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Section 10: Strings

There are two main ways to handle strings in C++: 1. Using char Array (Available in both C and C++) 2. class string (Only in C++)

1. Using Char Array

Key Concepts

  • Storage: Literals are created in the code section.
    • If you want a string in the Heap: Use a char pointer (char*).
    • If you want a string in the Stack: Use a char array (char[]).
  • Quotes:
    • Single quotes ' ' = char
    • Double quotes " " = string
  • Termination: Strings must end with a Null character '\0' (numeric value 0).

Warning (ISO C++11): Conversion from string literal to char* is not allowed. 

char *S = "Hello"; // Deprecated/Warning in modern C++
const char *S = "Hello"; // Correct way for literals

Reading Input

char name[20];
cin >> name;    // Reads only the first word (stops at space)

To read sentences (handling spaces):

  • cin.get(name, 20);
  • Note: get leaves the 'Enter' key (newline) in the buffer. The next string input will read that empty line.

  • Fix: Use cin.ignore(); after using cin.get().

  • cin.getline(name, 20);

  • Recommendation: Use this for multiple lines as it handles the newline character automatically.

Built-in Functions (<cstring> / string.h)

Function Usage Description
strlen(s) int len = strlen(s); Returns string length.
strcat(dest, src) strcat(s1, s2); Concatenates src to dest. dest becomes dest + src.
strncat(dest, src, n) strncat(s1, s2, 3); Concatenates only n letters of src to dest.
strcpy(dest, src) strcpy(s1, s2); Copies src string into dest.
strncpy(dest, src, n) strncpy(s1, s2, 5); Copies only n characters.
strstr(main, sub) if(strstr(s1, s2) != NULL) Finds a substring. Returns pointer to occurrence or NULL.
strchr(main, char) strchr(s1, 'a'); Finds first occurrence of a character.
strcmp(s1, s2) int res = strcmp(s1, s2); Compares 2 strings. Returns: -ve (s1 < s2), 0 (equal), +ve (s1 > s2).
strtol(s1, NULL, 10) long n = strtol(s, NULL, 10); String to Long Int (10 is the base/decimal).
strtof(s1, NULL) float f = strtof(s, NULL); String to Float.
strtok(s1, "=;") char* token = strtok(s, "=;"); Tokenizes string using delimiters (e.g., = or ;).

2. Class String (std::string)

Requires: #include <string>

Input

string str;
cin >> str;           // Reads only one word
getline(cin, str);    // Reads a full sentence

String Class Functions

Capacity & Size:

  • s.length() or s.size(): Returns length.
  • s.capacity(): Returns current allocated storage.
  • s.resize(30): Resizes the string (pads or truncates).
  • s.max_size(): Maximum size string can reach.
  • s.clear(): Clears the contents.
  • s.empty(): Returns boolean (true if empty).

Modification:

  • s.append("Bye"): Adds to end.
  • s.insert(3, "kk"): Inserts "kk" at index 3.

  • s.insert(3, "Apple", 2): Inserts first 2 chars of "Apple" at index 3.

  • s.replace(3, 5, "aa"): Starting at index 3, replace next 5 chars with "aa".

  • s.erase(): Same as clear().
  • s.push_back('z'): Insert char at end.
  • s.pop_back(): Delete last char.
  • s1.swap(s2): Swaps the contents of two strings.

Search & Copy:

  • s.copy(char des[]): Copy string to a char array.
  • s.find(str) or s.find(char): Returns index of first occurrence.
  • s.rfind(str): Search from the right (end).

  • Note: If not found, returns string::npos (which is effectively -1/largest unsigned int).

  • s.find_first_of('a', 3): Find 'a' starting search from index 3.

  • s.find_last_of('le'): Search from right side; returns index of any of the characters found first.
  • s.substr(start, number): Extract a portion of the string.
  • s.compare(str): Dictionary comparison (returns -ve, 0, +ve).

Operators

  • at(index)
  • front(): First character.
  • back(): Last character.
  • []: Overloaded array operator (access via index).
  • +: Concatenation.

3. String Class - Iterators

Iterators act like pointers to characters within the string (can read and modify).

  • begin(): Points to first char.
  • end(): Points to position after last char.
  • rbegin(): Reverse iterator start.
  • rend(): Reverse iterator end.

Iterator Diagram

graph LR
    subgraph Memory
    direction LR
    A[H] --- B[e] --- C[l] --- D[l] --- E[o]
    end

    ptr1["begin()"] --> A
    ptr2["end()"] -.-> F["null/end"]
    F --- E
    style F stroke-dasharray: 5 5

Examples

1. Basic Iterator (Modify string) 

string str = "today";
string::iterator it;

for(it = str.begin(); it != str.end(); it++) {
    // cout << *it;
    *it = *it - 32; // Convert to Uppercase
}
cout << str; // Output: TODAY

2. Reverse Iterator 

string str = "today";
string::reverse_iterator it;

for(it = str.rbegin(); it != str.rend(); it++) {
    cout << *it;
}
// Output: yadot

3. Standard Loop (Equivalent logic) 

string str = "today";
for (int i = 0; str[i] != '\0'; i++) {
    // cout << str[i];
    str[i] = str[i] - 32;
}
cout << str; // Output: TODAY

4. Create String of Specific Length 

string str = "";
str.resize(len);