gcd using args

gcd/src/main.rs

@@ -1,3 +1,8 @@
+// The below imported trait/extension is for adding a `from_str` method which will let us convert
+// strings to other types
+use std::env;
+use std::str::FromStr;
+
 fn gcd(mut a: u64, mut b: u64) -> u64 {
     while a > 0 {
         if a < b {
@@ -18,8 +23,22 @@ fn test_gcd() {
 }
 
 fn main() {
-    println!("gcd(4, 6) is {}", gcd(4, 6));
-    println!("gcd(7, 49) is {}", gcd(7, 49));
-    println!("gcd(3, 7) is {}", gcd(3, 7));
-    println!("gcd(128, 16) is {}", gcd(128, 16));
+    let mut numbers = Vec::new();
+
+    for arg in env::args().skip(1) {
+        numbers.push(u64::from_str(&arg).expect("error parsing argument"));
+    }
+
+    if numbers.len() == 0 {
+        eprintln!("Usage: gcd NUMBER ...");
+        std::process::exit(1);
+    }
+
+    let mut result = numbers[0];
+
+    for num in &numbers[1..] {
+        result = gcd(*num, result);
+    }
+
+    println!("The greatest common divisor of {:?} is {}", numbers, result);
 }