Characters and Strings

Real-world analogy

What is it?

Characters (char) are single letters, digits, or symbols stored as numbers using ASCII encoding. Strings are sequences of characters that represent words or text. In C++, strings can be accessed character-by-character using array-style indexing.

Real-world relevance

Every app that deals with text uses strings — search engines finding words in web pages, spell checkers comparing your typing against a dictionary, password validators checking if you used uppercase and digits. Even emoji are characters with special numeric codes!

Key points

• What is a Character (char)? — A char in C++ stores a single letter, digit, or symbol. You write it with single quotes: char c = 'A'; Characters are actually stored as numbers using the ASCII table. 'A' is 65, 'B' is 66, 'a' is 97, '0' is 48. You can do math with characters! 'A' + 1 gives 'B'.
• What is a String? — A string is a sequence of characters — basically a word or sentence. In C++, use the string type: string s = "hello"; Strings use double quotes (not single). You can read a string with cin >> s (reads one word) or getline(cin, s) (reads a whole line including spaces).
• String Length and Indexing — s.length() or s.size() tells you how many characters are in the string. You can access individual characters with s[0], s[1], etc., just like an array! The first character is s[0] and the last is s[s.length()-1]. Strings are basically arrays of characters.
• The ASCII Table — ASCII is how computers store characters as numbers. Key ranges: 'A' to 'Z' are 65-90, 'a' to 'z' are 97-122, '0' to '9' are 48-57. The difference between uppercase and lowercase is always 32. So 'a' - 'A' = 32. This is super useful for converting between cases!
• Useful Character Functions — C++ has handy functions for characters: islower(c) checks if lowercase, isupper(c) checks uppercase, isdigit(c) checks if it is a digit, isalpha(c) checks if it is a letter. toupper(c) converts to uppercase, tolower(c) converts to lowercase. These save you from memorizing ASCII values!
• Looping Through a String — You can loop through every character in a string just like an array: for(int i = 0; i < s.length(); i++) works great. Or use a range-based for loop: for(char c : s) which gives you each character one at a time. This is how you process strings character by character.
• String Concatenation and Comparison — You can join strings with +: string result = "hello" + " " + "world"; You can compare strings with ==, . Strings are compared letter by letter (like dictionary order). "apple" < "banana" is true because 'a' comes before 'b'. This is called lexicographic comparison.
• Reading Strings with Spaces — cin >> s only reads until the first space. To read a whole line (with spaces), use getline(cin, s). Warning: if you use cin >> before getline, you need to add cin.ignore() in between to eat the leftover newline character. This is a common gotcha!

Code example

#include <bits/stdc++.h>
using namespace std;

int main() {
    string s;
    cin >> s;  // Read one word
    
    cout << "Length: " << s.length() << endl;
    cout << "First char: " << s[0] << endl;
    cout << "Last char: " << s[s.length() - 1] << endl;
    
    // Count uppercase and lowercase letters
    int upper = 0, lower = 0, digits = 0;
    for (char c : s) {
        if (isupper(c)) upper++;
        else if (islower(c)) lower++;
        else if (isdigit(c)) digits++;
    }
    
    cout << "Uppercase: " << upper << endl;
    cout << "Lowercase: " << lower << endl;
    cout << "Digits: " << digits << endl;
    
    // Convert to uppercase
    string upperStr = s;
    for (int i = 0; i < upperStr.length(); i++) {
        upperStr[i] = toupper(upperStr[i]);
    }
    cout << "Uppercase version: " << upperStr << endl;
    
    return 0;
}
// Input:  Hello123World
// Output: Length: 13
//         First char: H
//         Last char: d
//         Uppercase: 2
//         Lowercase: 8
//         Digits: 3
//         Uppercase version: HELLO123WORLD

Line-by-line walkthrough

1. 1. We read a single word into string s using cin >>.
2. 2. s.length() tells us how many characters are in the string.
3. 3. s[0] is the first character, s[s.length()-1] is the last — just like arrays!
4. 4. We use a range-based for loop (for char c : s) to check each character.
5. 5. isupper(), islower(), isdigit() are built-in functions that check character types.
6. 6. To convert the string to uppercase, we loop through and apply toupper() to each character.
7. 7. toupper() converts a lowercase letter to uppercase. If it is already uppercase or a digit, it stays the same.

Spot the bug

#include <bits/stdc++.h>
using namespace std;

int main() {
    int n;
    cin >> n;
    string s;
    getline(cin, s);
    
    cout << "You entered: " << s << endl;
    cout << "Length: " << s.length() << endl;
    
    return 0;
}
// Input: 3
//        Hello World
// Expected: You entered: Hello World
//           Length: 11
// Actual: You entered: 
//         Length: 0

Explain like I'm 5

Fun fact

Hands-on challenge

More resources

• Strings in C++ — Full Course (CodeWithHarry)
• C++ Strings — cppreference (cppreference)
• String Problems for Beginners (Codeforces)