Chapter 19 - Class string and Stream Processing

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Chapter 19 - Class string and Stream Processing

Outline

19.1 Introduction

19.2 string Assignment and Concatenation 19.3 Comparing strings

19.4 Substrings

19.5 Swapping strings

19.6 string Characteristics

19.7 Finding Characters in a string 19.8 Replacing Characters in a string 19.9 Inserting Characters into a string 19.10 Conversion to C-Style char * Strings 19.11 Iterators

19.12 String Stream Processing

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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

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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')

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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'

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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

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Outline

1. Initialize strings 2. Modify strings 3. Print strings

1 // Fig. 19.1: fig19_01.cpp

2 // Demonstrating string assignment and concatenation 3 #include <iostream>

4

5 using std::cout;

6 using std::endl;

7

8 #include <string>

9

10 using std::string;

11

12 int main() 13 {

14 string s1( "cat" ), s2, s3;

15

16 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";

20

21 // modify s2 and s3 22 s2[ 0 ] = s3[ 2 ] = 'r';

23

24 cout << "After modification of s2 and s3:\n"

25 << "s1: " << s1 << "\ns2: " << s2 << "\ns3: ";

26

27 // demonstrating member function at() 28 int len = s3.length();

29 for ( int x = 0; x < len; ++x ) 30 cout << s3.at( x );

31

32 // concatenation

s1: cat s2: cat s3: cat

After modification of s2 and s3:

s1: cat s2: rat s3: car

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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: cat s2: cat s3: cat

After modification of s2 and s3:

s1: cat s2: rat s3: car

After concatenation:

s1: catacomb s2: rat

s3: carpet s4: catapult s5: comb

After concatenation:

s1: catacomb s2: rat

s3: carpet s4: catapult s5: comb

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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.

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Outline

1. Initialize strings 2. Compare strings

1 // Fig. 19.2: fig19_02.cpp

2 // Demonstrating string comparison capabilities 3 #include <iostream>

4

7

9

11

12 int main() 13 {

14 string s1( "Testing the comparison functions." ), 15 s2("Hello" ), s3( "stinger" ), z1( s2 );

16

17 cout << "s1: " << s1 << "\ns2: " << s2

18 << "\ns3: " << s3 << "\nz1: " << z1 << "\n\n";

19

20 // comparing s1 and z1 21 if ( s1 == z1 )

22 cout << "s1 == z1\n";

23 else { // s1 != z1 24 if ( s1 > z1 )

25 cout << "s1 > z1\n";

26 else // s1 < z1

27 cout << "s1 < z1\n";

28 } 29

30 // comparing s1 and s2 31 int f = s1.compare( s2 );

32

33 if ( f == 0)

s1: Testing the comparison functions.

s2: Hello s3: stinger z1: Hello

s1 > z1

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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

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Outline

Program Output

s1: Testing the comparison functions.

s2: Hello s3: stinger z1: Hello s1 > z1

s1.compare( s2 ) > 0

s1.compare( 2, 5, s3, 0, 5 ) == 0 z1.compare( 0, s2.size(), s2 ) == 0

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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

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19.5Swapping strings

• Function swap swaps the contents of two strings.

s1.swap(s2);

– switches s1 and s2

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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

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Outline

1. Initialize string 2. Function calls 3. Output

1 // Fig. 19.5: fig19_05.cpp

2 // Demonstrating functions related to size and capacity 3 #include <iostream>

4

7 using std::cin;

8

10

12

13 void printStats( const string & );

14

15 int main() 16 {

17 string s;

18

19 cout << "Stats before input:\n";

20 printStats( s );

21

22 cout << "\n\nEnter a string: ";

23 cin >> s; // delimited by whitespace 24 cout << "The string entered was: " << s;

25

26 cout << "\nStats after input:\n";

28

29 s.resize( s.length() + 10 );

30 cout << "\n\nStats after resizing by (length + 10):\n";

32

33 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

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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: 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

Stats after resizing by (length + 10):

capacity: 31

max size: 4294967293 size: 16

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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.

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Outline

1 // Fig. 19.6: fig19_06.cpp

2 // Demonstrating the string find functions 3 #include <iostream>

4

7

9

11

12 int main() 13 {

14 // compiler concatenates all parts into one string literal 15 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 102 22 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" );

27

28 // find 'p' in parent at locations 62 and 144

29 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

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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 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 (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

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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

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Outline

1 // Fig. 19.7: fig19_07.cpp

2 // Demonstrating functions erase and replace 3 #include <iostream>

4

7

9

11

12 int main() 13 {

14 // compiler concatenates all parts into one string 15 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 s

23 s.erase( 62 );

24

25 // output the new string

26 cout << "Original string after erase:\n" << s 27 << "\n\nAfter first replacement:\n";

28

29 // replace all spaces with a period 30 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

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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 subtree are less than the value in the After first replacement:

The.values.in.any.left.subtree are.less.than.the.value.in.the After second replacement:

The;;alues;;n;;ny;;eft;;ubtree are;;ess;;han;;he;;alue;;n;;he

After second replacement:

The;;alues;;n;;ny;;eft;;ubtree are;;ess;;han;;he;;alue;;n;;he

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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.

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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')

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Outline

1. Initialize strings 2. Insert strings 3. Print results.

1 // Fig. 19.9: fig19_09.cpp

2 // Converting to C-style strings.

3 #include <iostream>

4

7

9

11

12 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 null 18

19 // copy characters out of string into allocated memory 20 s.copy( ptr2, len, 0 );

21 ptr2[ len ] = 0; // add null terminator 22

23 // output

24 cout << "string s is " << s

25 << "\ns converted to a C-Style string is "

26 << s.c_str() << "\nptr1 is ";

27

28 // Assign to pointer ptr1 the const char * returned by 29 // function data(). NOTE: this is a potentially dangerous 30 // assignment. If the string is modified, the pointer 31 // 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

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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 STRINGS

s converted to a C-Style string is STRINGS ptr1 is STRINGS

ptr2 is STRINGS

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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)

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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

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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.

4

7

9

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

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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: 22

address of int: 0068FD0C

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

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

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Outline

1. Initialize objects

2. Input data into string1 and

string2

1 // Fig. 19.12: fig19_12.cpp

2 // Demonstrating input from an istringstream object.

4

7

9

11

12 #include <sstream>

13

14 using std::istringstream;

15

16 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;

24

25 inputString >> string1 >> string2 >> i >> d >> c;

26

27 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

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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 extracted from the istringstream object:

String: Input String: test int: 123 double: 4.7 char: A

inputString is empty