ch02 up through handling command line arguments

.gitignore

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+**/target
+**/.idea/*
+**/.vscode/*
+**/.DS_Store

ch02-a-tour-of-rust/gcd/Cargo.toml

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+[package]
+name = "gcd"
+version = "0.1.0"
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

ch02-a-tour-of-rust/gcd/src/main.rs

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+use std::env;
+use std::str::FromStr;
+
+fn main() {
+    let mut numbers = Vec::new();
+    for arg in env::args().skip(1) {
+        numbers.push(u64::from_str(&arg).expect("error parsing integer argument"));
+    }
+    if numbers.len() == 0 {
+        eprintln!("Usage: gcd NUMBER ...");
+        std::process::exit(1);
+    }
+
+    let mut d = numbers[0];
+    for m in &numbers[1..] {
+        d = gcd(d, *m);
+    }
+    println!("The greatest common divisor of {:?} is  {}", numbers, d);
+}
+
+fn gcd(mut m: u64, mut n: u64) -> u64 {
+    assert!(n != 0 && m != 0);
+    while m != 0 {
+        if m < n {
+            let t = m;
+            m = n;
+            n = t;
+        }
+        m = m % n;
+    }
+    n
+}
+
+#[test]
+fn test_gcd() {
+    assert_eq!(gcd(14, 15), 1);
+    assert_eq!(gcd(2 * 3 * 5 * 11 * 17, 3 * 7 * 11 * 13 * 19), 3 * 11,);
+}