Strings (data structure)

Data Structures C++

ASCII

🧠 What is ASCII?

• ASCII stands for American Standard Code for Information Interchange.
• It assigns numerical values (0–127) to characters, including:
  • Letters (A–Z, a–z)
  • Digits (0–9)
  • Punctuation (!, @, etc.)
  • Control characters (like newline \n, tab \t)

🔤 Character Encoding in C++

• C++ char types use ASCII values by default (in most systems).
• A char is just a small integer, so:

char c = 'A';
int val = c; // val = 65`

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💡 Important ASCII Facts for Strings/Chars in C++

Concept	Explanation
'A' to 'Z'	ASCII values from 65 to 90
'a' to 'z'	ASCII values from 97 to 122
'0' to '9'	ASCII values from 48 to 57
Case conversion	'a' - 'A' == 32, so you can flip case with +/- 32
Character checks	Use isalpha(c), isdigit(c), islower(c), etc. from <cctype>
Sorting/comparison	You can compare chars directly: 'a' < 'b', 'A' < 'a'
Looping through alphabet	Use ASCII values: for (char c = 'a'; c <= 'z'; ++c)

Stringify 🔢

The most efficient way to convert a digit x ∈ {0, 9} into a character is by adding its integer value to the ASCII value of '0', like so:

string ans = ""; 
int x = 5; 
ans += x + '0';`

Think of x + '0' as:

“Give me the ASCII value of '0', then add x to get the ASCII value of the digit character.”

This is much faster than the more familiar alternative using to_string:

`string ans = ""; int x = 5; ans = to_string(x) + ans;`

✅ Use x + '0' when working with single digits in performance-sensitive code, especially in tight loops or when building strings character by character. Using stringify (by adding a character to ‘0’) is faster than to_string in C++ because:

The Overhead of to_string: The to_string function is a more complex operation that:

1. Creates a new string object
2. Handles memory allocation
3. Performs the actual conversion from integer to string
4. Handles various edge cases and formatting

Simple Character Addition: The stringify approach using remainder + ‘0’ is much simpler because:

1. It directly converts a single digit (0-9) to its ASCII character equivalent
2. It’s a single arithmetic operation
3. No memory allocation is needed
4. No string object creation is required

String Comparison with <, >, == in C++

In modern C++, std::string supports direct comparison using relational operators, just like numbers:

std::string a = "apple";
std::string b = "banana";
 
if (a < b) std::cout << "apple comes before banana\n";
if (a != b) std::cout << "they are different\n";

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✅ Supported Operators:

Operator	Meaning	Example
==	Equal	"abc" == "abc" → true
!=	Not equal	"abc" != "xyz" → true
<	Lexicographically less than	"apple" < "banana"
>	Lexicographically greater than	"zebra" > "ant"
<=	Less than or equal	"abc" <= "abc"
>=	Greater than or equal	"cat" >= "ant"

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📘 Lexicographical Order:

• Comparison is done character by character using ASCII values.
• Stops at the first differing character.
• If one string is a prefix of the other, the shorter one is considered smaller:

"app" < "apple"  // true