Data Structures Using C: Arrays

© Oxford University Press 2011. All rights reserved.
Data Structures Using C
Reema Thareja, Assistant Professor, Institute of
Information Technology and Management,
New Delhi

CHAPTER 5
ARRAYS

INTRODUCTION
• An array is a collection of similar data elements.
• These data elements have the same data type.
• The elements of the array are stored in consecutive memory locations and are referenced by an
index (also known as the subscript).
• Declaring an array means specifying three things:
The data type- what kind of values it can store ex, int, char, float
Name- to identify the array
The size- the maximum number of values that the array can hold
• Arrays are declared using the following syntax.
type name[size];
1st
element
2nd
element
3rd
element
4th
element
5th
element
6th
element
7th
element
8th
element
9th
element
10th
element
marks[0] marks[1] marks[2] marks[3] marks[4] marks[5] marks[6] marks[7] marks[8] marks[9]

ACCESSING ELEMENTS OF THE ARRAY
To access all the elements of the array, you must use a loop. That is, we can access all the
elements of the array by varying the value of the subscript into the array. But note that the
subscript must be an integral value or an expression that evaluates to an integral value.
int i, marks[10];
for(i=0;i<10;i++)
marks[i] = -1;
CALCULATING THE ADDRESS OF ARRAY
ELEMENTS
Address of data element, A[k] = BA(A) + w( k – lower_bound)
Here, A is the array
k is the index of the element of which we have to calculate the address
BA is the base address of the array A.
w is the word size of one element in memory, for example, size of int is 2.
99 67 78 56 88 90 34 85
Marks[0] marks[1] marks[2] marks[3] marks[4] marks[5] marks[6] marks[7]
marks[7] 1000 1002 1004 1006 1008 1010 1012 1014
Marks[4] = 1000 + 2(4 – 0)
= 1000 + 2(4) = 1008

STORING VALES IN ARRAYS
Store values in the array
Initialize the elements
Input values for the elements
Assign values to the elements
Initialization of Arrays
Arrays are initialized by writing,
type array_name[size]={list of values};
int marks[5]={90, 82, 78, 95, 88};
int i, marks[10];
for(i=0;i<10;i++)
scanf(“%d”, &marks[i]);
Assigning Values
int i, arr1[10], arr2[10];
for(i=0;i<10;i++)
arr2[i] = arr1[i];
Inputting Values
CALCULATING THE LENGTH OF THE ARRAY
Length = upper_bound – lower_bound + 1
Where, upper_bound is the index of the last element
and lower_bound is the index of the first element in the array
99 67 78 56 88 90 34 85
Marks[0] marks[1] marks[2] marks[3] marks[4] marks[5] marks[6 marks[7]]
Here, lower_bound = 0, upper_bound = 7
Therefore, length = 7 – 0 + 1 = 8

WRITE A PROGRAM TO READ AND DISPLAY N NUMBERS USING
AN ARRAY
#include<stdio.h>
#include<conio.h>
int main()
{
int i=0, n, arr[20];
clrscr();
printf(“n Enter the number of elements : “);
scanf(“%d”, &n);
for(i=0;i<n;i++)
{
printf(“n Arr[%d] = “, i);
scanf(“%d”,&num[i]);
}
printf(“n The array elements are “);
for(i=0;i<n;i++)
printf(“Arr[%d] = %dt”, i, arr[i]);
return 0;
}

INSERTING AN ELEMENT IN THE ARRAY
Algorithm to insert a new element to the end of the array.
Step 1: Set upper_bound = upper_bound + 1
Step 2: Set A[upper_bound] = VAL
Step 3; EXIT
The algorithm INSERT will be declared as INSERT( A, N, POS, VAL).
Step 1: [INITIALIZATION] SET I = N
Step 2: Repeat Steps 3 and 4 while I >= POS
Step 3: SET A[I + 1] = A[I]
Step 4: SET I = I – 1
[End of Loop]
Step 5: SET N = N + 1
Step 6: SET A[POS] = VAL
Step 7: EXIT
Calling INSERT (Data, 6, 3, 100) will lead to the following processing in the array
45 23 34 12 56 20 20
Data[0] Data[1] Data[2] Data[3] Data[4] Data[5] Data[6]
45 23 34 12 56 56 20
45 23 34 12 12 56 20
45 23 34 100 12 56 20

DELETING AN ELEMENT FROM THE ARRAY
Algorithm to delete an element from the end of the array
Step 1: Set upper_bound = upper_bound - 1
Step 2: EXIT
Step 1: [INITIALIZATION] SET I = POS
Step 2: Repeat Steps 3 and 4 while I <= N - 1
Step 3: SET A[I] = A[I + 1]
Step 4: SET I = I + 1
[End of Loop]
Step 5: SET N = N - 1
Step 6: EXIT
The algorithm DELETE will be declared as DELETE( A, N, POS).
Calling DELETE (Data, 6, 2) will lead to the following processing in the array
45
23 12 12 56
20
Data[0] Data[1] Data[2] Data[3] Data[4] Data[5]
45 23 34 12 56 20
45
23 12 56 56
20
45
23 12 56 20
20
45
23 12 56 20
Data[0] Data[1] Data[2] Data[3] Data[4]

LINEAR SEARCH
LINEAR_SEARCH(A, N, VAL, POS)
Step 1: [INITIALIZE] SET POS = -1
Step 2: [INITIALIZE] SET I = 0
Step 3: Repeat Step 4 while I<N
Step 4: IF A[I] = VAL, then
SET POS = I
PRINT POS
Go to Step 6
[END OF IF]
[END OF LOOP]
Step 5: PRINT “Value Not Present In The Array”
Step 6: EXIT
BINARY SEARCH
BEG = lower_bound and END = upper_bound
MID = (BEG + END) / 2
If VAL < A[MID], then VAL will be present in the left segment of the array. So,
the value of END will be changed as, END = MID – 1
If VAL > A[MID], then VAL will be present in the right segment of the array. So,
the value of BEG will be changed as, BEG = MID + 1

BINARY_SEARCH(A, lower_bound, upper_bound, VAL, POS)
Step 1: [INITIALIZE] SET BEG = lower_bound, END = upper_bound, POS = -1
Step 2: Repeat Step 3 and Step 4 while BEG <= END
Step 3: SET MID = (BEG + END)/2
Step 4: IF A[MID] = VAL, then
POS = MID
PRINT POS
Go to Step 6
IF A[MID] > VAL then;
SET END = MID - 1
ELSE
SET BEG = MID + 1
[END OF IF]
[END OF LOOP]
Step 5: IF POS = -1, then
PRINTF “VAL IS NOT PRESENT IN THE ARRAY”
[END OF IF]
Step 6: EXIT
int A[] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
and VAL = 9, the algorithm will proceed in the following manner.
BEG = 0, END = 10, MID = (0 + 10)/2 = 5
Now, VAL = 9 and A[MID] = A[5] = 5
A[5] is less than VAL, therefore, we will now search for the value in the later half of the array. So, we change the values of
BEG and MID.
Now, BEG = MID + 1 = 6, END = 10, MID = (6 + 10)/2 =16/2 = 8
Now, VAL = 9 and A[MID] = A[8] = 8
A[8] is less than VAL, therefore, we will now search for the value in the later half of the array. So, again we change the
values of BEG and MID.
Now, BEG = MID + 1 = 9, END = 10, MID = (9 + 10)/2 = 9
Now VAL = 9 and A[MID] = 9.
Now VAL = 9 and A[MID] = 9.

ONE DIMENSIONAL ARRAYS FOR INTER FUNCTION
COMMUNICATION
1D Arrays For Inter Function Communication
Passing individual elements Passing entire array
Passing individual elements Passing entire array
Passing data values
main()
{
int arr[5] ={1, 2, 3, 4, 5};
func(arr[3]);
}
void func(int num)
{
printf("%d", num);
}
Passing addresses
main()
{
int arr[5] ={1, 2, 3, 4, 5};
func(&arr[3]);
}
void func(int *num)
{
printf("%d", num);
}
Passing the entire array
main()
{
int arr[5] ={1, 2, 3, 4, 5};
func(arr);
}
moid func(int arr[5])
{
int i;
for(i=0;i<5;i++)
printf("%d", arr[i]);

POINTERS AND ARRAYS
int arr[5] = {1, 2, 3, 4, 5};
int *ptr = &arr[0];
ptr++;
printf(“n The value of the second element of the array is %d”, *ptr);
Write a program to read and display an array of n integers
#include<stdio.h>
int main()
{
int i, n;
int arr[10], *parr;
parr = arr;
printf(“n Enter the number of elements : “);
scanf(“%d”, &n);
for(i=0; i <n; i++)
scanf(“%d”, (parr+i));
for(i=0; i <n; i++)
printf(“n arr[%d] = %d”, i, *(parr+i));
}

ARRAY OF POINTERS
An array of pointers can be declared as
int *ptr[10]
The above statement declares an array of 10 pointers where each of the pointer points to an
integer variable. For example, look at the code given below.
int *ptr[10];
int p=1, q=2, r=3, s=4, t=5;
ptr[0]=&p;
ptr[1]=&q;
ptr[2]=&r;
ptr[3]=&s;
ptr[4]=&t
Can you tell what will be the output of the following statement?
printf(“n %d”, *ptr[3]);
Yes, the output will be 4 because ptr[3] stores the address of integer variable s and *ptr[3] will
therefore print the value of s that is 4.

TWO DIMENSIONAL ARRAYS
• A two dimensional array is specified using two subscripts where one subscript denotes row
and the other denotes column.
• C looks a two dimensional array as an array of a one dimensional array.
Second Dimension
A two dimensional array is declared as:
data_type array_name[row_size][column_size];
Therefore, a two dimensional mXn array is an
array that contains m*n data elements and each
element is accessed using two subscripts, i and j
where i<=m and j<=n
int marks[3][5]
Rows/Columns
Col 0 Col 1 Col2 Col 3 Col 4
Row 0 Marks[0][0] Marks[0][1] Marks[0][2] Marks[0][3] Marks[0][4]
Two Dimensional Array

MEMORY REPRESENTATION OF A TWO DIMENSIONAL ARRAY
There are two ways of storing a 2-D array can be stored in memory. The first way is row major
order and the second is column major order.
In the row major order the elements of the first row are stored before the elements of the second
and third row. That is, the elements of the array are stored row by row where n elements of the
first row will occupy the first nth locations.
(0,0) (0, 1) (0,2) (0,3) (1,0) (1,1) (1,2) (1,3) (2,0) (2,1) (2,2) (2,3)
However, when we store the elements in a column major order, the elements of the first column
are stored before the elements of the second and third column. That is, the elements of the
array are stored column by column where n elements of the first column will occupy the first nth
locations.
(0,0) (1,0) (2,0) (3,0) (0,1) (1,1) (2,1 (3,1) (0,2) (1,2) (2,2) (3,2)
Address(A[I][J] = Base_Address + w{M ( J - 1) + (I - 1)}, if the array elements are stored in column
major order.
And, Address(A[I][J] = Base_Address + w{N ( I - 1) + (J - 1)}, if the array elements are stored in row
major order.
Where, w is the number of words stored per memory location
m, is the number of columns
n, is the number of rows
I and J are the subscripts of the array element

TWO DIMENSIONAL ARRAYS CONTD..
• A two dimensional array is initialized in the same was as a single dimensional array is initialized. For example,
int marks[2][3]={90, 87, 78, 68, 62, 71};
int marks[2][3]={{90,87,78},{68, 62, 71}};
Write a program to print the elements of a 2D array
• #include<stdio.h>
• #include<conio.h>
• main()
• {
• int arr[2][2] = {12, 34, 56,32};
• int i, j;
• for(i=0;i<2;i++)
• {
• printf("n");
• for(j=0;j<2;j++)
• printf("%dt", arr[i][j]);
• }
• return 0;
• }

TWO DIMENSIONAL ARRAYS FOR INTER FUNCTION
COMMUNICATION
Passing individual elements Passing a row
2D Array for Inter Function Communication
Passing the entire 2D array
There are three ways of passing parts of the two dimensional array to a function. First, we can pass
individual elements of the array. This is exactly same as we passed element of a one dimensional
array.
Passing a row
main()
{
int arr[2][3]= ( {1, 2, 3}, {4, 5, 6} };
func(arr[1]);
}
void func(int arr[])
{
int i;
for(i=0;i<5;i++)
printf("%d", arr[i] * 10);
}
Passing the entire 2D array
To pass a two dimensional array to a function, we use the array name as the actual parameter.
(The same we did in case of a 1D array). However, the parameter in the called function must
indicate that the array has two dimensions.

PROGRAM ILLUSTRATING PASSING ENTIRE ARRAY TO A FUNCTION
• void read_matrix(int mat[][], int, int);
• void sum_matrix(int mat1[][], int mat2[][], int, int);
• void display_matrix(int mat[5][5], int r, int c);
• int main()
• { int row, col, mat1[5][5], mat2[5][5];
• printf(“n Enter the number of rows and columns of the matrix : “);
• scanf(“%d %d”, row, col);
• read_matrix(mat1, row, col);
• printf(“n Enter the second matrix : “);
• read_matrix(mat2, row, col);
• sum_matrix(mat1, mat2, row, col);
• }
• void read_matrix(int mat[5][5], int r, int c)
• { int i, j;
• for(i=0;i<r;i++)
• { for(j=0;j<c;j++)
• { printf(“n mat[%d][%d] = “);
• scanf(“%d“, &mat[i][j]);
• }
• }
• }
• void sum_matrix(int mat1[5][5], mat2[5][5], int r, int c)
• { int i, j, sum[5][5];
• for(i=0;i<r;i++)
• { for(j=0;j<c;j++)
• sum[i][j] = mat1[i][j] + mat2[i][j];
• }
• display_matrix(sum, r, c);
• }
• void display_matrix(int mat[5][5], int r, int c)
• { int i, j;
• for(i=0;i<r;i++)
• { printf(“n”);
• for(j=0;j<c;j++)
• printf(“t mat[%d][%d] = %d“, mat[i][j]);

ARRAY OF FUNCTION POINTERS
When an array of function pointers is made the appropriate function is selected using an index. The
code given below shows the way to define and use an array of function pointers in C.
Step 1: Use typedef keyword so that 'fp' can be used as type
typedef int (*fp)(int, int);
Step2: Define the array and initialize each element to NULL. This can be done in two ways // with
10 pointers to functions which return an int and take two ints
First Way: fp funcArr[10] = {NULL};
Second Way: int (*funcArr[10])(int, int) = {NULL};
Step 3: Assign the function's address - 'Add' and 'Subtract'
funcArr1[0] = funcArr2[1] = &Add;
funcArr[0] = &Add; funcArr[1] = &Subtract;
Step 4: Call the function using an index to address the function pointer
printf("%dn", funcArr[1](2, 3)); // short form
printf("%dn", (*funcArr[0])(2, 3)); // "correct" way Let us now write a program that uses array of
function pointers

• #include <stdio.h>
int sum(int a, int b);
int subtract(int a, int b);
int mul(int a, int b);
int div(int a, int b);
int (*fp[4]) (int a, int b);
int main(void)
{ int result;
int num1, num2, op;
p[0] = sum;
p[1] = subtract;
p[2] = mul;
p[3] = div;
printf("n Enter the numbers: ");
• scanf("%d %d", &num1, &num2);
• do
• { printf("n 0: Add n 1: Subtract n 2: Multiply n 3: Divide n 4. EXIT");
• printf("Enter the operation: ");
• scanf("%d", &op);
result = (*fp[op]) (num1, num2);
printf("%d", result);
} while(op!=4);
}
int sum(int a, int b)
{ return a + b; }
int subtract(int a, int b)
{ return a - b; }
int mul(int a, int b)
{ return a * b; }
int div(int a, int b)
{ if(b)
return a / b;

POINTERS AND TWO DIMENSIONAL ARRAY
• Individual elements of the array mat can be accessed using either:
mat[i][j] or *(*(mat + i) + j) or*(mat[i]+j);
• See pointer to a one dimensional array can be declared as,
int arr[]={1,2,3,4,5};
int *parr;
parr=arr;
• Similarly, pointer to a two dimensional array can be declared as,
int arr[2][2]={{1,2},{3,4}};
int (*parr)[2];
parr=arr;
• Look at the code given below which illustrates the use of a pointer to a two dimensional array.
main()
{ int arr[2][2]={{1,2}.{3,4}};
int i, (*parr)[2];
parr=arr;
for(i=0;i<2;i++)
{ for(j=0;j<2;j++)
printf(" %d", (*(parr+i))[j]);
}
}
OUTPUT

MULTI DIMENSIONAL ARRAYS
• A multi dimensional array is an array of arrays.
• Like we have one index in a single dimensional array, two indices in a two dimensional array, in
the same way we have n indices in a n-dimensional array or multi dimensional array.
• Conversely, an n dimensional array is specified using n indices.
• An n dimensional m1 x m2 x m3 x ….. mn array is a collection m1*m2*m3* ….. *mn elements.
• In a multi dimensional array, a particular element is specified by using n subscripts as
A[I1][I2][I3]…[In], where,
I1<=M1 I2<=M2 I3 <= M3 ……… In <= Mn

PROGRAM TO READ AND DISPLAY A 2X2X2 ARRAY
#include<stdio.h>
int main()
{ int array1[3][3][3], i, j, k;
printf(“n Enter the elements of the matrix”);
printf(“n ******************************”);
for(i=0;i<2;i++)
{ for(j=0;j<2;j++)
{
for(k=0;k<2;k++)
{
printf(“n array[%d][ %d][ %d] = ”, i, j, k);
scanf(“%d”, &array1[i][j][k]);
}
}
}
printf(“n The matrix is : “);
printf(“n *********************************”)l
for(i=0;i<2;i++)
{ printf(“nn”);
for(j=0;j<2;j++)
{
printf(“n”);
for(k=0;k<2;k++)
printf(“t array[%d][ %d][ %d] = %d”, i, j, k, array1[i][j][k]);
}
}

SPARSE MATRIX
• Sparse matrix is a matrix that has many elements with a value zero.
• In order to efficiently utilize the memory, specialized algorithms and data structures that take
advantage of the sparse structure of the matrix should be used. Otherwise, execution will
slow down and the matrix will consume large amounts of memory.
• There are two types of sparse matrices. In the first type of sparse matrix, all elements above
the main diagonal have a value zero. This type of sparse matrix is also called a (lower)
triagonal matrix. In a lower triangular matrix, Ai,j = 0 where i<j.
• An nXn lower triangular matrix A has one non zero element in the first row, two non zero
element in the second row and likewise, n non zero elements in the nth row.
1
5 3
2 7 -1
3 1 4 2
-9 2 -8 1 7
• In an upper triangular matrix Ai,j = 0 where i>j.
•An nXn upper triangular matrix A has n non zero element in the first row, n-1 non zero element in
the second row and likewise, 1 non zero elements in the nth row.
1 2 3 4 5
3 6 7 8
-1 9 1
9 3
7

SPARSE MATRIX CONTD.
• In the second variant of a sparse matrix, elements with a non-zero value can appear only on the
diagonal or immediately above or below the diagonal. This type of matrix is also called a
tridiagonal matrix.
• In a tridiagonal matrix, Ai,j = 0 where | i – j| > 1. Therefore, if elements are present on
• the main diagonal the, it contains non-zero elements for i=j. In all there will be n elements
• diagonal below the main diagonal, it contains non zero elements for i=j+1. In all there will be n-1
elements
• diagonal above the main diagonal, it contains non zero elements for i=j-1. In all there will be n-1
elements
4 1
5 1 2
9 3 1
4 2 2
5 1 9
6 7

Data Structures Using C: Arrays

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