, this is rather tedious and impracticable. These types of problem can be handled in C programming using arrays.

An array in C Programing can be defined as number of memory locations, each of which can store the same data type and which can be references through the same variable name. It is a collective name given to a group of similar quantities. These similar quantities could be marks of 500 students, number of chairs in university, salaries of 300 employees or ages of 250 students. Thus we can say array is a sequence of data item of homogeneous values (same type). These values could be all integers, floats or characters etc.

We have two types of arrays:

1. One-dimensional arrays.
2. Multidimensional arrays.

One Dimensional Arrays:

A one-dimensional array is a structured collection of components (often called array elements ) that can be accessed individually by specifying the position of a component with a single index value. Arrays must be declared before they can be used in the program. Here is the declaration syntax of one dimensional array:

data_type array_name[array_size];

Here “data_type” is the type of the array we want to define, “array_name” is the name given to the array and “array_size” is the size of the array that we want to assign to the array. The array size is always mentioned inside the “[]”.

For example: Int age[5];

int age [5];

Here int is the data type

Age is the name of the array

[5] is the size of the array

Example of array in C programming

/* C program to find the sum marks of n students using arrays */

#include <stdio.h>

int main(){

int i,n; int marks[n]; int sum= 0 ;

printf("Enter number of students: "); scanf("%d",&n);

for(i= 0 ;i<n;i++){ printf("Enter marks of student%d: ",i+ 1 ); scanf("%d",&marks[i]); //saving the marks in array sum+=marks[i]; }

printf("Sum of marks = %d",sum);

return 0 ;

Output :

Enter number of students: (input by user) Enter marks of student1: (input by user) 10 Enter marks of student2: (input by user) 29 Enter marks of student3: (input by user) 11

Sum of marks = 50

Important thing to remember in C arrays

Suppose, you declared the array of 10 students. For example: students[10]. You can use array members from student[0] to student[9]. But, what if you want to use element student[10], student[100] etc. In this case the compiler may not show error using these elements but, may cause fatal error during program execution.

Multidimensional Arrays:

C programming language allows the user to create arrays of arrays known as multidimensional arrays. To access a particular element from the array we have to use two subscripts one for row number and other for column number. The notation is of the form array [i] [j] where i stands for row subscripts and j stands for column subscripts. The array holds ij elements. Suppose there is a multidimensional array array[i][j][k][m]. Then this array can hold ijkm numbers of data. In the same way, the array of any dimension can be initialized in C programming. For example :

int smarks[3][4];

Here, smarks is an array of two dimension, which is an example of multidimensional array. This array has 3 rows and 4columns. For better understanding of multidimensional arrays, array elements of above example can be as below:

Col 1 Col 2 Col 3 Col 4

Row 1 smakrs[0][0] smarks[0][1] smarks[0][2] smarks[0][3]

Row 2 smarks[1][0] smarks[1][1] smarks[1][2] smarks[1][3]

Row 3 smarks[2][0] smarks[2][1] smarks[2][2] smarks[ 2 ][ 3 ]

Make sure that you remember to put each subscript in its own, correct pair of brackets. All three examples below are wrong.

int a2[5, 7]; /* XXX WRONG */

a2[i, j] = 0; /* XXX WRONG */

a2[j][i] = 0; /* XXX WRONG */

would do anything remotely like what you wanted

Initialization of Multidimensional Arrays

In C, multidimensional arrays can be initialized in different number of ways.

int smarks[2][3]={{1,2,3}, {-1,-2,-3}}; OR int smarks[][3]={{1,2,3}, {-1,-2,-3}}; OR int smarks[2][3]={1,2,3,-1,-2,-3};

printf("\nSum of two matrix is: \n\n"); for(int i= 0 ;i<r;++i){ for(int j= 0 ;j<c;++j){ printf("%d ",sum[i][j]); if(j==c- 1 ) printf("\n\n"); }

return 0 ;

Program Output:

Enter number of rows (between 1 and 100): 3 Enter number of rows (between 1 and 100): 2

Enter elements of 1st matrix: Enter element a11: 4 Enter element a12: - 4 Enter element a21: 8 Enter element a22: 5 Enter element a31: 1 Enter element a32: 0 Enter elements of 2nd matrix: Enter element a11: 4 Enter element a12: - 7 Enter element a21: 9 Enter element a22: 1 Enter element a31: 4 Enter element a32: 5

Sum of two matrix is:

Dynamic Arrays and Resizing

Arrays by definition are static structures, meaning that size cannot be changed during run time. When an array is defined as

int A[n];

then A is considered a static array and memory is allocated from the run time stack for A. When A goes out of scope, the memory is deallocated and A no longer can be referenced. C allows dynamic declaration of an array as follows:

int* A = (int)malloc(n sizeof(int))

The above code declares a memory block of size n*sizeof(int) that can be accessed using the pointer A. For example, A can be initialized as follows:

int i; for (i=0; i<n; i++) A[i] = 0;

Note that although A was declared as a pointer, A can be treated as an array. The difference between

int A[10] and int* A = malloc(10*sizeof(int)) is that latter is assigned memory in the dynamic heap (and

hence must be managed by the programmer) and former is assigned memory from the run time stack (and hence managed by the compiler). Static arrays are used when we know the amount of bytes in array at compile time while the dynamic array is used where we come to know about the size on run time. Arrays can also be initialized using the calloc() functions instead of the the malloc(). Calloc function is used to reserve space for dynamic arrays. Has the following form.

array =(cast-type*)calloc(n,element-size);

Single element of an array can be passed in similar manner as passing variable to a function.

Passing entire one-dimensional array to a function

While passing arrays to the argument, the name of the array is passed as an argument(,i.e, starting address of memory area is passed as argument).

C program to pass an array containing age of person to a function. This function will return average age and display the average age in main function.

#include <stdio.h> float average(float a[]); int main(){ float avg, c[]={23.4, 55 , 22.6, 3 , 40.5, 18 }; avg=average(c); /* Only name of array is passed as argument. */ printf("Average age=%.2f",avg); return 0 ; } float average(float a[]){ int i; float avg, sum=0.0; for(i= 0 ;i< 6 ;++i){ sum+=a[i]; } avg =(sum/ 6 ); return avg; }

Output Average age=27.

Passing Multi-dimensional Arrays to Function

To pass two-dimensional array to a function as an argument, starting address of memory area reserved is passed as in one dimensional array

Example to pass two-dimensional arrays to function #include void Function(int c[ 2 ][ 2 ]); int main(){ int c[ 2 ][ 2 ],i,j; printf("Enter 4 numbers:\n"); for(i= 0 ;i< 2 ;++i) for(j= 0 ;j< 2 ;++j){

scanf("%d",&c[i][j]); } Function(c); /* passing multi-dimensional array to function / return 0 ; } void Function(int c[ 2 ][ 2 ]){ / Instead to above line, void Function(int c[][2]){ is also valid */ int i,j; printf("Displaying:\n"); for(i= 0 ;i< 2 ;++i) for(j= 0 ;j< 2 ;++j) printf("%d\n",c[i][j]);

Output

Enter 4 numbers: 2 3 4 5 Displaying: 2 3 4 5

Introduction to Pointers

A variable in a program is something with a name, the value of which can vary. The way the compiler and linker handles this is that it assigns a specific block of memory within the computer to hold the value of that variable. The size of that block depends on the range over which the variable is allowed to vary. For example, on 32 bit PC's the size of an integer variable is 4 bytes. On older 16 bit PCs integers were 2 bytes. In C the size of a variable type such as an integer need not be the same on all types of machines. We have integers, long integers and short integers which you can read up on in any basic text on C. This document assumes the use of a 32 bit system with 4 byte integers.

When we declare a variable we inform the compiler of two things, the name of the variable and the type of the variable. For example, we declare a variable of type integer with the name k by writing:

int k;

will copy 7 to the address pointed to by ptr. Thus if ptr "points to" (contains the address of) k, the above statement will set the value of k to 7. That is, when we use the '*' this way we are referring to the value of that which ptr is pointing to, not the value of the pointer itself.

Similarly, we could write:

printf("%d\n",*ptr);

to print to the screen the integer value stored at the address pointed to by ptr.

Arrays and Pointers

Arrays and pointers are closely related in C. In fact an array declared as

int A[10];

can be accessed using its pointer representation. The name of the array A is a constant pointer to the first element of the array. So A can be considered a const int*. Since A is a constant pointer, A = NULL would be an illegal statement. Arrays and pointers are synonymous in terms of how they use to access memory. But, the important difference between them is that, a pointer variable can take different addresses as value whereas, in case of array it is fixed.

Consider the following array:

Int age[5];

age[0] age[1] age[2] age[3] age[4]

In C , name of the array always points to the first element of an array. Here, address of first element of an array is &age[0]. Also, age represents the address of the pointer where it is pointing. Hence, &age[0] is equivalent to age. Note, value inside the address &age[0] and address age are equal. Value in address &age[0] is age[0] and value in address age is *age. Hence, age[0] is equivalent to *age.

C arrays can be of any type. We define array of ints, chars, doubles etc. We can also define an array of pointers as follows. Here is the code to define an array of n char pointers or an array of strings.

char* A[n];

each cell in the array A[i] is a char* and so it can point to a character. Now if you would like to assign a string to each A[i] you can do something like this.

A[i] = malloc(length_of_string + 1);

Here ‘age’ points

to the first

element of the

array

Again this only allocates memory for a string and you still need to copy the characters into this string. So if you are building a dynamic dictionary (n words) you need to allocate memory for n char*’s and then allocate just the right amount of memory for each string.

In C, you can declare an array and can use pointer to alter the data of an array. This program declares the array of six element and the elements of that array are accessed using pointer, and returns the sum.

//Program to find the sum of six numbers with arrays and pointers. #include <stdio.h> int main(){ int i,class[ 6 ],sum= 0 ; printf("Enter 6 numbers:\n"); for(i= 0 ;i< 6 ;++i){ scanf("%d",(class+i)); // (class+i) is equivalent to &class[i] sum += *(class+i); // *(class+i) is equivalent to class[i] } printf("Sum=%d",sum); return 0 ; }

Output

Enter 6 numbers: 2 3 4 5 3 4

Sum=

Bibliography

 "The C Programming Language" 2nd Edition B. Kernighan and D. Ritchie Prentice Hall ISBN 0-13-110362-8 document.

 "Algorithms in C" Robert Sedgewick Addison-Wesley ISBN 0-201-51425-

 http://www.programiz.com

 http://www.lysator.liu.se/c/bwk-tutor.html#array

 http://www.eskimo.com/~scs/cclass/int/sx9.html