05 strings

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More on C++ strings

CSIS1117 Computer Programming

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c1117 lecture 5 2

Contents Comparing C++

strings

Simple data type char Member functions provided by C++ strings

Build-in function getline(), getchar()

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c1117 lecture 5 3

C++ String Recall that: C++ has a

string

library that providesmore convenient ways to handle texts.

To use the string, we need to include the header:#include

We can declarestring variables to store string values

We can assign astring constant

, or the value ofanother

string

to astring

.

We can concatenate two strings by the operator "+"

string str1 = "I loves";

string str2 = HKU";

cout "

I loves HKU :->

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c1117 lecture 5 4

C++ strings can also be comparedlexicographically (dictionary order) by relationaloperators. See name.cc & monthdays.cc

string str1 = "Apple"; string str2 = "apple";

string str3 = "apples"; string str4 = "orange";bool t1 = (str1 == str2);

bool t2 = (str1 < str2);


bool t4 = (str3 != str4);bool t5 = (str4 > str3);

string str1 = "Apple"; string str2 = "apple";

string str3 = "apples"; string str4 = "orange";bool t1 = (str1 == str2);



bool t4 = (str3 != str4);bool t5 = (str4 > str3);

t1 : false

t2 : truet3 : truet4 : truet5 : true

t1 : false

t2 : truet3 : truet4 : truet5 : true

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c1117 lecture 5 5

Never compare two string literals directly!

At least one of the operands must be a C++str n

.

See string-cmp.cc

if("zebra" < "ant")

cout

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c1117 lecture 5 6

In fact, astring

is composed of a sequence ofcharacters.

Characters in a string can be specified by theirpositions. For a str n with length x, the positions of its

characters are ranging from 0 to x-1

Given astring variable

, we can access anindividual character of it using the subscriptoperator[] (square bracket).

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c1117 lecture 5 7

Error message may begiven if the program

accesses a character outside the range of a string.

cout

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c1117 lecture 5 8

Simple data type (char

)

myMsg is of type string but myMsg[4] is just acharacter!

A character is a single letter, digit, punctuation, orsymbol (mostly) can be seen on the keyboard.

string myMsg = "How are you doing?";

myMsg[4] = "d";


myMsg[4] = "d";It is wrong, you will get a type mismatch error.

Can we change the characters in a string?

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c1117 lecture 5 9


) In C++ language, we use data type

char

torepresent characters.

char : representing value of one character

Character constants are enclosed by a pair of singlequotes, e.g. 'A', 'a', '+', '!', '9',

Non-printable characters, e.g. '\n', '\t',

Uppercase and lowercase of letters are considered to bedifferent.

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c1117 lecture 5 10


)

int Ival = 100; // any integers

bool Bval = true; // or falsestring Sval = "100"; // use double quotes

char Cval = '1'; // use single quotes

int Ival = 100; // any integers

bool Bval = true; // or falsestring Sval = "100"; // use double quotes

char Cval = '1'; // use single quotes

Constant value 4

How to represent the following in program?4

Constant char 4 '4'

Constant string 4 "4"

Print a new line cont

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c1117 lecture 5 11


) Input/output operations are similar toint and

double

.

It can be compared using relational operators.

cout choice;

cout

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c1117 lecture 5 12

We can change the characters in a string as follow:


myMsg[0] = 'h';

cout

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c1117 lecture 5 13

Member functions of string The

string

library is very flexible for handingoperations related to string/text manipulation.

e.g. finding the length of a string, comparing two strings,searching/extracting of sub-string, concatenation,

Each string variable is associated with a number

of member functions, e.g.s.length ), s.substr

)

Member functions are applied to individual variableusing dot operator.

Unlike the predefined functions provided in library, e.g.sqrt ), pow )

, member functions cannot be invokedwithout a variable.

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c1117 lecture 5 14

Member functions of strings.length ) return the no. of characters in the

string

string s = "Hello World!";

unsigned int len_s = s.length();

cout

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c1117 lecture 5 15

unsigned int There are two types of int: signed and unsigned.

If neither is specified theint

issigned

.signed int

: ranging from -231 to 231-1 inclusively (inmost 32-bit CPU).

unsigned int

: only consider non-negative integers,ranging from 0 to 232-1 inclusively.

As the length of a string must be a non-negativenumber, we should use an

unsigned int

torepresent it.

unsigned int len_s = s.length();unsigned int len_s = s.length();

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c1117 lecture 5 16

unsigned int An overflow occurs when the result of an

arithmetic operation exceeds the representablerange.

In C++, no warning or error message will be given forinteger overflow, but incorrect result may obtained.

unsigned int x = pow(2,31) + 2;

int y = -1;

if(x > y)

cout

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c1117 lecture 5 17

What is the shortest length of a string?

An empty string is a string containing zero no. of

character. e.g. string s = ""

How about string r = " " ? Is it still an empty string?

See string-ex.cc as an example

No, r is a string containing one character the space

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c1117 lecture 5 18

Member functions of string A substring is part of a

string

. Substrings can beextracted using the member function

s.substr

)

.

s.substr(x, y) extracting a substring with length

y

starting at positionx

.

If the second argument y is omitted, a substring up to

the end of thestring

is extracted.string s (attending lecture");

string r = s.substr(0,6);

string t = s.substr(11);

string u = s.substr(11,99);

cout

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c1117 lecture 5 19

Member functions of string The member functions.find ) can be used to

look for the occurrence of one string in another. s.find(r) checking if string r occurs in the string s.

The starting position of the first occurrence is reported.

If there is no such occurrence, the constant

string::npos

defined in is returned.

string r = "programming";

int a = r.find("gram");

cout

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c1117 lecture 5 20

Reading a line from input We use cin >> v1 >> v2 >> >> vn to get data

from input.

cin makes use of whitespace characters as inter-data separator. So

cin

will skip all the whitespacesyou typed. See cin-input.cc as an example.

As a result, we cannot read a string involvingspace character into a

string variable

.

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c1117 lecture 5 21

Instead, we can use the library functiongetline() to read a line from the current

position till the end of the current line.

See read-a-line.cc for an example.

string s;

getline(cin, s);

cout

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c1117 lecture 5 22

Reading characters from input Similarly, if we want to read characters one by one

from input, including the whitespace characters,we can use the library function

getchar )

.

char a, b, c;

cout

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