File reading with standard fs API

[learn-rust.git] / src / main.rs

diff --git a/src/main.rs b/src/main.rs
index 058f8584cf31063cdef5dc9cd61a9daf38bfce00..c07c4051165861365a2176a3c1edec6957d16903 100644 (file)
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,8 +1,12 @@
 mod word_utils;
 
+use std::collections::HashMap;
+use std::io::{prelude::*, ErrorKind};
+use std::fs::{self, File};
+
 use word_utils::{first_word, second_word};
 
-fn main() {
+fn test_strings() {
     let s = String::from("Hello world");
     println!("first word: '{}'", first_word(&s));
     println!("second word: '{}'", second_word(&s).unwrap());
@@ -15,3 +19,106 @@ fn main() {
         None => println!("match: second word of '{}' does not exist", s2),
     }
 }
+
+fn test_vectors() {
+    let v1 = vec![1, 2, 3];
+    println!("statically initialized vector: {:?}", v1);
+
+    let mut v2: Vec<String> = Vec::new();
+    v2.push("Hello".to_string());
+    v2.push(String::from("world"));
+    println!("dynamically built vector: {:?}", v2);
+    println!("first element: {}", v2[0]);
+    for el in &mut v2 {
+        *el += "xx";
+    }
+    for el in &v2 {
+        println!("{}", el);
+    }
+}
+
+fn test_hashmaps() {
+    let mut scores = HashMap::new();
+    scores.insert("john", 10);
+    scores.insert("mary", 20);
+
+    println!("scores: {:?}", scores);
+
+    // hash map with .collect()
+    let persons = vec![("homer", 42), ("marge", 30)];
+    let collect_scores: HashMap<_, _> = persons.into_iter().collect();
+    println!("collect_scores: {:?}", collect_scores);
+
+    for (p, s) in &collect_scores {
+        println!("person {}: score {}", p, s);
+    }
+
+    println!("john's score: {}", scores.get("john").unwrap());
+    println!("jake's score: {}", scores.get("jake").unwrap_or(&-1));
+
+    // double scores
+    for (_, v) in scores.iter_mut() {
+        *v *= 2;
+    }
+    println!("scores after doubling: {:?}", scores);
+
+    // double scores of immutable hashmap (rebuild it)
+    let collect_scores: HashMap<_, _> = collect_scores.iter()
+        .map(|(k, v)| (k, 2 * v))
+        .collect();
+    println!("collect_scores after rebuilding with doubling: {:?}", collect_scores);
+}
+
+fn read_file(path: &str) -> Result<String, std::io::Error> {
+    let mut s = String::new();
+    File::open(path)?
+        .read_to_string(&mut s)?;
+    Ok(s)
+}
+
+fn test_files() {
+    if let Ok(mut f) = File::open("Cargo.toml") {
+        let mut contents = String::new();
+        match f.read_to_string(&mut contents) {
+            Ok(len) => println!("successfully opened Cargo.toml: {:?}, contents {} bytes:\n{}\n----------", f, len, contents),
+            Err(e) => panic!("could not read file: {:?}", e)
+        }
+    } else {
+        println!("could not open Cargo.toml");
+    }
+
+    // alternative form, more specific error checking
+    let mut f = File::open("Cargo.toml").unwrap_or_else(|e| {
+        if e.kind() == ErrorKind::NotFound {
+            println!("Cargo.toml not found, falling back to /dev/null");
+            // need to return a File
+            File::open("/dev/null").unwrap()
+        } else {
+            panic!("Could not open Cargo.toml: {:?}", e);
+        }
+    });
+    let mut contents = String::new();
+    let len = f.read_to_string(&mut contents).unwrap_or_else(|e| {
+        panic!("Could not read file: {:?}", e);
+    });
+    println!("successfully opened Cargo.toml with unwrap_or_else: {:?}, contents {} bytes:\n{}\n----------", f, len, contents);
+
+    // using the '?' operator
+    match read_file("Cargo.toml") {
+        Ok(s) => println!("Cargo.toml contents:\n{}\n-------------", s),
+        Err(e) => println!("Could not open Cargo.toml: {:?}", e)
+    }
+
+    // using std API
+    match fs::read_to_string("Cargo.toml") {
+        Ok(s) => println!("Cargo.toml contents:\n{}\n-------------", s),
+        Err(e) => println!("Could not open Cargo.toml: {:?}", e)
+    }
+}
+
+fn main() {
+    test_strings();
+    test_vectors();
+    test_hashmaps();
+    test_files();
+}