chapter8 arrays

7/29/2019 Chapter8 Arrays

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Principles of Programming - NI2005 1

Chapter 8: Arrays

In this chapter, you will learn aboutIntroduction to Array

Array declaration

Array initialization

Assigning values to array elementsReading values from array elements

Relationship with pointers

Passing array to function

2 Dimensional arrays

Simple Searching

Simple Sorting


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Introduction to Array

In C, a group of items of the same type can beset up using Array

An array is a group of consecutive memorylocations related by the fact that they all have thesame name and the same type.

The compiler must reserve storage (space) foreach element/item of a declared array.

The size of an array is static (fixed) throughoutprogram execution.

To refer to a particular location or element in thearray, we specify the name of the array (index orsubscript) and the position number of theparticular element in the array.


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Let say we have an array called a.

-10

99

-8

100

27

10

1976

-2020

1

a[0]

a[1]

a[2]

a[3]

a[4]

a[5]

a[6]

a[7]

a[8]

Name of the array

The position number within

the square brackets is formally

called a subscript. A subscript

can be an integer or an integer

expression. For example if

x = 1 and y = 2, then a[x+y]

is equal to a[3].

Notice that the position

starts from 0.


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Array Declaration

Array declaration is made by specifying thedata type, its name and the number ofspace (size) so that the computer mayreserve the appropriate amount of memory.

General syntax:data_type array_name[size];

Examples:

int my_array[100];

char name[20];

double bigval[5*200];

int a[27], b[10], c[76];


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Array Initialization

There are 2 ways to initialize an array: duringcompilation and during execution.

During compilation:int arr[ ] = {1, 2, 3, 4, 5}; unsized array

We can define how many elements that we want since

the array size is not given.int arr[3] = {90, 21, 22};We can define only 3 elements since the array size isalready given.

int arr[5] = {2,4}Initialize the first two elements to the value of 2 and 4respectively, while the other elements are initialized tozero.

int arr[5] = {0}Initialize all array elements to zero.


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Array Initialization cont

During execution:Using loop to initialize all elements to zero

int arr[3], index;

for (index = 0; index < 3; index++)

arr[index] = 0;Using loop and asking the user to specify the

value for each element.

int arr[3], index;

for (index = 0; index < 3; index++){

printf (arr[%d]:,index);

scanf(%d,&arr[index]);

}


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Assigning value to array element

We can assign a value to a specific array element byusing its index number.

Example: lets say we have an array that representthe number of inhabitant in 5 unit apartments.

int apartment[5]={3,2,6,4,5};

The above initialization indicates that there are 3people living in apartment 0, 2 people living inapartment 1 and so on.

Let say that we have a new born in apartment 3, sowe need to change the number of inhabitant living in

apartment three.apartment[3] = apartment[3] + 1;

Now, we have the following values in our array:

3,2,6,5,5


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Reading values from array elements

We can read a value from a specific array elementby referring to the index.

For example, lets say we want to know how manypeople leaving in apartment 3, we could simple dothis:

int apartment[5] = {3,2,6,4,5};int no_of_people;

no_of_people = apartment[3];

printf(Apartment 3 has %d people, no_of_people);

The above C code will produce the following output:Apartment 3 has 4 people.

Hint!!! Remember that array index starts at 0


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Example 1: finding total inhabitants#include

#define size 5

void main(void){

int apartment[size] = {3,2,6,4,5};int index, total = 0;

for (index = 0; index < size; index++){

total = total + apartment[index];}

printf("There are total of %d inhabitants",total);}

Output:There are total of 20 inhabitants


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Example 2: list down number of inhabitants in each apartment

#include

void main(void){

int apartment[5] = {3,2,6,4,5};int index, total = 0;

printf("%-7s %-15s\n","Apt No", "No of people");

for (index = 0; index < 5; index++){

printf("%4d %10d\n",index, apartment[index]);}

}


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Example 2 output

Apt No No of people0 3

1 2

2 6

3 44 5


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Relationship with pointers

The name of an array is actually a pointer tothe first element in the array.

Therefore, if we have:int test[3] = {9, 10, 11};

printf(%d, *test);The output would be: 9

There are a few ways to traverse an array:

int test[3] = {9, 10, 11}, k;for (k = 0; k < 3; k++)

printf(%d\n, test[k]);

Using indexint test[3] = {9, 10, 11}, k;int *ptr = test;for (k = 0; k < 3; k++, ptr++)

printf(%d\n, *ptr);

Using pointers


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Passing Array to a Function

When we pass an array to a function, we areactually passing the pointerto the first elementin the array to the function. Therefore, anychanges to the array inside the function will alsochange the actual array inside the calling

function.

When we want to pass an array to a function, weneed to know these 3 things.

How to write the function prototype?How to do function call?

How does the function header would look like?


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Passing Array to a FunctionAssume that we have the following array declaration.

flaot marks[10] = {0.0};

Say for example we want to write a function, called get_marks,which will read marks from the user and store the marks insidethe array.

Function prototype:/* data type with square bracket */

void get_marks(float [ ]);

void get_marks(float *); /*treating array as pointer */

Function call:get_marks(marks); /* just pass the array name */

Function header:

void get_marks(float marks[ ])

void get_marks(float *marks) /*treating array as pointers */


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Example 1: parameter received as an array

#include #define size 10

void get_marks(float [ ]);

float calc_average(float [ ]);

void main(void){

float marks[size] = {0.0}; /*initializing the array

get_marks(marks); /* function call */

printf(Average for marks given is %f, calc_average(marks));

}


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Example 1: parameter received as an array

void get_marks(float marks[ ])

{int i;

for (i = 0; i < size; i++){

printf("Marks student %d:",i + 1);

scanf("%f",&marks[i]);}

} float calc_average(float marks[ ]){

float total = 0.0;int i;

for (i = 0; i < size; i++){

total = total + marks[i];}return (total / size);

}


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Example 2: parameter received as pointers

A function could also receive/treat array parameter as pointer.

#include

#define size 10

void get_marks(float *);

float calc_average(float *);

void main(void)

{

float marks[size] = {0.0};

get_marks(marks);

printf("Average for marks given is %f\n", calc_average(marks));

}

Observe the

function prototypes


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Example 2: parameter received as pointers

void get_marks(float *marks)

{int i;

for (i = 0; i < size; i++, marks++){

printf("Marks student %d: ", i + 1);scanf("%f", marks);

}} float calculate_average(float *marks)

{float total = 0.0;int i;

for (i = 0; i < size; i++, marks++){total = total + *marks;

}

return (total / size);

}

Manipulating the memory address

Pointer variable


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2-Dimensional Array

It is possible to create an array which hasmore than one dimension.

For example:

2D array: int array[4][2];

3D array: int array[4][2][10];Graphical representation of a 2D array:

int myarray[4][2] = {1, 2, 3, 4, 5, 6, 7, 8};

1 2

3 4

5 6

7 8

This array has 4 rows and 2

columns.Four

rows

Col 1 Col2


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2-Dimensional Array contVariable initialization can also be done this way:

int myarray[4][2] = {{1, 2}, {3, 4}, {5, 6}, {7, 8}};

This method is less confusing since we can see the rowsand columns division more clearly.

To initialize a 2D array during execution, we need to use anested for loop:

for (row = 0; row < 4; row++)

{

for (column = 0; column < 2; column++)

myarray[row][column] = 0;}

Although it is possible to create a multi-dimensional array,arrays above 2-dimensions are rarely used.


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Passing a 2D array to a function

When a 2D (or higher dimensional) array ispassed to a function, the size of the second (orsubsequent) subscript needs to be specified.

For example, if we have:

int twoD[4][5];

Then the function header which would taketwoD as an

argument should be declared like this:

void Process2D(int twoD[ ][5])

An array is stored consecutively in memory

regardless of the number of dimensions.Therefore, specifying the subscripts in thefunction parameter will help the compiler to knowthe boundary of the different dimensions.


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Simple Searching

Searching is the process of determiningwhether an array contains a value thatmatches a certain key value/search key.

The process of finding a particular element ofan array is called searching.

Same as in sort, there are more than onealgorithms that can be used to do a search.

The most commonly used searchingtechniques are linear search and binary

search.Here, we will discuss to do searching byusing linear search on an array.


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Simple Searching cont

Search keyis a data element of the sametype as the list elements.

If search key = list element value, the search

is said to be successful

Otherwise, it is unsuccessful.Linear search is a simple searching algorithmwhere:

data are stored in an array

a search key is compared with each elements

in the array starting from the first element


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Example

void main(void){

int list[ ] = {34, 53, 21, 23, 4};int i, search_key, found = 0;

printf(Enter the number that you want to find: );scanf(%d, &search_key);

for (i = 0; i < 5; i++){

if (list[i] = = search_key){

found = 1;

printf(The number %d is found at index %d\n, search_key, i);}}if (found = = 0)

printf(The number %d cannot be found in the list\n,search_key);}

Sample Output:

Enter the number that you want to find: 53

The number 53 is found at index 1Press any key to continue


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Sorting

Sorting is the process of placing data into aparticular order such as ascending ordescending.

The following example shows the C code forsorting unsorted list to a list sorted in

ascending order.Explanation for the working program and theconcept behind it will be done during lecturehour (so please attend the class!!!!).


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Example using Simple Sort#include

#define size 10void sort(int [ ]);

void main(void){

int index, list[size] = {0};

for (index = 0; index < size; index++) /*array initialisation by user */{printf("list[%d]:",index);scanf("%d",&list[index]);

}

sort(list); /* calling function to sort the array

printf("\nList sorted in ascending order:\n\n");

for (index = 0; index < size; index++){

printf("%d\t",list[index]); /* printing the array element */}

}


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Sorting example contvoid sort(int list[ ])

{int pivot, checker, temp;

for (pivot = 0; pivot < (size - 1); pivot++){

for (checker = (pivot + 1); checker < size; checker++){

if (list[checker] < list[pivot]){

/* swap the elements */temp = list[pivot] ;list[pivot] = list[checker];list[checker] = temp;

}}

}}


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Summary

In this chapter, we have looked at:Array declaration and initialization

Reading and writing from/to array elements

Passing array to function

2 dimensional array

Simple search

Array sorting

chapter8 arrays

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