chapter 19 - class string and stream processing

2000 Deitel & Associates, Inc. All rights reserved. 2000 Deitel & Associates, Inc. All rights reserved.

Chapter 19 - Class string and Stream Processing

Outline19.1 Introduction19.2 string Assignment and Concatenation19.3 Comparing strings19.4 Substrings19.5 Swapping strings19.6 string Characteristics19.7 Finding Characters in a string19.8 Replacing Characters in a string19.9 Inserting Characters into a string19.10 Conversion to C-Style char * Strings19.11 Iterators19.12 String Stream Processing


19.1Introduction

• template class basic_string– typedef basic_string< char > string;

• creates alias string for basic_string< char > which we use

– include <string>

• Initialization– string s1( "Hi");– string s2( 8, 'x');– string name = "Jim";

• not truly assignment, calls copy constructor

WRONG:string s1 = 'c';string s2( 'u' );string s3 = 34;string s4( 8 );

• no automatic conversion between int or char to string


19.1Introduction (II)

• strings do not always end in '\0'• use the [] operator to access elements

– first subscript 0– string not a pointer. &string1[0] != string1

• String length - s1.size() or s1.length() • Inputting strings

string myString;

cin >> myString;• delimited by whitespace characters

string myString;

getline(cin, myString);• overloaded getline function

• delimited by newline ('\n')


19.2string Assignment and Concatenation

• String assignment– = operator: s2 = s1;– assign(): s2.assign(s1);

• copies s1 into s2– s2.assign(s1, startSubscript, numberOfChars);

• copies numberOfChars characters in s1 to s2, starting from startSubscript

– s2[0] = 'm'; • sets first character of s2 to 'm'


19.2string Assignment and Concatenation (II)

• More string operations– s2.at(2);

• range checked access

– s2 += "hi" • concatenates s2 and "hi"

– s2.append("_again"); • concatenates "_again" and s2

– s2.append(s1, startSubscript, numberOfChars);• concatenates numberOfChars characters in s1 with s2,

starting from startSubscript


Outline

1. Initialize strings

2. Modify strings

3. Print strings

1 // Fig. 19.1: fig19_01.cpp2 // Demonstrating string assignment and concatenation3 #include <iostream>45 using std::cout;6 using std::endl;78 #include <string>910 using std::string;1112 int main()13 {14 string s1( "cat" ), s2, s3;1516 s2 = s1; // assign s1 to s2 with =17 s3.assign( s1 ); // assign s1 to s3 with assign()18 cout << "s1: " << s1 << "\ns2: " << s2 << "\ns3: "19 << s3 << "\n\n";2021 // modify s2 and s3 22 s2[ 0 ] = s3[ 2 ] = 'r';2324 cout << "After modification of s2 and s3:\n"25 << "s1: " << s1 << "\ns2: " << s2 << "\ns3: "; 2627 // demonstrating member function at()28 int len = s3.length();29 for ( int x = 0; x < len; ++x ) 30 cout << s3.at( x );3132 // concatenation

s1: cat

s2: cat

s3: cat

After modification of s2 and s3:

s1: cat

s2: rat

s3: car


Outline

3. Print strings

Program Output

33 string s4( s1 + "apult" ), s5; // declare s4 and s5

34

35 // overloaded +=

36 s3 += "pet"; // create "carpet"

37 s1.append( "acomb" ); // create "catacomb"

38

39 // append subscript locations 4 thru the end of s1 to

40 // create the string "comb" (s5 was initially empty)

41 s5.append( s1, 4, s1.size() );

42

43 cout << "\n\nAfter concatenation:\n" << "s1: " << s1

44 << "\ns2: " << s2 << "\ns3: " << s3 << "\ns4: " << s4

45 << "\ns5: " << s5 << endl;

46

47 return 0;

48 }

s1: cats2: cats3: cat After modification of s2 and s3:s1: cats2: rats3: car After concatenation:s1: catacombs2: rats3: carpets4: catapults5: comb

After concatenation:

s1: catacomb

s2: rat

s3: carpet

s4: catapult

s5: comb


19.3Comparing strings

• overloaded ==, !=, <, >, <= and >= operators – for string comparisons – return bool values

• result = s1.compare(s2);– lexicographical comparison– if s1 > s2, result > 0– if s1 < s2, result < 0– if s1 == s2, result = 0

s1.compare(sub1, sub2, s2, sub3, sub4)compares s1 from subscript sub1 to sub2 with s2 from subscript sub3 to sub4.

s1.compare(sub1, number, s2)compares number characters in s1 starting from subscript sub1 with s2.


Outline


2. Compare strings

1 // Fig. 19.2: fig19_02.cpp2 // Demonstrating string comparison capabilities3 #include <iostream>45 using std::cout;6 using std::endl;78 #include <string>910 using std::string;1112 int main()13 {14 string s1( "Testing the comparison functions." ),15 s2("Hello" ), s3( "stinger" ), z1( s2 );16 17 cout << "s1: " << s1 << "\ns2: " << s218 << "\ns3: " << s3 << "\nz1: " << z1 << "\n\n";1920 // comparing s1 and z121 if ( s1 == z1 )22 cout << "s1 == z1\n";23 else { // s1 != z1 24 if ( s1 > z1 )25 cout << "s1 > z1\n";26 else // s1 < z127 cout << "s1 < z1\n";28 }2930 // comparing s1 and s231 int f = s1.compare( s2 );3233 if ( f == 0)

s1: Testing the comparison functions.

s2: Hello

s3: stinger

z1: Hello

s1 > z1


Outline

2. Compare strings

34 cout << "s1.compare( s2 ) == 0\n";

35 else if ( f > 0 )

36 cout << "s1.compare( s2 ) > 0\n";

37 else // f < 0

38 cout << "s1.compare( s2 ) < 0\n";

39

40 // comparing s1 (elements 2 - 5) and s3 (elements 0 - 5)

41 f = s1.compare( 2, 5, s3, 0, 5 );

42

43 if ( f == 0 )

44 cout << "s1.compare( 2, 5, s3, 0, 5 ) == 0\n";

45 else if ( f > 0 )

46 cout << "s1.compare( 2, 5, s3, 0, 5 ) > 0\n";

47 else // f < 0

48 cout << "s1.compare( 2, 5, s3, 0, 5 ) < 0\n";

49

50 // comparing s2 and z1

51 f = z1.compare( 0, s2.size(), s2 );

52

53 if ( f == 0 )

54 cout << "z1.compare( 0, s2.size(), s2 ) == 0" << endl;

55 else if ( f > 0 )

56 cout << "z1.compare( 0, s2.size(), s2 ) > 0" << endl;

57 else // f < 0

58 cout << "z1.compare( 0, s2.size(), s2 ) < 0" << endl;

59

60 return 0;

61 }

s1.compare( s2 ) > 0

s1.compare( 2, 5, s3, 0, 5 ) == 0

z1.compare( 0, s2.size(), s2 ) == 0


Outline

Program Output

s1: Testing the comparison functions.s2: Hellos3: stingerz1: Hello s1 > z1s1.compare( s2 ) > 0s1.compare( 2, 5, s3, 0, 5 ) == 0z1.compare( 0, s2.size(), s2 ) == 0


19.4Substrings

• Function substr retrieves a substring from a string.

s1.substr( startSubScript, number );

– startSubscript - starting subscript– number - number of characters to extract

– returns the substring


19.5Swapping strings

• Function swap swaps the contents of two strings.

s1.swap(s2);– switches s1 and s2


19.6string Characteristics

• s1.size() and s1.length()– current number of characters in string

• s1.capacity() – number of elements that can be stored without reallocation

• s1.max_size() – maximum possible string size

• str.empty() – if no characters, returns true

• s1.resize(newlength)– resizes string to newlength


Outline

1. Initialize string

2. Function calls

3. Output

1 // Fig. 19.5: fig19_05.cpp2 // Demonstrating functions related to size and capacity3 #include <iostream>45 using std::cout;6 using std::endl;7 using std::cin;89 #include <string>1011 using std::string;1213 void printStats( const string & );1415 int main()16 {17 string s;18 19 cout << "Stats before input:\n";20 printStats( s );2122 cout << "\n\nEnter a string: ";23 cin >> s; // delimited by whitespace24 cout << "The string entered was: " << s;2526 cout << "\nStats after input:\n";27 printStats( s );2829 s.resize( s.length() + 10 );30 cout << "\n\nStats after resizing by (length + 10):\n";31 printStats( s );3233 cout << endl;

Stats before input:

capacity: 0

max size: 4294967293

size: 0

length: 0

empty: true

Enter a string: tomato soup

The string entered was: tomato

Stats after input:

capacity: 31

max size: 4294967293

size: 6

length: 6

empty: false

Stats after resizing by (length + 10):

capacity: 31

max size: 4294967293

size: 16

length: 16

empty: false


Outline

3.1 Function definition

Program Output

34 return 0;

35 }

36

37 void printStats( const string &str )

38 {

39 cout << "capacity: " << str.capacity()

40 << "\nmax size: " << str.max_size()

41 << "\nsize: " << str.size()

42 << "\nlength: " << str.length()

43 << "\nempty: " << ( str.empty() ? "true": "false" );

44 }

Stats before input:capacity: 0max size: 4294967293size: 0length: 0empty: true Enter a string: tomato soupThe string entered was: tomatoStats after input:capacity: 31max size: 4294967293size: 6length: 6empty: false Stats after resizing by (length + 10):capacity: 31max size: 4294967293size: 16length: 16empty: false


19.7Finding Characters in a string

• Class string find functions: – if found, subscript of match returned

– if not found, value of string::npos is returned

– s1.find(string) - searches s1 for string– s1.rfind(string) - like find, but searches s1 for

string backwards.

– s1.find_first_of(string) - finds first occurrence in s1 of any character in string

– s1.find_last_of(string)- as above, but last occurrence. Searches s1 backwards.

– s1.find_first_not_of(string)- finds first character in s1 NOT in s2.

– s1.find_last_not_of(string) - as above, but last character. Searches s1 backwards.


Outline


2. Function calls

3. Output

1 // Fig. 19.6: fig19_06.cpp2 // Demonstrating the string find functions3 #include <iostream>45 using std::cout;6 using std::endl;78 #include <string>910 using std::string;1112 int main()13 {14 // compiler concatenates all parts into one string literal15 string s( "The values in any left subtree"16 "\nare less than the value in the"17 "\nparent node and the values in"18 "\nany right subtree are greater"19 "\nthan the value in the parent node" );20 21 // find "subtree" at locations 23 and 10222 cout << "Original string:\n" << s 23 << "\n\n(find) \"subtree\" was found at: " 24 << s.find( "subtree" ) 25 << "\n(rfind) \"subtree\" was found at: " 26 << s.rfind( "subtree" );2728 // find 'p' in parent at locations 62 and 14429 cout << "\n(find_first_of) character from \"qpxz\" at: " 30 << s.find_first_of( "qpxz" ) 31 << "\n(find_last_of) character from \"qpxz\" at: " 32 << s.find_last_of( "qpxz" );33

Original string:

The values in any left subtree

are less than the value in the

parent node and the values in

any right subtree are greater

than the value in the parent node

(find) "subtree" was found at: 23

(rfind) "subtree" was found at: 102

(find_first_of) character from "qpxz" at: 62

(find_last_of) character from "qpxz" at: 144


Outline

3. Output

Program Output

34 // find 'b' at location 25

35 cout << "\n(find_first_not_of) first character not\n"

36 << " contained in \"heTv lusinodrpayft\": "

37 << s.find_first_not_of( "heTv lusinodrpayft" );

38

39 // find '\n' at location 121

40 cout << "\n(find_last_not_of) first character not\n"

41 << " contained in \"heTv lusinodrpayft\": "

42 << s.find_last_not_of( "heTv lusinodrpayft" ) << endl;

43

44 return 0;

45 }

Original string:The values in any left subtreeare less than the value in theparent node and the values inany right subtree are greaterthan the value in the parent node (find) "subtree" was found at: 23(rfind) "subtree" was found at: 102(find_first_of) character from "qpxz" at: 62(find_last_of) character from "qpxz" at: 144(find_first_not_of) first character not contained in "heTv lusinodrpayft": 25(find_last_not_of) first character not contained in "heTv lusinodrpayft": 121

(find_first_not_of) first character not

contained in "heTv lusinodrpayft": 25

(find_last_not_of) first character not

contained in "heTv lusinodrpayft": 121


19.8Replacing Characters in a string

• s1.erase(number);– erases everything after element number to the end

• s1.replace(sub1, numberToReplace, replacementString, sub2, numberToUse);

– sub1 - subscript in s1 where replacement occurs– numberToReplace - number of characters being replaced in s1,

starting from sub1– replacementString - string containing replacement characters– sub2 - beginning subscript of replacement characters in

replacementString– numberToUse - number of replacement characters to use in

replacementString, starting with sub2


Outline


2. Function calls

3. Output

1 // Fig. 19.7: fig19_07.cpp2 // Demonstrating functions erase and replace3 #include <iostream>45 using std::cout;6 using std::endl;78 #include <string>910 using std::string;1112 int main()13 {14 // compiler concatenates all parts into one string15 string s( "The values in any left subtree"16 "\nare less than the value in the"17 "\nparent node and the values in"18 "\nany right subtree are greater"19 "\nthan the value in the parent node" );20 21 // remove all characters from location 62 22 // through the end of s23 s.erase( 62 );2425 // output the new string26 cout << "Original string after erase:\n" << s 27 << "\n\nAfter first replacement:\n";28 29 // replace all spaces with a period30 int x = s.find( " " );31 while ( x < string::npos ) {32 s.replace( x, 1, "." );33 x = s.find( " ", x + 1 );

Original string after erase:

The values in any left subtree

are less than the value in the

Remove all characters from location 62 (the 63rd element) to the end.

After first replacement:

The.values.in.any.left.subtree

are.less.than.the.value.in.the


Outline

3. Output

Program Output

34 }

35

36 cout << s << "\n\nAfter second replacement:\n";

37

38 // replace all periods with two semicolons

39 // NOTE: this will overwrite characters

40 x = s.find( "." );

41 while ( x < string::npos ) {

42 s.replace( x, 2, "xxxxx;;yyy", 5, 2 );

43 x = s.find( ".", x + 1 );

44 }

45

46 cout << s << endl;

47 return 0;

48 }

Original string after erase:The values in any left subtreeare less than the value in the After first replacement:The.values.in.any.left.subtreeare.less.than.the.value.in.the After second replacement:The;;alues;;n;;ny;;eft;;ubtreeare;;ess;;han;;he;;alue;;n;;he

After second replacement:

The;;alues;;n;;ny;;eft;;ubtree

are;;ess;;han;;he;;alue;;n;;he


19.9Inserting Characters into a string

• s1.insert(sub1, string)– inserts string before element sub1 in s1

• s1.insert(sub1, string, sub2, numChars);– selects numChars characters, starting from sub2 in string, and

inserts them into s1.


19.10 Conversion to C-Style char * Strings

• strings can be converted to C-style char * strings– s1.data() - conversion to const char * string, not

null terminated– s1.c_str() - conversion to const char * string, null

terminated (ends with '\0')


Outline


2. Insert strings

3. Print results.

1 // Fig. 19.9: fig19_09.cpp2 // Converting to C-style strings.3 #include <iostream>45 using std::cout;6 using std::endl;78 #include <string>910 using std::string;1112 int main()13 {14 string s( "STRINGS" );15 const char *ptr1 = 0;16 int len = s.length();17 char *ptr2 = new char[ len + 1 ]; // including null18 19 // copy characters out of string into allocated memory20 s.copy( ptr2, len, 0 );21 ptr2[ len ] = 0; // add null terminator2223 // output 24 cout << "string s is " << s 25 << "\ns converted to a C-Style string is "26 << s.c_str() << "\nptr1 is ";2728 // Assign to pointer ptr1 the const char * returned by29 // function data(). NOTE: this is a potentially dangerous30 // assignment. If the string is modified, the pointer31 // ptr1 can become invalid.32 ptr1 = s.data(); 33

Notice the format for inserting strings.

string s is STRINGS

s converted to a C-Style string is STRINGS

ptr1 is STRINGS


Outline

3. Print results.

Program Output

34 for ( int k = 0; k < len; ++k )

35 cout << *( ptr1 + k ); // use pointer arithmetic

36

37 cout << "\nptr2 is " << ptr2 << endl;

38 delete [] ptr2;

39 return 0;

40 }

string s is STRINGSs converted to a C-Style string is STRINGSptr1 is STRINGSptr2 is STRINGS

ptr2 is STRINGS


19.11 Iterators

• iterators– forwards and backwards traversal of strings

– access to individual characters

– similar to pointer operations

• Basic coverage here - more next chapter– type const_iterator - iterator that cannot modify the string– s.begin() - returns iterator to front of string– s.end() - returns iterator to end– *myIterator - dereferences iterator (like pointer)– ++myIterator - advances iterator one position (character)


19.12 String Stream Processing

• in-memory I/O (string stream processing)– headers <sstream> and <iostream>

• Inputting strings to memory– create objects of class istringstream

istringstream myInputString;– works the same as input from a file

– can feed input string into other strings

• Output to a string– objects of class ostringstream

ostringstream myOutputString;

- myOutputString.str() - outputs the string


Outline

1. Initialize ostringstream object

1.1 Initialize strings

1.2 Initialize variables

2. Print outputString

2.1 Modify outputString

1 // Fig. 19.11: fig19_11.cpp

2 // Using a dynamically allocated ostringstream object.

3 #include <iostream>

4

5 using std::cout;

6 using std::endl;

7

8 #include <string>

9

10 using std::string;

11

12 #include <sstream>

13

14 using std::ostringstream;

15

16 int main()

17 {

18 ostringstream outputString;

19 string s1( "Output of several data types " ),

20 s2( "to an ostringstream object:" ),

21 s3( "\n double: " ),

22 s4( "\n int: " ),

23 s5( "\naddress of int: " );

24 double d = 123.4567;

25 int i = 22;

26

27 outputString << s1 << s2 << s3 << d << s4 << i << s5 << &i;

28 cout << "outputString contains:\n" << outputString.str();

29

30 outputString << "\nmore characters added";

output strings to outputString

outputString contains:

Output of several data types to an ostringstream object:

double: 123.457

int: 22

address of int: 0068FD0C

Add more characters to outputString


Outline

2.2 Print outputString

Program Output

31 cout << "\n\nafter additional stream insertions,\n"

32 << "outputString contains:\n" << outputString.str()

33 << endl;

34

35 return 0;

36 }

outputString contains:Output of several data types to an ostringstream object: double: 123.457 int: 22address of int: 0068FD0C after additional stream insertions,outputString contains:Output of several data types to an ostringstream object: double: 123.457 int: 22address of int: 0068FD0Cmore characters added

after additional stream insertions,

outputString contains:

Output of several data types to an ostringstream object:

double: 123.457

int: 22

address of int: 0068FD0C

more characters added


Outline

1. Initialize objects

2. Input data into string1 and string2

1 // Fig. 19.12: fig19_12.cpp2 // Demonstrating input from an istringstream object.3 #include <iostream>45 using std::cout;6 using std::endl;78 #include <string>910 using std::string;1112 #include <sstream>1314 using std::istringstream;1516 int main()17 {18 string input( "Input test 123 4.7 A" );19 istringstream inputString( input );20 string string1, string2;21 int i;22 double d;23 char c;2425 inputString >> string1 >> string2 >> i >> d >> c;2627 cout << "The following items were extracted\n"28 << "from the istringstream object:"29 << "\nstring: " << string1 30 << "\nstring: " << string2 31 << "\n int: " << i 32 << "\ndouble: " << d 33 << "\n char: " << c;

Notice how inputString inputs data to string1 and string2 (similar format as cin)

The following items were extracted

from the istringstream object:

String: Input

String: test

int: 123

double: 4.7

char: A


Outline

2. Function calls

Program Output

34

35 // attempt to read from empty stream

36 long x;

37

38 inputString >> x;

39

40 if ( inputString.good() )

41 cout << "\n\nlong value is: " << x << endl;

42 else

43 cout << "\n\ninputString is empty" << endl;

44

45 return 0;

46 }

The following items were extractedfrom the istringstream object:String: InputString: test int: 123double: 4.7 char: AinputString is empty

inputString is empty

chapter 19 - class string and stream processing

Documents