CS 192 Lecture 11 Winter 2003 December 29-30, 2003 Dr. Shafay Shamail.

CS 192

Lecture 11

Winter 2003

December 29-30, 2003

Dr. Shafay Shamail

Strings• A collection of items of same type in contiguous memory

• One-dimensional array of type char e.g. ”hello”• A string is in double quotes and is called literal string constant

• A string is terminated by the null character: ‘\0’ (a zero)

• Compiler automatically appends it at the end of string; signifies end of string for the compiler

• char str[6] = “hello”;• str[5] is what?

• char a1[]=“abc”;• char a2[]={‘a’, ‘b’, ‘c’};• char a3[]={‘a’, ‘b’, ‘c’, ‘\0’};• cout<<a1; cout<<a2; cout<<a3;• Is a2[] a string?

• char b[4]=“hi”; b[3] set to zero (null) as is b[2]

Reading Strings from Keyboard• The easiest way to read a string entered from

the keyboard is to make a character array

int main()

{

char str[80];

cout << "Enter a string: ";

cin >> str; // read string from keyboard

cout << "Here is your string: ";

cout << str<<"\n";

return 0;

}

Reading Strings from Keyboard• There is a problem with previous program, if the string has

whitespace characters (space, tab, newline)

• Use gets() to read a string from the keyboard. #include <stdio.h>

int main()

{

char str[80];


cout.flush();

gets(str); // read a string from the keyboard

cout << "Here is your string: ";

cout << str<<“\n”;

return 0;

}

/*

The gets function reads a line,

which consists of all characters up to and including the first newline character ('\n').

gets then replaces the newline character with a null character ('\0') before returning the line.

*/

gets() Library Function

String Library FunctionsUseful for string manipulation

#include <string.h>

l=strlen(s) Returns string length excluding NULL

strcat(s1,s2) Concatenates s2 to s1

strcpy(s1,s2) Copies s2 to s1

strncpy(s1,s2,n) Copies n characters from s2 to s1

strcmp(s1,s2) Returns 0 if equal, -ve if s1<s2, +ve if s1>s2

strstr(s1,s2) Searches first occurrence of s2 in s1, if found returns pointer otherwise returns NULL

String Library Functions

==========================

strcpy(s1, s2)

Copies until \0 is moved. Contents of s1 overwritten

s1 should have enough space. The value of s1 is returned

==========================int main()

{ char str[80];

strcpy(str, "hello");

cout << str;

return 0;

} // Output is: hello


==========================

strcat(s1, s2)

Appends s2 to the end of s1 and returns s1. s2 is unchanged

==========================

int main()

{ char s1[20], s2[10];

strcpy(s1, "hello");

strcpy(s2, " there");

strcat(s1, s2);

cout << s1<<“\n”;

return 0;

} //output?

Output is:hello there

String Library Functions==========================

strmp(s1, s2)

==========================

• compares two strings lexicographically• returns 0 if they are equal (dictionary order)• returns a +ve number if s1 is greater than s2• returns a -ve number if s1 is less than s2

strcmp(“Zoo”, “ape”);

ASCII code has uppercase before lowercase

strcmp(“Zoo”, “zoo”); // Output ?

strcmp(s1, s2) …

bool password();

int main(){

if ( password() ) cout << "Logged on.\n";

else cout << "Access denied.\n"; return 0;

}

// Return true if password accepted; false otherwise.bool password()

{

char s[80];

cout << "Enter password: ";

gets(s);

if ( strcmp(s, "password") ) // strings differ

{

cout << "Invalid password.\n";

return false;

}

// strings compared the same

return true;

}

strcmp(s1, s2) …

int main()

{

char s[80];

for(;;)

{


cout.flush();

gets(s);

if( !strcmp("quit", s) )

break;

}

return 0;

}

strcmp(s1, s2) …

int main()

{

char s[80];

do

{ cout << "Enter a string:"; gets(s); } while(strcmp("quit",s));

//other than 0 is true

return 0;

}


==========================strlen(s)

returns the length of s

==========================int main()

{

char str[80];


gets(str);

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

return 0;

}

//what is the output if we enter: this is cs192

Write your own

• strLength(s)

• strCompare(s1, s2)

• strCopy(s1, s2);

• strConcatenate(s1, s2)

String/Numeric Conversion n=atoi(s) Converts s to integer

ln=atol(s) Converts s to a long integer

fn=atof(s) Converts s to a float

itoa(v,s,b) Converts v integer of base b to a string s

To Upper

char mystring[80] = "i am a string";int i=0;

while ( mystring[i] ){ // mystring[i] - ’a’ + ’A’

char upper = mystring[i] - (97 - 65);cout << upper;i++;

}

#include <string.h> void main(){ char str[80]; int i;

strcpy(str, "this is a test");

for(i=0; str[i]; i++)str[i] = toupper(str[i]);

cout << str << endl;

}

Using Null Terminator

• Test condition of the for loop is simply the array indexed by i

• Loop runs until it encounters the null terminator.

==================================

toupper() tolower() isalpha()

isdigit() isspace() ispunct()

Using Null Terminator

• The following declares an array of 30 strings• Each having a maximum length of 80 characters

char str_array[30][80];

• In order to access an individual string, specify only the left index.

Arrays of Strings

int main()

{

char city[80] = "Islamabad";

char places[4][80] = {"Lahore", "Hanoi", "Karachi", "Madrid"};

cout << places[3] << '\n';

cout << places[2][4] << '\n';

cout << places[1][6] << '\n';

cout << city << '\n';

cout << city[4] << '\n';

return 0;

}

Example

Madrid

c

?

Islamabad

m

Example

int main() {

int t, i; char text[100][80];

for(t=0; t<100; t++) {

cout << t << ": ";cout.flush();

gets ( text[t] ); if ( !text[t][0] )

break; // quit on blank line } // redisplay the strings for(i=0; i<t; i++) cout << text[i] << '\n'; return 0; }

Arrays of Strings

const int EMPLOYEES = 10; // this array holds employee names

char name[EMPLOYEES][80]; // their phone numbers

char phone[EMPLOYEES][20]; // hours worked per week

float hours[EMPLOYEES]; // wage

float wage[EMPLOYEES];

Example using String Arrays

void enter() // Enter information.{ int i; for(i=0; i<EMPLOYEES; i++) {

cout << "Enter last name: "; cin >> name[i]; cout << "Enter phone number: "; cin >> phone[i]; cout << "Enter number of hours worked: "; cin >> hours[i]; cout << "Enter wage: "; cin >> wage[i];

}}


// Display report.

void report()

{

int i;

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

{

cout << name[i] << ' ' << phone[i] << '\n';

cout << "Pay for the week: " << wage[i] * hours[i];

cout << '\n';

}

} //employee example


Reference

Dietel & Dietel

Fundamentals of Strings

Series of characters treated as one unit• Can include letters, digits, special characters +, -, * ...• String literal (string constants)

– Enclosed in double quotes, for example:"I like C++"

• Array of characters, ends with null character '\0'• Strings are constant pointers (like arrays)

– Value of string is the address of its first character


• String assignment– Character array:

char color[] = "blue";

• Creates 5 element char array, color, (last element is '\0')

– variable of type char *char *colorPtr = "blue";

• Creates a pointer to string “blue”, colorPtr, and stores it somewhere in memory

Fundamentals of Strings• Reading strings

– Assign input to character array word[ 20 ]cin >> word

• Reads characters until whitespace or EOF• String could exceed array size

cin >> setw( 20 ) >> word;

• Reads 19 characters (space reserved for '\0')

• cin.getline– Reads a line of text– Using cin.getline

cin.getline( array, size, delimiter character);


• cin.getline– Copies input into specified array until either

• One less than the size is reached

• The delimiter character is input

– Examplechar sentence[ 80 ];

cin.getline( sentence, 80, '\n' );

String Manipulation Functions of the String-handling Library

• String handling library <cstring> provides functions to– Manipulate strings

– Compare strings

– Search strings

– Tokenize strings (separate them into logical pieces)

• ASCII character code– Strings are compared using their character codes

– Easy to make comparisons (greater than, less than, equal to)

• Tokenizing – Breaking strings into tokens, separated by user-specified characters

– Tokens are usually logical units, such as words (separated by spaces)– "This is my string" has 4 word tokens (separated by spaces)


char *strcpy

( char *s1, const char *s2 );

Copies the string s2 into the character

array s1. The value of s1 is returned.

char *strncpy

( char *s1, const char *s2, size_t n );

Copies at most n characters of the string s2 into the character array s1. The value of s1 is returned.

char *strcat


Appends the string s2 to the string s1. The first character of s2 overwrites the terminating null character of s1. The value of s1 is returned.

char *strncat

( char *s1, const char *s2, size_t n );

Appends at most n characters of string s2 to string s1. The first character of s2 overwrites the terminating null character of s1. The value of s1 is returned.

int strcmp

( const char *s1, const char *s2 );

Compares the string s1 with the string s2. The function returns a value of zero, less than zero or greater than zero if s1 is equal to, less than or greater than s2, respectively.


int strncmp

( const char *s1, const char *s2, size_t n );

Compares up to n characters of the string s1 with the string s2. The function returns zero, less than zero or greater than zero if s1 is equal to, less than or greater than s2, respectively.

char *strtok


A sequence of calls to strtok breaks string s1 into “tokens”—logical pieces such as words in a line of text—delimited by characters contained in string s2. The first call contains s1 as the first argument, and subsequent calls to continue tokenizing the same string contain NULL as the first argument. A pointer to the current to ken is returned by each call. If there are no more tokens when the function is called, NULL is returned.

size_t strlen

( const char *s );

Determines the length of string s. The number of characters preceding the terminating null character is returned.

CS 192 Lecture 11 Winter 2003 December 29-30, 2003 Dr. Shafay Shamail.

Documents

Transcript of CS 192 Lecture 11 Winter 2003 December 29-30, 2003 Dr. Shafay Shamail.