Strings

• We have char array which is available in c & c++.
• We have class String which is only available in c++.
• Let's first understand char array:

Char Array

• Different ways of declaring and initializing string using char array:

  // char_array.cpp
  
  char arr1 = 'a';
  char arr2[10] = "Hello";
  char arr3[] = "Hello";
  char arr4[] = {'H', 'e', 'l', 'l', 'o', '\0'};
  char arr5[] = {65, 66, 67, 68, 69, '\0'};
  char *arr6 = "Hello";   // warning: ISO C++ forbids converting a string constant to 'char*'
  const char *arr7 = "Hello"; // to fix arr6 warning, add const
  
  cout << "arr1 = " << arr1 << endl;      // a
  cout << "arr2 = " << arr2 << endl;      // Hello
  cout << "arr3 = " << arr3 << endl;      // Hello
  cout << "arr4 = " << arr4 << endl;      // Hello
  cout << "arr5 = " << arr5 << endl;      // ABCDE
  cout << "arr6 = " << arr6 << endl;      // Hello
  cout << "arr7 = " << arr7 << endl;      // Hello
  

• Use Single Quotes for char ''
• use Double Quotes for string ""
• Declaring & Initializing String:
  
• Literals are created in code section.
  • a literal is a notation for representing a fixed value in source code.
  • literals can be used to represent constants of various types, such as integers, floating-point numbers, characters, and more
• If we want a string in heap, go for character pointer.
• If you want in stack, then go for character array.
• Null character = '\0' (or 0 numeric)
• just cin only takes characters to the first white space.
• cin.getline() reads input up to '\n' and stops
• cin.get() reads input up to '\n' and keeps '\n' in the stream
• cin.getline() reads a whole line, up to, but not including the Enter key.
• cin.get() is the same, except it keeps the newline character.
• Example:

  // char_array.cpp
  
  char name[20];
  cout << "Enter name: "; 
  cin >> name;    // Reads until the first whitespace character, read only first word not whole sentence
  cout << "name = " << name << endl;  // Rajat
  cin.ignore();   // to handle any leftover newline characters or other unwanted characters in the input buffer
  
  // cin.get() is used for accessing character array
  char data[20];
  cout << "Enter Something for cin.get(): ";
  cin.get(data, 20);   //  get will not read enter key, second string will take that enter as '\n' string
  cout << "data = " << data << endl;      // How are you?
  cin.ignore();   // to handle any leftover newline characters or other unwanted characters in the input buffer
  
  // `cin.getline()` reads input up to `'\n'` and stops (no need to use cin.ignore())
  char data2[20];
  cout << "Enter Something for cin.getline(): ";
  cin.getline(data2, 20);
  cout << "data2 = " << data2 << endl;    // I am fine
  

• This will also work:

  char *s;
  cout<<"Enter a String";
  cin>>s;
  cin.getline(s, 100);
  

Char Array/String built-in functions

• Need to include #include <ctring> or #include <string.h>

  • '<cstring>' is the C++ version of the C header '<string.h>'. It is part of the C++ Standard Library and contains the same functions as , but they are encapsulated within the std namespace. So, when you use , you should use the std namespace or include it explicitly:

    #include <cstring>
    
    // Or use std::function
    std::size_t length = std::strlen("Hello");
    

  • '<string.h>' is the C version of the header and is typically used in C programs. If you are writing C++ code, it is generally recommended to use '<cstring>' for better compatibility with C++ and to take advantage of the std namespace.

    #include <string.h>
    
    const char *source = "Hello, World!";
    char destination[20];
    
    strcpy(destination, source);
    printf("Copied string: %s\n", destination);
    

• strlen(s) // for string length

  // strlen: String Length
  
  char str[] = "Hello";
  int length = strlen(str);
  cout << "Length of string: " << length << endl;     // 5
  

• strcat(destination, source) // for concatenate strings, source string will added in destination string, destination will become destination + source.

  // strcat: Concatenate strings
  
  char source[] = " World";
  char destination[20] = "Hello";
  strcat(destination, source);
  cout << "Concatenated string: " << destination << endl;     // Hello World
  

• strncat(destination, source, number of letter of second string to concatenate with first) // Concatenate with limited characters

  // strncat: Concatenate with limited characters
  
  char source2[] = " Universe";
  strncat(destination, source2, 4); // Concatenate only the first 4 characters of source2
  cout << "Partially concatenated string: " << destination << endl;   // Hello World Uni
  

• strcpy(destination, source) // copy source string to destination

  // strcpy: Copy string
  
  char source3[] = "Copy";
  char destination2[20];
  strcpy(destination2, source3);
  cout << "Copied string: " << destination2 << endl;      // Copy
  

• strncpy(destination,source, length) // Copy string with limited characters

  // strncpy: Copy string with limited characters
  char source4[] = "Partial Copy";
  char destination3[20];
  strncpy(destination3, source4, 7); // Copy only the first 7 characters of source4
  cout << "Partially copied string: " << destination3 << endl;    // Partial
  

• strstr(main, sub) // to find substring, will crash if not found. Use if(strstr(s1,s2)!=NULL) {…}

  // strstr: Find substring
  
  char mainStr[] = "Hello World";
  char subStr[] = "World";
  if (strstr(mainStr, subStr) != NULL) {
      cout << "Substring found!" << endl;     // Substring found!
  } else {
      cout << "Substring not found." << endl;
  }
  

• strchr(main, char); // find occurrence of a character in string

  // strchr: Find character occurrence
  
  char searchChar = 'o';
  char* charPtr = strchr(mainStr, searchChar);
  if (charPtr != NULL) {
      cout << "Character found at position: " << (charPtr - mainStr) << endl;     // 4
  } else {
      cout << "Character not found." << endl;
  }
  

• strcmp(str1, str2) ; // compare 2 string, return -ve, 0 (when strings are same), +ve

  // strcmp: Compare strings
  
  char str1[] = "Hello";
  char str2[] = "World";
  int comparisonResult = strcmp(str1, str2);
  cout << "Comparison result: " << comparisonResult << endl;      // -1
  

• strtol(str1, NULL, 10) // string to long int, where 10 (decimal) is base

  // strtol: String to long int
  
  char str3[] = "123";
  long intResult = strtol(str3, NULL, 10);
  cout << "String to long int: " << intResult << endl;    // 123
  

• strtof(str1, NULL) // string to float

  // strtof: String to float
  
  char str4[] = "3.14";
  float floatResult = strtof(str4, NULL);
  cout << "String to float: " << floatResult << endl;     // 3.14
  

• strtok(str1, "=;") // to tokenize a string, where =; is token/delimiter.

  // Example for strtok
  
  char str5[] = "key1=value1;key2=value2";
  char* token = strtok(str5, "=;");
  while (token != NULL) {
      cout << "Token: " << token << endl;     // Token: key1 
      token = strtok(NULL, "=;");             // Token: value1 .....
  }
  

• Working code with all above example is in char_functions.cpp

Class String

• How to define a string:
  • string str
• Declaration and Initialization of String:
  • std::string newStr = "You are learning C++";
• How to input a string:
  • cin >> std // take only one word
  • getline(cin, str) // can take sentence

String Class Functions / Operations on Strings

Input Functions:

• getline(cin, str) : store a stream of characters as entered by the user
• push_back('c') : input a character at the end of the string
• pop_back() : delete the last character from the string.

  string str1;
  cout << "Enter str1: ";
  getline(cin, str1);     // Taking string input
  cout << "str1 = " << str1 << endl;
  
  str1.push_back('r');    // Inserting a character
  cout << "str1.push_back('r') = " << str1 << endl;
  
  str1.pop_back();        // Deleting a character
  cout << "str1.pop_back() = " << str1 << endl;
  

String Length:

• str.size() : Returns the size of the string
• str.length() : Alias of size, returns the size of the string
• str.capacity() : Returns the capacity allocated to a string by the compiler
• str.reserve(100) : Changes the capacity of the string

• shrink_to_fit() : reduces the capacity of the string object to fit its actual length

  cout << "str1.size() = " << str1.size() << endl;
  cout << "str1.length() = " << str1.length() << endl;
  cout << "str1.capacity() = " << str1.capacity() << endl;
  str1.resize(100);
  cout << "str1.capacity() after str1.resize(100); = " << str1.capacity() << endl;
  str1.shrink_to_fit();
  cout << "str1.capacity() after str1.shrink_to_fit() = " << str1.capacity() << endl;
  

String class provides a variety of member functions:

• Constructor:

  std::string str1;                   // Default constructor
  std::string str2("Hello, World!");  // Constructor with a C-string
  std::string str3(5, 'A');           // Constructor with repetition 
  

• Assignment:

  std::string str4 = "C++";
  std::string str5;
  str5 = str4;  // Assignment
  

• Concatenation:

  std::string str6 = "Hello, ";
  std::string str7 = "World!";
  std::string result = str6 + str7;  // Concatenation
  

• Accessing Characters:

  std::string str9 = "Example";
  char firstChar = str9[0];    // Accessing first character
  char lastChar = str9.back();  // Accessing last character
  

• Substring:

  std::string str10 = "Programming";
  std::string subStr = str10.substr(3, 5);  // Get substring from position 3 with length 5
  

• Find:

  std::string str11 = "C++ is powerful";
  size_t pos = str11.find("power");  // Find the position of the substring "power"
  

• Replace:

  std::string str12 = "C++ is great";
  str12.replace(7, 5, "awesome");  // Replace "great" with "awesome"
  

• Erase:

  std::string str13 = "Remove this";
  str13.erase(7, 5);  // Erase 5 characters starting from position 7
  

• Compare:
  • similar to strcmp, compare string in dictionary order and return result as -ve, 0, +ve.

    std::string str14 = "apple";
    std::string str15 = "orange";
    int result = str14.compare(str15);  // Compare strings lexicographically
    

• There are more functions in string class:
  • s.max_size()
  • s.clear()
  • s.empty()
  • s.append( “Bye")
  • s.insert(3,"kk" )
  • s.insert(3, "Apple", 2)
  • s.replace(3,5,"aa" ) // 3 is starting index, 4 is length from starting index to replace with aa
  • s1.swap(s2) // swap 2 strings
  • s.copy(char des[]) // copy string char array des[]
  • s.find(str) or char // to find occurrence of string or char and return index
  • s.rfind(str) // to find occurrence of string or char from end/right hand side and return index. If return index is greater than length of string it means it didn't find the string or char
  • s.find_first_of('a', 3) // a character to find from last side and start finding from index 3 onwards
  • s.find_last_of('le') // search from right hand side, will return index of any of character found first

Some operators defined upon string class:

• str.at(5) // similar to str[5]
• str.front() // return first character of string
• str.back() // return last character of string

Iterators

• string::iterator // iterator object will work like a pointer to a character in a string (can read and modify)
• begin()
• end()
• reverse_iterator
• rbegin()
• rend()

• Example: ```cpp // Part 1: Convert each character to uppercase using iterators string str1 = "today"; string::iterator it1; for (it1 = str1.begin(); it1 != str1.end(); it1++) { it1 = it1 - 32; // Convert to uppercase } cout << str1 << endl; // Output: TODAY

  // Part 2: Print the reversed string string str2 = "today"; string::reverse_iterator it2; for (it2 = str2.rbegin(); it2 != str2.rend(); it2++) { cout << *it2; } cout << endl; // Output: yadot

  // Part 3: Convert each character to uppercase using array indexing string str3 = "today"; for (int i = 0; str3[i] != '\0'; i++) { cout << str3[i]; // Output: today str3[i] = str3[i] - 32; // Convert to uppercase } cout << endl << str3; // Output: TODAY ```