array.ppt

2
Array
 Many applications require multiple data
items that have common characteristics
In mathematics, we often express such
groups of data items in indexed form:
 x1, x2, x3, …, xn
 Array is a data structure which can
represent a collection of data items which
have the same data type (float/int/char/…)

3
int a, b, c;
scanf(“%d”, &a);
scanf(“%d”, &b);
scanf(“%d”, &c);
printf(“%d ”, c);
printf(“%d ”, b);
printf(“%d n”, a);
int a, b, c, d;
scanf(“%d”, &a);
scanf(“%d”, &b);
scanf(“%d”, &c);
scanf(“%d”, &d);
printf(“%d ”, d);
printf(“%d ”, c);
printf(“%d ”, b);
printf(“%d n”, a);
3 numbers
4 numbers
Example: Printing Numbers in
Reverse

4
The Problem
 Suppose we have 10 numbers to handle
 Or 20
 Or 100
 Where do we store the numbers ? Use
100 variables ??
 How to tackle this problem?
 Solution:
Use arrays

Printing in Reverse Using Arrays
void main()
{
int n, A[100], i;
printf(“How many numbers to read? “);
scanf(“%d”, &n);
for (i = 0; i < n; ++i)
scanf(“%d”, &A[i]);
for (i = n -1; i >= 0; --i)
printf(“%d ”, A[i]);
printf(“n”);
}

6
Using Arrays
 All the data items constituting the group
share the same name
int x[10];
 Individual elements are accessed by
specifying the index
x[0] x[1] x[2] x[9]
X is a 10-element one
dimensional array

7
A first example
void main()
{
int i;
int data[10];
for (i=0; i<10; i++) data[i]= i;
i=0;
while (i<10)
{
printf("Data[%d] = %dn", i, data[i]);
i++;
}
}
“data refers to a block of 10
integer variables, data[0], data[1],
…, data[9]

8
The result
void main()
{
int i;
int data[10];
for (i=0; i<10; i++) data[i]= i;
i=0;
while (i<10)
{
printf("Data[%d] = %dn", i, data[i]);
i++;
}
}
Data[0] = 0
Data[1] = 1
Data[2] = 2
Data[3] = 3
Data[4] = 4
Data[5] = 5
Data[6] = 6
Data[7] = 7
Data[8] = 8
Data[9] = 9
Array size should be a constant
Output

9
Declaring Arrays
 Like variables, the arrays used in a program must be
declared before they are used
 General syntax:
type array-name [size];
 type specifies the type of element that will be
contained in the array (int, float, char, etc.)
 size is an integer constant which indicates the
maximum number of elements that can be stored
inside the array
 marks is an array that can store a maximum of 5
integers
int marks[5];

10
 Examples:
int x[10];
char line[80];
float points[150];
char name[35];
 If we are not sure of the exact size of the array,
we can define an array of a large size
int marks[50];
though in a particular run we may only be using,
say, 10 elements

11
Accessing Array Elements
 A particular element of the array can be
accessed by specifying two things:
Name of the array
Index (relative position) of the element in the
array
 In C, the index of an array starts from zero
 Example:
An array is defined as int x[10];
The first element of the array x can be
accessed as x[0], fourth element as x[3], tenth
element as x[9], etc.

12
Contd.
 The array index must evaluate to an integer
between 0 and n-1 where n is the maximum
number of elements possible in the array
a[x+2] = 25;
b[3*x-y] = a[10-x] + 5;
 Remember that each array element is a
variable in itself, and can be used anywhere a
variable can be used (in expressions,
assignments, conditions,…)

13
How is an array stored in
memory?
 Starting from a given memory location, the
successive array elements are allocated space
in consecutive memory locations
 x: starting address of the array in memory
 k: number of bytes allocated per array element
a[i]  is allocated memory location at
address x + i*k
Array a

14
Storage
void main()
{
int i;
int data[10];
for(i=0; i<10; i++)
printf("&Data[%d] = %un", i, &data[i]);
}
&Data[0] = 3221224480
&Data[1] = 3221224484
&Data[2] = 3221224488
&Data[3] = 3221224492
&Data[4] = 3221224496
&Data[5] = 3221224500
&Data[6] = 3221224504
&Data[7] = 3221224508
&Data[8] = 3221224512
&Data[9] = 3221224516
Output

15
Initialization of Arrays
 General form:
type array_name[size] = { list of values };
 Examples:
int marks[5] = {72, 83, 65, 80, 76};
char name[4] = {‘A’, ‘m’, ‘i’, ‘t’};
 The size may be omitted. In such cases the
compiler automatically allocates enough space
for all initialized elements
int flag[ ] = {1, 1, 1, 0};
char name[ ] = {‘A’, ‘m’, ‘i’, ‘t’};

16
How to read the elements of an
array?
 By reading them one element at a time
for (j=0; j<25; j++)
scanf (“%f”, &a[j]);
 The ampersand (&) is necessary
 The elements can be entered all in one line or in
different lines

17
A Warning
 In C, while accessing array elements, array
bounds are not checked
 Example:
int marks[5];
:
:
marks[8] = 75;
The above assignment would not necessarily
cause an error
Rather, it may result in unpredictable program
results

18
Reading into an array
void main()
{
const int MAX_SIZE = 100;
int i, size;
float marks[MAX_SIZE];
float total;
scanf("%d",&size);
for (i=0, total=0; i<size; i++)
{
scanf("%f",&marks[i]);
total = total + marks[i];
}
printf("Total = %f n Avg = %fn", total,
total/size);
}
4
2.5
3.5
4.5
5
Total = 15.500000
Avg = 3.875000
Output

19
How to print the elements of an
array?
 By printing them one element at a time
for (j=0; j<25; j++)
printf (“n %f”, a[j]);
The elements are printed one per line
printf (“n”);
for (j=0; j<25; j++)
printf (“ %f”, a[j]);
The elements are printed all in one line
(starting with a new line)

20
How to copy the elements of one
array to another?
 By copying individual elements
for (j=0; j<25; j++)
a[j] = b[j];
 The element assignments will follow the rules
of assignment expressions
 Destination array must have sufficient size

21
Example 1: Find the minimum of a
set of 10 numbers
void main()
{
int a[10], i, min;
for (i=0; i<10; i++)
scanf (“%d”, &a[i]);
min = a[0];
for (i=1; i<10; i++)
{
if (a[i] < min)
min = a[i];
}
printf (“n Minimum is %d”, min);
}

22
const int size = 10;
void main()
{
int a[size], i, min;
for (i=0; i<size; i++)
min = a[0];
{
if (a[i] < min)
min = a[i];
}
}
Alternate Version 1
Change only one
line to change the
problem size

23
#define size 10
void main()
{
int a[size], i, min;
min = a[0];
{
if (a[i] < min)
min = a[i];
}
}
Alternate Version 2
Change only one
line to change the
problem size
Used #define macro

24
#define macro
 #define X Y
 Preprocessor directive
 Compiler will first replace all occurrences of
string X with string Y in the program, then
compile the program
 Similar effect as read-only variables (const), but
no storage allocated
 We prefer you use const instead of #define

25
void main()
{
int a[100], i, min, n;
scanf (“%d”, &n); /* Number of elements */
for (i=0; i<n; i++)
min = a[0];
for (i=1; i<n; i++)
{
if (a[i] < min)
min = a[i];
}
}
Alternate Version 3
Define an array of
large size and use
only the required
number of elements

26
Example 2:
Computing
cgpa
const int nsub = 6;
void main()
{
int grade_pt[nsub], cred[nsub], i,
gp_sum=0, cred_sum=0;
double gpa;
for (i=0; i<nsub; i++)
scanf (“%d %d”, &grade_pt[i], &cred[i]);
for (i=0; i<nsub; i++)
{
gp_sum += grade_pt[i] * cred[i];
cred_sum += cred[i];
}
gpa = ((float) gp_sum) / cred_sum;
printf (“n Grade point average: is %.2lf”, gpa);
}
Handling two arrays
at the same time

27
Example: Binary Search
 Searching for an element k in a sorted array A with n
elements
 Idea:
 Choose the middle element A[n/2]
 If k == A[n/2], we are done
 If k < A[n/2], search for k between A[0] and A[n/2 -1]
 If k > A[n/2], search for k between A[n/2 + 1] and
A[n-1]
 Repeat until either k is found, or no more elements
to search
 Requires less number of comparisons than linear
search in the worst case (log2n instead of n)

28
void main() {
int A[100], n, k, i, mid, low, high;
scanf(“%d %d”, &n, &k);
for (i=0; i<n; ++i) scanf(“%d”, &A[i]);
low = 0; high = n – 1; mid = low + (high – low)/2;
while (high >= low) {
printf(“low = %d, high = %d, mid = %d, A[%d] = %dn”,
low, high, mid, mid, A[mid]);
if (A[mid] == k) {
printf(“%d is foundn”, k);
break;
}
if (k < A[mid]) high = mid – 1;
else low = mid + 1;
mid = low + (high – low)/2;
}
If (high < low) printf(“%d is not foundn”, k);
}

29
8 21
9 11 14 17 19 20 23 27
low = 0, high = 7, mid = 3, A[3] = 17
low = 4, high = 7, mid = 5, A[5] = 20
low = 6, high = 7, mid = 6, A[6] = 23
21 is not found
8 14
9 11 14 17 19 20 23 27
low = 0, high = 7, mid = 3, A[3] = 17
low = 0, high = 2, mid = 1, A[1] = 11
low = 2, high = 2, mid = 2, A[2] = 14
14 is found
Output

30
Example: Selection Sort
 Sort the elements of an array A with n
elements in ascending order
 Basic Idea:
Find the min of the n elements, swap it with
A[0] (so min is at A[0] now)
Now find the min of the remaining n-1
elements, swap it with A[1] (so 2nd min is at
A[1] now)
Continue until no more elements left

31
void main() {
int A[100], n, i, j, k, min, pos, temp;
scanf(“%d”, &n);
for (i=0; i<n; ++i) scanf(“%d”, &A[i]);
for (i = 0; i < n - 1; ++i) {
min = A[i]; pos = i;
for (j = i + 1; j < n; ++j) {
if (A[j] < min) {
min = A[j];
pos = j;
}
}
temp = A[i];
A[i] = A[pos];
A[pos] = temp;
for (k=0; k<n; ++k) printf(“%d ”, A[k]);
printf(“n”);
}
}

32
6
7 12 5 15 17 9
5 12 7 15 17 9
5 7 12 15 17 9
5 7 9 15 17 12
5 7 9 12 17 15
5 7 9 12 15 17
8
9 8 7 6 5 4 3 2
2 8 7 6 5 4 3 9
2 3 7 6 5 4 8 9
2 3 4 6 5 7 8 9
2 3 4 5 6 7 8 9
2 3 4 5 6 7 8 9
2 3 4 5 6 7 8 9
2 3 4 5 6 7 8 9
Output

33
Things you cannot do
 You cannot
use = to assign one array variable to another
a = b; /* a and b are arrays */
use == to directly compare array variables
if (a = = b) ………..
directly scanf or printf arrays
printf (“……”, a);

34
Character Arrays and Strings
char C[8] = { 'a', 'b', 'h', 'i', 'j', 'i', 't', '0' };
 C[0] gets the value 'a', C[1] the value 'b', and so on.
The last (7th) location receives the null character ‘0’
 Null-terminated (last character is ‘0’) character arrays
are also called strings
 Strings can be initialized in an alternative way. The
last declaration is equivalent to:
char C[8] = "abhijit";
 The trailing null character is missing here. C
automatically puts it at the end if you define it like this
 Note also that for individual characters, C uses single
quotes, whereas for strings, it uses double quotes

35
Reading strings: %s format
void main()
{
char name[25];
scanf("%s", name);
printf("Name = %s n", name);
}
%s reads a string into a character array
given the array name or start address.
It ends the string with ‘0’

36
An example
void main()
{
#define SIZE 25
int i, count=0;
char name[SIZE];
scanf("%s", name);
printf("Name = %s n", name);
for (i=0; name[i]!='0'; i++)
if (name[i] == 'a') count++;
printf("Total a's = %dn", count);
}
Satyanarayana
Name = Satyanarayana
Total a's = 6
Note that character strings read
in %s format end with ‘0’
Seen on screen
Typed as input
Printed by program

37
Palindrome Checking
void main()
{
const int SIZE = 25;
int i, flag, count=0;
char name[SIZE];
scanf("%s", name); /* Read Name */
for (i=0; name[i]!='0'; i++); /* Find Length of String */
printf("Total length = %dn",i);
count=i; flag = 0;
/* Loop below checks for palindrome by comparison*/
for(i=0; i<count; i++) if (name[i]!=name[count-i-1]) flag = 1;
if (flag ==0) printf ("%s is a Palindromen", name);
else printf("%s is NOT a Palindromen", name);
}

38
Some Exercises
1. Write a C program that reads an integer n and stores the
first n Fibonacci numbers in an array.
2. Write a C program that reads an integer n and uses an
array to efficiently find out the first n prime numbers.
3. Read in an integer n, read in n integers and print the integer
with the highest frequency.
4. Read in an integer n, read in n numbers and find out the
mean, median and mode.
5. Read in two names and compare them and print them in
lexicographic (dictionary) order.
6. Read in an integer n, read in n names and print the last
name when compared in lexicographic order.

array.ppt

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