chapter 5. arrays and strings

Chapter 5. ARRAY AND STRING


Minh Quang NGUYEN

Hanoi National University of Education

September 2015

Paris Saclay Minh Quang NGUYEN 1


ARRAYs

STRING

Two-Dimensional Arrays

Initialization

You do it

Q & A



ARRAYs

What is array?

I An array is a collection of variables of the same type that arereferred to by a common name.

I Every member of an array is allocated together in a memorylocation.



ARRAYs

What is array?





ARRAYs

One-Dimensional Arrays

A one-dimensional array is a list of related variables. The generalform of a one-dimensional array declaration is:

type variable_name[size];

wheretype: base type of the array, determines the data type of eachelement in the arrayvariable name: the name of the arraysize: how many elements the array will hold



ARRAYs

Example

Declaration withoutinitialization

int sample[10];

float float_numbers[100];

char last_name[40];

Declaration with initialization

int scores[7] = {1, 3, 4, 5,

1, 3, 2};

char alphabet[3] = {’A’, ’B’,

’E’};



ARRAYs

Accessing Array Elements

I An individual element within an array is accessed by use of anindex. An index describes the position of an element within anarray.

I This consists of the name of the array, followed by bracketsdelimiting a integer or a variable i.

Example

score[1] = 9;

for(int i = 0; i < 7; i++)

cout << score[i] << endl;

Note: In C++ the first element has the index zero!



ARRAYs




Example

score[1] = 9;

for(int i = 0; i < 7; i++)





ARRAYs

No Array-to-Array Assignments

You cannot assign one array to another in C++.

Example

int a[10], b[10];

// do something

// assign all elements

// of array b to array a

a = b;

Instead, you have to do theassignments for each element

int i;



for(i=0; i<10; i++)

a[i] = b[i];

Or copy all of the memory bytes

memcpy();



ARRAYs

No Array-to-Array Assignments

You cannot assign one array to another in C++.

Example

int a[10], b[10];

// do something



a = b;

Instead, you have to do theassignments for each element

int i;



for(i=0; i<10; i++)

a[i] = b[i];

Or copy all of the memory bytes

memcpy();



ARRAYs

No Bounds Checking

C++ performs no bounds checking on arrays. Nothing will stopyou from overrunning the end of an array.

I You will assign values to some other variables’ data!

I You might even write into a piece of the program code!

Example

int crash[10], i;

for(i = 0; i < 11; i++)

crash[i] = i;



STRING

What is string?

The most common use for one-dimensional arrays is to storestrings of characters.In C++, a string is defined as a character array terminated by anull symbol.



STRING

Declaration

I To declare an array str that could hold a 10-character string,one would write:char str[11];

I Specifying the size as 10 makes room for the null at the endof the string.



STRING

Declaration





STRING

Reading a String from the Keyboard

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

cout << "Enter a string: ";

cin >> str;

cout << "Your string:";

cout << str;



STRING

Some C++ Library Functions for Strings

C++ supports a range of string-manipulation functions

I strcpy(): copy characters from one string to another

I strcat(): concatenation of strings

I strlen(): length of a string

I strcmp(): comparison of strings



STRING









STRING

Example: strcpy() and strlen()

Example 1: strcpy()

char a[10];

strcpy(a, "hello");

Example 2: strlen()

cout << "Enter a string: ";

gets(str);

cout << "Length is: " << strlen(str);

Figure: Result of example 1



STRING

Example: strcat()

Example 3: strcpy()

char s1[21], s2[11];

strcpy(s1, "hello");

strcpy(s2, " there");

strcat(s1, s2);

The first string array has to belarge enough to hold bothstrings:



STRING

About strcmp()

The strcmp(str1, str2)

function compares two stringsand returns the following result:

I str1 == str2: 0

I str1 > str2 : positivenumber

I str1 < str2 : negativenumber

The strings are comparedlexicographically (i.e., accordingto dictionary order):

a < aa < aaa < . . . < b < ba < bb < . . . < bz < baa < . . . < abca < abd < ...

Example 4

cout << "Enter password: ";

gets(str);

if(strcmp(str, "password"))

{

cout << "Invalid password.\n";

}

else

cout << "Logged on.\n";




Two-Dimensional Arrays: declaration

I A two-dimensional array isa list of one-dimensionalarrays.

I To declare atwo-dimensional integerarray two dim of size 10,20we would write:int matrix[3][4];

I This corresponds to a tablewith 3 rows and 4 columns




Memory Allocation for Two-Dimensional Arrays


I Storage for all array elements is determined at compile time.

I The memory used to hold an array is required the entire timethat the array is in existence.

I The following formula determines the number of bytes ofmemory that will be allocated:bytes = rows × columns × number of bytes in type

I For example, an integer array (with two-byte integers) withdimensions 100,100 would require 100 * 100 * 2 = 20,000bytes.




Multidimensional Arrays


I C++ allows arrays with more than two dimensions. Thegeneral form of an N-dimensional array declaration is:type array name [size 1] [size 2] ... [size N];

I For example, the following declaration creates a 4 characterarray, or a matrix of strings:char string matrix[4][10][20];



Initialization

Character Array InitializationC++ supports a range of string-manipulation functions

I Character arrays that will hold strings allow a shorthandinitialization that takes this form:char array-name[size] = "string";

I For example, the following code fragment initializes str to thephrase ”hello”:char str[6] = "hello";

I This is the same as writing:char str[6] = ’h’, ’e’, ’l’, ’l’, ’o’, ’

0’;

Note: Remember that one has to make sure to make the arraylong enough to include the null terminator.



Initialization





0’;




Initialization

Multi-Dimensional Array Initialization

I Multi-dimensional arrays are initialized the same way asonedimensional arrays

I For example, the following code fragment initializes an arraysquares with the numbers 1 through 10 and their squares:int squares[10][2] = {{1, 1}, {2, 4}, {3, 9}, {4, 16},{5, 25}, {6, 36}, {7, 49}, {8, 64}, {9, 81}, {10,100}};



Initialization






Initialization

Unsized Array Initializations

I It is possible to let C++ automatically dimension the arraysthrough the use of unsized arrays. C++ will automaticallycreate arrays large enough to hold all the initializers present.char error1[] = "Divide by 0";

char error2[] = "End-of-File";

char error3[] = "Access Denied";

I For a multi-dimensional array, all but the leftmost dimensionhave to be specified. So we can write:char errors[][20] = {"Divide by 0", "End-of-File",

"Access Denied" };



Initialization






"Access Denied" };



Initialization

Arrays of Strings

I An array of strings is a special form of a two-dimensionalarray.

I The size of the left index determines the number of strings.I The size of the right index specifies the maximum length of

each string.

For example, the following declares an array of 30 strings, eachhaving a maximum length of 80 characters (with one extracharacter for the null terminator):char string array[30][81];



You do it



Q & A

QUESTION and ANSWER


chapter 5. arrays and strings

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