read_to_string reads a file during program execution and places its text into a String. The filesystem version takes a path; the File method fills a mutable String supplied by the program. section 18.3.2 section 18.3.3
use std::fs; use std::fs::File; use std::io::Read; #1 fn main() -> std::io::Result<()> { fs::write("calvin_with_dad.txt", "Calvin: Dad, how come old photographs are always black and white? Didn't they have color film back then? Dad: Sure they did. In fact, those photographs are in color. It's just the world was black and white then.")?; let mut calvin_file = File::open("calvin_with_dad.txt")?; #2 let mut calvin_string = String::new(); #3 calvin_file.read_to_string(&mut calvin_string)?; #4 calvin_string.split_whitespace().for_each(|word| print!("{} ", word.to_uppercase())); #5 Ok(()) } #1 This is to use the function .read_to_string(). #2 Opens the file we just made #3 This String will hold the contents of the file. #4 Reads the file into the String using the read_to_string method #5 Now that we have it as a String, we’ll capitalize the whole thing just for fun.
A complete example creates a text file, opens it, reads it into calvin_string, and prints the words in uppercase. section 18.3.2
A typical read_to_string workflow is:
- Import the filesystem function or the Read trait needed for the File method. section 18.3.2
- Provide a path to std::fs::read_to_string, or open a File and pass a mutable String to read_to_string. section 18.3.2 section 23.2.2
- Handle the returned error with ? or unwrap before using the resulting text. section 18.3.3
Loading text for a typing exercise
let file_content = read_to_string("typing.txt").unwrap(); loads the target text into a String, which the program prints beside the user's typed input. section 23.2.2
Unlike include_str!, read_to_string accesses the file at runtime, so the file must be available then and errors must be handled. The runtime approach keeps the file out of the compiled binary. section 18.3.3
