class: center, middle # CMPE 30: Lecture 3 Strings --- # On a system with 4-byte int, what is sizeof(scores) for int scores[10]? 1. 4 1. 10 1. 14 1. 40 1. Ben got this wrong --- # Learning Objectives - Create `std::string` and use `.size()`, `.length()`, `.empty()` - Concatenate with `+` and `+=` - Compare strings (character by character, ASCII) - Access characters with `[]` and `.at()` - Iterate with range-based `for` - `.find()` and `.substr()` to search and extract - Read lines with `std::getline` - Convert strings to/from numbers --- # Why std::string? - Raw `char` arrays are painful: - you track the length - cannot resize - `==` compares pointers, not content - missing `'\0'` crashes you - `std::string` manages its own memory - Always `#include <string>` --- # Creating Strings .lc[ ```cpp #include <iostream> #include <string> int main() { std::string empty; std::string greeting = "Hola"; std::string copy = greeting; std::string repeat(5, '!'); std::cout << greeting << "\n"; } ``` ] .rc[ - Default-constructed string is `""` (**not** garbage) - `std::string(count, char)` repeats the character - Copies are deep --- each has its own buffer ] --- # Length, Size, Empty .lc[ ```cpp std::string title = "Ice Ice Baby"; std::cout << title.size() << "\n"; // 12 std::cout << title.length() << "\n"; // 12 std::string nada; if (nada.empty()) { std::cout << "nothing here\n"; } ``` ] .rc[ - `.size()` and `.length()` are **identical** - `.empty()` returns `true` for zero length - Return type is `size_t` (**unsigned**) --- careful with signed comparisons ] --- # Concatenation .lc[ ```cpp std::string first = "Baby"; std::string second = " One More Time"; std::string hit = first + second; std::string lyrics = "Bailamos"; lyrics += ", te quiero"; ``` ] .rc[ - `+` produces a new string - `+=` modifies in place - At least **one side** of `+` must be a real `std::string` **Trap:** `"hello" + " world"` does **not** compile --- both are `const char*`. ] --- # Comparing Strings .lc[ ```cpp std::string a = "Hanson"; std::string b = "Vanilla Ice"; if (a < b) { std::cout << a << " first\n"; } ``` ] .rc[ - `==`, `!=`, `<`, `>`, `<=`, `>=` all work - Character by character using **ASCII values** - `'Z'` (90) < `'a'` (97) - So `"Zebra" < "apple"` is **true** ] --- # Accessing Characters .lc[ ```cpp std::string song = "MMMBop"; std::cout << song[0] << "\n"; // M std::cout << song.at(3) << "\n"; // B std::string shout = "hey!"; shout[0] = 'H'; // now "Hey!" ``` ] .rc[ - `[]` is fast, **no bounds check** - `.at()` throws `std::out_of_range` on bad index - Both are writable **Tip:** Use `.at()` when you are not 100% sure the index is valid. ] --- # Iterating .lc[ ```cpp std::string word = "Iris"; for (char c : word) { std::cout << c << ' '; } for (size_t i = 0; i < word.size(); ++i) { std::cout << word[i] << ' '; } ``` ] .rc[ - Range-based `for` is cleanest - Use `size_t` for the index --- matches `.size()` - Avoid `int` vs `size_t` signed/unsigned warnings ] --- # Finding and Extracting .lc[ ```cpp std::string line = "Ice Ice Baby"; size_t pos = line.find("Baby"); if (pos != std::string::npos) { std::cout << pos << "\n"; // 8 } std::string part = line.substr(8, 4); // "Baby" ``` ] .rc[ - `.find(needle)` returns position or `std::string::npos` - **Always** check for `npos` - `.substr(pos, len)` extracts; omit `len` for "to the end" ] --- # Reading Whole Lines .lc[ ```cpp std::string full_name; std::cout << "full name: "; std::getline(std::cin, full_name); int age; std::string name; std::cin >> age; std::cin.ignore(); std::getline(std::cin, name); ``` ] .rc[ - `std::getline(cin, s)` reads **up to the newline** - `cin >> s` stops at whitespace **Trap:** Mixing `>>` and `getline` without `cin.ignore()` gives an empty line. ] --- # Strings to/from Numbers .lc[ ```cpp std::string y = "1997"; int year = std::stoi(y); std::string p = "9.99"; double price = std::stod(p); int track = 7; std::string label = "Track " + std::to_string(track); ``` ] .rc[ - `std::stoi` --- string to int - `std::stod` --- string to double - `std::to_string` --- any number to string **Trap:** `std::stoi("abc")` **throws** an exception --- crash unless you handle it. ] --- # What does this print? .lc[ ```cpp std::string a = "Ice"; std::string b = a + " " + a + " Baby"; std::cout << b.size() << "\n"; ``` ] .rc[ 1. 8 1. 11 1. 12 1. 13 1. Ben got this wrong ] --- # Where is the bug? ```cpp std::string greeting = "Hello, " + "world!"; ``` 1. Missing `#include <string>` 1. `std::string` does not allow `+` 1. Both operands are literals; one side must be `std::string` 1. `+` should be `+=` 1. Ben got this wrong --- # Why is getline returning empty after cin >>? .lc[ ```cpp int age; std::string name; std::cin >> age; std::getline(std::cin, name); // name is empty! ``` ] .rc[ 1. `getline` only reads one word 1. The newline left by `>>` was consumed by `getline` 1. `std::cin` was closed 1. `getline` does not work with string 1. Ben got this wrong ] --- # Key Points - `std::string` lives in `<string>` - `.size()` / `.length()` are equal; both return `size_t` - `+` needs at least one `std::string` operand - Comparisons are ASCII, character by character - `[]` is fast, `.at()` is safe - `std::getline` reads lines; watch the `cin >>` newline trap - `std::stoi`/`std::stod`/`std::to_string` bridge strings and numbers **Read:** chapter 4 of *Gorgo Starting C++*. **Do:** exercises 1-9.