Strings, Tuples and Arrays
A catalogue entry is more than one number. It has a title (text), maybe a title and page count travelling together, and a week of daily loan counts. Rust has a specific type for each shape, and text comes in two kinds that everyone mixes up at first.
The rule
Stringowns text,&strborrows it; tuples mix types, arrays repeat one.
String is text the variable owns. It lives on the heap and can grow.
let mut title = String::from("Iron Tides");
title.push_str(" (2nd edition)");&str (a "string slice") is a borrowed view into text stored somewhere else. Every literal in quotes is a &str, and it is read-only.
let shelf_name: &str = "Fiction";
let first_word: &str = &title[0..4]; // "Iron"Convert between them: "Fiction".to_string() or String::from(...) makes a String; &title gives a &str view of a String. Functions that only read text should take &str, so they accept both.
Strings are UTF-8, so .len() counts bytes, not characters. "café".len() is 5. Walk characters with .chars(). For the same reason you cannot write title[0]; Rust forbids indexing a string by position.
Tuples group a fixed number of values that may differ in type. Read them by position:
let entry: (&str, u32) = ("Night Garden", 288);
println!("{} has {} pages", entry.0, entry.1);Arrays hold a fixed number of values of one type. The length is part of the type:
let week: [u32; 7] = [4, 2, 5, 3, 6, 9, 1];
let monday = week[0];
let blank = [0; 7]; // seven zerosIndexing past the end, like week[7], panics instead of reading stray memory.
The shortcut
Ask two questions: Is it text? Then "do I need to own or change it?" Yes → String, no → &str. Not text? "Same type every time?" Yes → array, no → tuple.
The search term
Text the function only reads.
&str.
Drills
Answer fast — trust the rule.
What type is the literal "Fiction"?
What does "café".len() return?
After let entry = ("Night Garden", 288);, how do you read 288?
Which type holds exactly seven u32 values?
Remember
Stringowns and can grow;&strborrows and reads. Functions that only read take&str.- Text? Own or change it →
String, else&str. Not text? Same type → array, mixed → tuple. - Trap:
.len()counts bytes, and strings cannot be indexed by position.