C-strings and

C++ string Class

Topics

• C-StringsC-Strings

• Library Functions for Working with C-StringsLibrary Functions for Working with C-Strings

• Conversions Between Numbers and StringsConversions Between Numbers and Strings

• Character TestingCharacter Testing

• Character Case ConversionCharacter Case Conversion

• Writing Your Own C-String Handling FunctionsWriting Your Own C-String Handling Functions

• C++ C++ stringstring Class Class

Strings• C-stringC-string: array of characters terminated by : array of characters terminated by NULLNULL character character

• The C-string The C-string

"Hi there!""Hi there!"

would be stored in memory as shown: would be stored in memory as shown:

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H i t h e r e ! \0

Array of char

• Array of char can be defined and initialized Array of char can be defined and initialized to a C-stringto a C-string

char str1[20] = "hi there!";char str1[20] = "hi there!";• Array of char can be defined and later have Array of char can be defined and later have

a string copied into ita string copied into it

char str2[20];char str2[20];

strcpy(str2, "hi there!");strcpy(str2, "hi there!");

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Pointer to char• Pointer to Pointer to charchar can refer to C-strings defined can refer to C-strings defined

as arrays of charas arrays of char

char str[20] = "hi";char str[20] = "hi";

char *pStr = str;char *pStr = str;

cout << pStr; // prints hicout << pStr; // prints hi• Can dynamically allocate memory to be used Can dynamically allocate memory to be used

for C-string using for C-string using newnew

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Library Functions for C-Strings(Requires #include <cstring>)(Requires #include <cstring>)• int strlen(char *str)int strlen(char *str) // // Returns length of a C-string:Returns length of a C-string:

cout << strlen("hello");cout << strlen("hello"); // // Prints: Prints: 55

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const int LENGTH = 30;char line[LENGTH];

cout << "Enter a string (no more than " << LENGTH - 1 << " characters.)\n";cin.getline(line, LENGTH);

cout << "The C-string read was " << "\"" << line << "\"" << " whose length was " << strlen(line) << endl;

strcpy

• char name[20] = “Mary” char name[20] = “Mary” // This is OK// This is OK

char name2[20];char name2[20];name2 = “Nancy”; name2 = “Nancy”; // Now allowed// Now allowed

char name3[20];char name3[20];strcpy(name3, “Nancystrcpy(name3, “Nancy”); //”); // CopyCopy

cout << name3; cout << name3; // Prints Nancy// Prints Nancy

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strcpy

• strcpy(charstrcpy(char *dest, char *source) *dest, char *source)

// Copies a string from a source address // Copies a string from a source address // to a destination address // to a destination address

char name[15];char name[15];

strcpy(name, "Deborah");strcpy(name, "Deborah");

cout << name; // cout << name; // prints Deborahprints Deborah

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strcat• strcat(char *dest, char *source)strcat(char *dest, char *source) //// Takes two C-strings as input. Takes two C-strings as input. // Adds the contents of the second string to the // Adds the contents of the second string to the // end of the first string: // end of the first string:

char str1[15] = "Good ";char str1[15] = "Good "; char str2[30] = "Morning!";char str2[30] = "Morning!"; strcat(str2, str1);strcat(str2, str1); cout << str1; // cout << str1; // prints: Good Morning!prints: Good Morning!

• No automatic bounds checking: programmer must No automatic bounds checking: programmer must ensure that str1 has enough room for resultensure that str1 has enough room for result

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strcmp

• intint strcmp(charstrcmp(char *str1,*str1, char*str2)char*str2) //// Compares strings stored at two addressesCompares strings stored at two addresses

• Returns a value:Returns a value:

less than 0 if less than 0 if str1str1 precedes precedes str2str2

equal to 0 if equal to 0 if str1str1 equals equals str2str2

greater than 0 if greater than 0 if str1str1 succeeds succeeds str2str2

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strcmp• Often used to test for equalityOften used to test for equality

if(strcmp(str1, str2) == 0)if(strcmp(str1, str2) == 0)

cout << "equal"; cout << "equal";

else else

cout << "not equal";cout << "not equal";

• Also used to determine ordering of C-strings in sorting applicationsAlso used to determine ordering of C-strings in sorting applications• Note that C-strings cannot be compared using == or >Note that C-strings cannot be compared using == or >

if (str1 > str2) -- this is not allowed if (str1 > str2) -- this is not allowed(compares addresses of C-strings, not contents)(compares addresses of C-strings, not contents)

• If If •

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Swapping

const int NAME_LENGTH = 20;const int NAME_LENGTH = 20;char name1[NAME_LENGTH];char name1[NAME_LENGTH];char name2[NAME_LENGTH];char name2[NAME_LENGTH];char temp[NAME_LENGTH];char temp[NAME_LENGTH];

strcpy(name1, "William"); strcpy(name1, "William"); strcpy(name2, "Catherine"); strcpy(name2, "Catherine"); cout << "Originally...\n";cout << "Originally...\n";cout << "Name1: " << name1 << endl;cout << "Name1: " << name1 << endl;cout << "Name2: " << name2 << endl;cout << "Name2: " << name2 << endl;

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Swapping (cont.)

if (strcmp(name1, name2) > 0)if (strcmp(name1, name2) > 0){{ strcpy(temp, name1); strcpy(temp, name1); strcpy(name1, name2); strcpy(name1, name2); strcpy(name2, temp); strcpy(name2, temp);

}}

cout << "\nAfter switch...\n";cout << "\nAfter switch...\n";cout << "Name1: " << name1 << endl;cout << "Name1: " << name1 << endl;cout << "Name2: " << name2 << endl;cout << "Name2: " << name2 << endl;

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Character Testing

Requires #include <Requires #include <cctype>cctype>

FUNCTION MEANING

isalpha() true if arg. is a letter, false otherwise

isalnum() true if arg. is a letter or digit, false otherwise

isdigit() true if arg. is a digit 0-9, false otherwise

islower() true if arg. is lowercase letter, false otherwise

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Character Testing

Require #include<Require #include<cctype>cctype>

FUNCTION MEANING

isprint() true if arg. is a printable character, false otherwise

ispunct() true if arg. is a punctuation character, false otherwise

isupper() true if arg. is an uppercase letter, false otherwise

isspace() true if arg. is a white space character, false otherwise

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C++ String Class

The C++ string Class

• The string class offers several advantages over The string class offers several advantages over C-style strings:C-style strings:

large body of member functionslarge body of member functionsoverloaded operators to simplify expressionsoverloaded operators to simplify expressions

• Requires #include <Requires #include <stringstring>>

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string class constructors

• string()string()• string(string str)string(string str) initializes string initializes string

object with strobject with str• string(char *cstr)string(char *cstr) initializes string initializes string

ojbect with C-stringojbect with C-string

• Various other constructorsVarious other constructorswww.cplusplus.com

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String Class Member Functions• int find(string str)int find(string str)

// returns the position of str in the string object // returns the position of str in the string object

• int find (char ch)int find (char ch) // returns the position of ch in the string object // returns the position of ch in the string object

• int find (string str, int x)int find (string str, int x) // returns the position of str beyond x // returns the position of str beyond x

• int find (ch ch, intx)int find (ch ch, intx) // returns the position of ch beyond // returns the position of ch beyond

String Class Member Functions (cont.)• void insert (int x, string str)void insert (int x, string str)

// inserts str at position x // inserts str at position x

• void insert (int x, char ch)void insert (int x, char ch) // inserts char at position x // inserts char at position x

• string replace(int pos, int n, string str)string replace(int pos, int n, string str) // replaces n characters starting at pos with // replaces n characters starting at pos with // substring str // substring str

• Other member functions Other member functions www.cplusplus.com

Overloaded string Operators

OPERATOR MEANING

>> reads whitespace-delimited strings into string object

<< outputs string object to a stream

= assigns string on right to string object on left

+= appends string on right to end of contents of string on left

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Overloaded string Operators (continued)

OPERATOR MEANING

+ Returns concatenation of the two strings

[] references character in string using array notation

>, >=, <, <=, ==, !=

relational operators for string comparison. Return true or false

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Overloaded string Operators

string word1, phrase;string word1, phrase;string word2 = " Dog";string word2 = " Dog";cin >> word1; // user enters "Hot"cin >> word1; // user enters "Hot" // word1 has "Hot"// word1 has "Hot"phrase = word1 + word2; // phrase hasphrase = word1 + word2; // phrase has // "Hot Dog"// "Hot Dog"phrase += " on a bun";phrase += " on a bun";for (int i = 0; i < 16; i++)for (int i = 0; i < 16; i++) cout << phrase[i]; // displayscout << phrase[i]; // displays // "Hot Dog on a bun"// "Hot Dog on a bun"

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Swapping Two Strings string name1 = "William"; string name2 = "Catherine"; string temp;

cout << "Originally...\n"; cout << "Name 1: " << name1 << endl; cout << "Name 2: " << name2 << endl;

if (name1 > name2){ temp = name1; name1 = name2; name2 = temp; } cout << "\nAfter swap...\n"; cout << "Name 1: " << name1 << endl; cout << "Name 2: " << name2 << endl;

Conversion to C-strings

• data()data() and and c_str()c_str() both return the C- both return the C-string equivalent of a string equivalent of a stringstring object object

• Useful when using a string object with a Useful when using a string object with a function that is expecting a C-stringfunction that is expecting a C-string

char greeting[20] = "Have a ";char greeting[20] = "Have a ";

string str("nice day");string str("nice day");

strcat(greeting, str.data());strcat(greeting, str.data());

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Modification of string objects

• str.insert(int pos, string s)str.insert(int pos, string s)

inserts inserts ss at position at position pospos in in strstr• Convert constructor for Convert constructor for stringstring allows a allows a

C-string to be passed in place of C-string to be passed in place of ss

string str("Have a day");string str("Have a day");

str.insert(7, "nice ");str.insert(7, "nice ");• insertinsert is overloaded for flexibility is overloaded for flexibility

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Modification of string objects

• str.append(string s)str.append(string s)

appends contents of appends contents of ss to end of to end of strstr• Convert constructor for Convert constructor for stringstring allows a allows a

C-string to be passed in place of C-string to be passed in place of ss

string str("Have a ");string str("Have a ");

str.append("nice day");str.append("nice day");• appendappend is overloaded for flexibility is overloaded for flexibility

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Your Turn(Formatting Number)

• Problem: Design a function which formats a Problem: Design a function which formats a string like “1234567.89” to string like “1234567.89” to “$1,234,567.89”.“$1,234,567.89”.

• Plan:Plan:– Find the position dp of the decimal pointFind the position dp of the decimal point– Starting at dp, count backwards and insert a Starting at dp, count backwards and insert a

comma every 3 places.comma every 3 places.– At position 0, insert ‘$’.At position 0, insert ‘$’.

Refinement

• dp dp position of decimal point in string position of decimal point in stringpos pos dp dp

• Loop (while dp is greater than 3)Loop (while dp is greater than 3) go back 3 places in the string go back 3 places in the string insert ‘,’ there insert ‘,’ thereEnd LoopEnd Loop

• Insert “$” at front of stringInsert “$” at front of string

0 1 2 3 4 5 6 7 8 9

1 2 3 4 5 6 7 . 9 5

dp

Your Turn

• Write a program which inputs a long text Write a program which inputs a long text and counts the frequency of occurrence of and counts the frequency of occurrence of each letter in the text.each letter in the text.

0 1 2 3 4 5 … 23 24 25

8 1 2 2 20 3 … 1 7 0

letters

frequency

In Main() // Declarations const int MAX = 26; int frequency[MAX]; string text; cout << "Please enter a “ + “substantially long text.\n"; getline(cin, text); string input(text); // convert to // c-string letterFrequency(frequency, MAX, input);

Letter Frequency

• The quick brown fox jumps over the lazy dog.

• Pack my box with five dozen liquor jugs.

• Now is the time for all good people to come to the aid of their country.

letterFrequency()void letterFrequency(int frequency[], int count, string input) { // Initialize requency for (int i = 0; i < count; i++){ frequency[i] = 0; } // Count frequency int length = input.length(); for (int i = 0; i < length; i++) { switch(input[i]) { case 'a': case 'A': frequency[0]++; break; case 'b': case 'B': frequency[1]++; break;

letterFrequency() (cont.) case 'c': case 'C': frequency[2]++; break; case 'd': case 'D': frequency[3]++; break; case 'e': case 'E': frequency[4]++; break; . . . . . .; case 'x': case 'X': frequency[23]++; break; case 'y': case 'Y': frequency[24]++; break; case 'z': case 'Z': frequency[25]++; break; default:; } // switch }}

String I/O

• string str1;string str1;cin >> str1; // delimitted bycin >> str1; // delimitted by // white-space // white-spacecout << str1;cout << str1;

• string str2;string str2;getline(cin, str2);getline(cin, str2); // delimitted by // delimitted by // by eol // by eol

Your Turn

• Problem:Problem:– Input a long sentence and count the number of Input a long sentence and count the number of

words. words.

• Assumption:Assumption:– All words are delimitted by a single space.All words are delimitted by a single space.

In Main()

• // Input text// Input text

• // wordCount = countWords(text)// wordCount = countWords(text)

• // Output results// Output results

countWords()

• int countWords(string text)int countWords(string text) Loop (while length > 0 && !done) Loop (while length > 0 && !done) ps ps position of ‘ ‘ in text position of ‘ ‘ in text If (ps = 0) If (ps = 0) done done true true End If End If Add one to count Add one to count Replace first part of text with “” Replace first part of text with “”

• End LoopEnd Loop