Lecture 5. Arrays and Strings

1Arrays and Strings

Lecture 5

Computer Programming(MEng 1052)

Mechanical Engineering

Department

Prepared by: Addisu D. & Beza T. April, 2015

Part I: Arrays

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Introduction

An array is used to process a collection of data all of which is

of the same type, such as a list of temperatures or a list of

names.

Suppose we wish to write a program that reads in 5 test scores

and performs some manipulations on these scores.

To retain the 5 scores, we will need something equivalent to 5

variables of type int. But 5 variables are hard to keep track of,

and we may later want to change our program to handle 100

scores; certainly, 100 variables are impractical.

An array is the perfect solution for such kind of situations.

An array behaves like a list of variables with a uniform

naming mechanism that can be declared in a single line of

simple code.

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In C++, an array consisting of five variables of type int can be

declared as follows:

int score[5];

This declaration is like declaring the following five variables

to all be of type int:

score[0], score[1], score[2], score[3], score[4]

The individual variables that together make up the array are

called indexed variables. They are also sometimes called

subscripted variables or elements of the array.

The number in square brackets is called an index or a

subscript.

In C++, indexes are numbered starting with 0, not starting

with 1 or any other number except 0.

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Declaring and Referencing Arrays

The number of indexed variables in an array is called the

declared size of the array, or sometimes simply the size of the

array.

When an array is declared, the size of the array is given in

square brackets after the array name.

The indexed variables are then numbered (also using square

brackets), starting with 0 and ending with the integer that is one

less than the size of the array.

An array can have indexed variables of any type. For example,

to declare an array with indexed variables of type double, simply

use the type name double instead of int in the declaration of the

array.

All the indexed variables for one array are, however, of the same

type. This type is called the base type of the array. Thus, in our

example of the array score, the base type is int.5


You can declare arrays and regular variables together. For

example, the following declares the two int variables next

and max in addition to the array score:

int next, score[5], max;

Do not confuse the two ways to use the square brackets [ ]

with an array name.

When used in a declaration, such as int score[5]; the number

enclosed in the square brackets specifies how many indexed

variables the array has.

When used anywhere else, the number enclosed in the square

brackets tells which indexed variable is meant. For example,

score[0] through score[4] are indexed variables.

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The index inside the square brackets need not be given as an

integer constant. You can use any expression in the square

brackets as long as the expression evaluates to one of the

integers 0 through the integer that is one less than the size of

the array.

For example, the following will set the value of score[3]

equal to 99:

int n = 2;

score[n + 1] = 99;

Although they may look different, score[n + 1] and score[3]

are the same indexed variable in the above code. That is

because n + 1 evaluates to 3.

Lets look at the following example

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Initializing Arrays

An array can be initialized when it is declared. When initializing

the array, the values for the various indexed variables are

enclosed in braces and separated with commas. For example,

int children[3] = {2, 12, 1};

This declaration is equivalent to the following code:

int children[3];

children[0] = 2;

children[1] = 12;

children[2] = 1;

If you list fewer values than there are indexed variables, those

values will be used to initialize the first few indexed variables,

and the remaining indexed variables will be initialized to a zero

of the array base type.

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The number of elements in the array that we initialized within

curly brackets { } must be equal or less than the length in elements

that we declared for the array enclosed within square brackets [ ].

If we have less number of items for the initialization, the rest will

be filled with zero.

If you initialize an array when it is declared, you can omit the size

of the array, and the array will automatically be declared to have

the minimum size needed for the initialization values.

For example, the following declaration

int b[ ] = {5, 12, 11};

is equivalent to

int b[3] = {5, 12, 11};

When initializing an array, we can provide fewer values than the

array elements. E.g. int a [10] = {10, 2, 3}; in this case the

compiler sets the remaining elements to zero. 10

Initializing Arrays

Partial Array Initialization

When an array is being initialized, C++ does not require a

value for every element

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Program Output

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Here are the contents of the array:

1

2

4

8

0

0

0

Partial Array Initialization

Accessing and processing array elements

In any point of the program in which the array is visible we can

access individually anyone of its elements for reading or

modifying it as if it was a normal variable.

To access individual elements, index or subscript is used. The

format is the following:

name [ index ]

In c++ the first element has an index of 0 and the last element has

an index, which is one less the size of the array (i.e. arraysize-1).

Thus, from the following declaration,

int day [5];

day[0] is the first element and day[4] is the last element.

For example, to store the value 75 in the third element of the

array variable day a suitable sentence would be:

day[2] = 75; //as the third element is found at index 213

To pass the value of the third element of the array variable

day to the variable a , we could write:

a = day[2];

Other valid operations with arrays in accessing and

assigning:

int a=1;

day [0] = a;

day[a] = 5;

b = day [a+2];

day [day[a]] = day [2] + 5;

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Accessing and processing array elements

Array Examples

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Program Output

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Month 1 has 31 days.












Two Dimensional Array

The simplest form of multidimensional array is the two

dimensional array.

A twodimensional array is, in essence, a list of onedimensional arrays.

The general form of a twodimensional array declaration is

type name[size1][size2];

Pay careful attention to the declaration. Unlike some other

computer languages, which use commas to separate the array

dimensions, C++ places each dimension in its own set of

brackets. Similarly, to access an element, specify own set of

brackets

Twodimensional arrays are stored in a rowcolumn matrix, where the first index indicates the row and the second indicates

the column25

In preceding example, nums[0][0] will have the value 1,

nums[0][1] the value 2, nums[0][2] the value 3, and so on.

The value of nums[2][3] will be 12. Conceptually, the array

will look like that shown here

Twodimensional arrays are used to create matrix, where the first index indicates the row and the second indicates

the column27

Two Dimensional Array

Part II: Strings

29

Strings

The most common use for one-dimensional arrays is to store

strings of characters.

In C++, a string is defined as a character array terminated by

a null symbol (\0).

To declare an array str that could hold a 10-character string,

one would write:

char str[11];

Specifying the size as 11 makes room for the null at the end

of the string.

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Strings

Declaration of Strings

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Similar to declaration of any array

char name[30];

// no initialization

char title [20] = "Le Grande Fromage";

// initialized at declaration

// with a string

char chList [10] = {'a', 'b', 'c', 'd'};

// initialized with list of char

// values

How to read a string entered from the keyboard?

Make an array, that will receive the string, the target of a cin

stream.

The following program reads (part of) a string entered by the

user:

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Reading a String from the Keyboard

Problem: Entering the string This is a test, the above

program only returns This, not the entire sentence.

Reason: The C++ input/output system stops reading a string

when the first whitespace character is encountered.

Solution: Use another C++ library function, gets( ).

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Reading a String from the Keyboard

Some C++ Library Functions for Strings

C++ supports a range of string-manipulation functions.

The most common are:

strcpy ( ) : copy characters from one string to another

strcat ( ) : concatenation of strings

strlen ( ) : length of a string

strcmp ( ) : comparison of strings

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String manipulation

strlen (str) = tells the length of a string; the total number

of characters in the string.

strcpy (str1, str2) = copies string str2 to string str1.

strcat (st1, str2) = appends string str2 to string st1.

strncat (str1, str2, n) = appends only n characters of str2

to str1.

strrev (str) = reverses the string str.

strcmp (str1, str2) = compares the two strings.

strncmp (str1, str2, n) = compares only the first n

characters of the two strings.

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strcpy (to_string, from_string ) String Copy:

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strlen(string) String Length

strlen(str) returns the length of the string pointed to by str ,

i.e., the number of characters excluding the null terminator.

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strcat(string_1, string_2) Concatenation of Strings

The strcat ( ) function appends s2 to the end of s1. String s2

is unchanged.

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Note: The first string array has to be large enough to hold both

strings:

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strcmp(string_1, string_2) Comparison of Strings

The strcmp(str_1, str_2) function compares two strings and

returns the following result:

str_1 == str_2 : 0

str_1 > str_2 : positive number

str_1 < str_2 : negative number

The strings are compared lexicographically (i.e., according

to dictionary order):

a < aa < aaa < < b < ba < bb < < bz < baa <

< abca < abd < ...

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You also cannot use the operator == in an expression to test

whether two strings are the same.

To test whether two C strings are the same, you can use the

predefined function strcmp. For example:

Note that the function strcmp works differently than you

might guess.

The comparison is true if the strings do not match. The

function strcmp compares the characters in the string

arguments a character at a time43


If at any point the numeric encoding of the character from

c_string1 is less than the numeric encoding of the

corresponding character from c_string2, the testing stops, and

a negative number is returned.

If the character from c_string1 is greater than the character

from c_string2, then a positive number is returned.

If the C strings are the same, a 0 is returned.

The ordering relationship used for comparing characters is

called lexicographic order.

The important point to note is that if both strings are all in

uppercase or all in lowercase, then lexicographic order is just

alphabetic order.

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Arrays of Strings

An array of strings is a special form of a two-dimensional

array.

The size of the left index determines the number of strings.

The size of the right index specifies the maximum length of

each string.

For example, the following declares an array of 30 strings,

each having a maximum length of 80 characters (with one

extra character for the null terminator):

char string_array[30][81];

For accessing an individual string, one simply specifies only

the left index:

firstString = string_array[0];

sixthString = string_array[5];47

The following example calls the gets ( ) function with the

third string in the array:

gets(string_array[2]);

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Arrays of Strings

Arrays of strings are commonly used for handling tables of

information.

One such application would be an employee database that

stores

the name

telephone number

hours worked per pay period, and

hourly wage.

These data we could store in arrays:

char name[20][80]; // employee names

int phone[20]; // phone numbers

float hours[20]; // hours worked

float wage[20]; // wage49

Programming Example_ Functions containing

arrays and strings

The full program is shown below

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Exercises

1. Write a C++ program that accepts 10 integers from the user

and finally displays the smallest value and the largest value.

2. Write a program that accepts ten different integers from the

user and display these numbers after sorting them in increasing

order.

3. Write a C++ program that calculates the letter grades of 20

students. The program should accept the mid result and the

final result from the students. Use the appropriate validity

control mechanism to prevent wrong inputs.

4. Write a C++ program that accepts a word from the user and

then displays the word after reversing it.

5. Develop a C++ program that accepts the name of a person and

then counts how many vowels the persons name have.

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End of course

Thank You!!!

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Lecture 5. Arrays and Strings

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