Characters and Strings

Overview

• Character functions
• C-style Strings
• Working with C-style Strings
• C++ Strings
• Working with C++ Strings

Character Functions

#include <cctype>

function_name (char)

• Functions for testing characters
• Functions for converting character case

Testing characters

| isalpha(c) | True if c is a letter | | isalnum(c) | True if c is a letter of digit | | isdigit(c) | True if c is a digit | | islower(c) | True if c is a lowercase letter | | isprint(c) | True if c is a printable character | | ispunct(c) | True if c is a punctuation character | | isupper(c) | True if c is an uppercase letter | | isspace(c) | True if c is whitespace |

Converting characters

| tolower(c) | return lowercase of c | | toupper(c) | return uppercase of c |

C-Style Strings

• Sequence of characters

  • Contiguous in memory
  • Implemented as an array of characters
  • Fixed in size
  • Terminated by a null character (null)
    • null - character with a value of zero
  • Referred to as zero or null terminated strings

• String literal

  • Sequence of characters in double quote
    • e.g. "Anthony"
  • Constant
  • Terminated with null character

Example "C++ is fun" is stored: | C | + | + | | | i | s| | f | u | n | \0 |

• Needed null character in the end
• size of 11 in memory instead of 10

Declaring variables

char my_name [] {"Frank"};

my_name[5] = 'y';   // Problem since not dynamic size

char my_name [8];
| ? | ? | ? | ? | ? | | ? | ? | ? | 

my_name = "Frank";          // Error

strcpy(my_name, "Frank");   // OK

Functions that work with C-style Strings

• Copying
• Concatenation
• Comparison
• Searching
• and others

All functions rely on one factor: that the sequence of characters that they are working on are terminated by the null character.

Example

char str[80];

strcpy(str, "Hello ");      // copy

strcat(str, "there");       // concatenate

cout << strlen(str);        // length - Output: 11

strcmp(str, "Another");     // compare - Ouput: 0

General purpose functions

• C standard library: #include <cstdlibb>
• Includes functions to convert C-style Strings to
  • integer
  • float
  • long
  • etc.

C++ Strings

• std::string is a Class in the Standard Template Library
  • #include <string>
  • std namespace
  • contiguous in memory
  • dynamic size
  • work with input and output streams
  • lots of useful member functions
  • our familiar operators can be used (+, =, <, <=, >, >=, +=, ==, !=, [], ...)
  • can be easily converted to C-style Strings if needed
  • safer

Declaring and initializing

#include <string>
using namespace std;

string s1;                  // empty
// automatically empty - no need to worry about garbage data and initialization
string s2 {"Frank"};        // Frank
string s3 {s2};             // Frank: different memories as s2
string s4 {"Frank", 3};     // Fra
string s5 {s3, 0, 2};       // Fr: string s3 and starting at index 0 and copy length of 2
string s6 (3, 'X');         // XXX

Assignment

#include <string>
using namespace std;

string s1;
s1 = "C++ Rocks!";

string s2 {"Hello"};
s2 = s1;

Concatenation

#include <string>
using namespace std;

string part1 {"C++"};
string part2 {"is a powerful"};

string sentence;

sentence = part1 + " " + part2 + " language";
// Output: C++ is a powerful language

// Below is ILLEGAL!!!
sentence = "C++" + " is powerful";

Accessing charaacters [] and at() method

#include <string>
using namespace std;

string s1;
string s2 {"Frank"};

cout << s2[0] << endl;      // Output: F
cout << s2.at(0) << endl;   // Output: F

s2[0] = 'f';        // frank
s2.at(0) = 'p'      // prank

// ========= For loop to access each character ========= 
string s1 {"Frank"};

for (char c : s1) {
    cout << c << endl;
}

// Output:
// F
// r
// a
// n
// k
// 

// ========= For loop to access each character as integer ========= 
string s1 {"Frank"};

for (int c : s1) {
    cout << c << endl;
}

// Output: ascii code of the characters
// 70   // F
// 114  // r
// 97   // a
// 110  // n
// 107  // k
// 0    // null character

Comparing

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

The objects are compared character by character lexically.

Can compare:

• two std::string objects
• std::stringobject and C-style string literal
• std::string object and C-style string variable

Example

string s1{"Apple"};
string s2{"Banana"};
string s3{"Kiwi"};
string s4{"apple"};
string s5{s1};      // Apple

s1 == s5;       // True
s1 == s2;       // False
s1 != s2;       // True
s1 < s2;        // True
s2 > s1;        // True
s4 < s5;        // False
s1 == "Apple";  // True

Substrings - substr

Extracts a substring from a std::string

object.substr(start_index, length)

string s1 {"This is a test"};

cout << s1.substr(0, 4);    // Output: This
cout << s1.substr(5, 2);    // Output: is
cout << s1.substr(10, 4);   // Output: test

Searching - find

Returns the index of a substring in a std::string

obect.find(search_string)

string s1 {"This is a test"};

cout << s1.find("This");    // Output: 0
cout << s1.find("is");      // Output: 2
cout << s1.find("test");    // Output: 10
cout << s1.find("e");       // Output: 11
cout << s1.find("is", 4);   // Output: 5
// start find at specified index, in this case, index 4
cout << s1.find("XX");      // Output: string::npos
// no position available

Removing characters - erase() and cleaer()

Removes a substring of characters from a std::string

object.erase(start_index, length)

string s1 {"This is a test"};

cout << s1.erase(0, 5);     // Output: is a test
cout << s1.erase(5, 4);     // Output: is a
s1.clear();                 // empties string s1

Other useful methods

string s1 {"Frank"};
cout << s1.length() << endl;    // Output: 5

s1 += " James";
cout << s1 << endl;             // Output: Frank James

many more...

Input with C++ Strings

Inut >> and getline()

Reading std::string from cin

string s1;

cin >> s1;
// Input: "Hello there"

cout << s1 << endl;
// Output: "Hello"

// Input only accepts    up to the first <SPACE>

// Read entire line until \n
getline(cin, s1);

cout << s1 << endl;
// Output: "Hello there"

getline(cin, s1, 'x');
// Input: this isx

cout << s1 << endl;
// Output: "this is"