Confusing Output to a HashMap iterable from input in Rust

Question

This program takes as input a sentence " Add name to dept", passes the string into a function that splits the string by whitespace into a vector that then is inserted into a hashmap that is suppose to retain the values of name and dept, which it does for 1 input. On the second input only the first word "Add" is printed. Are there any glaring missteps that may cause this odd output?

use std::io; use std::collections::HashMap; fn main() { let mut input = String::new(); let mut db: HashMap<String,String> = HashMap::new(); loop { println!("'Add <name> to <department>'"); io::stdin().read_line(&mut input).expect("Not input"); add_to_hashmap(&input, &mut db); } } fn add_to_hashmap(input: &String, db: &mut HashMap<String,String>){ let v: Vec<&str> = input.split(" ").collect(); db.insert(v[1].to_string(),v[3].to_string()); for (name, dept) in db{ println!("{}, {}", name, dept); } }

John Kugelman · Accepted Answer · 2022-01-04 01:50:00Z

Investigation

To diagnose this I added a dbg! call to check the value of input each time add_to_hashmap is called.

let v: Vec<&str> = dbg!(input).split(" ").collect();

The first time it prints:

'Add <name> to <department>' Add john to math [src/main.rs:13] input = "Add john to math\n" john, math

The second time:

'Add <name> to <department>' Add bill to science [src/main.rs:13] input = "Add john to math\nAdd bill to science\n" john, math

Diagnosis

input isn't being cleared. read_line doesn't erase the input buffer; it just appends to it.

In the documentation you can see that the example code clears the buffer after each call:

use std::io::{self, BufRead}; let mut cursor = io::Cursor::new(b"foo\nbar"); let mut buf = String::new(); // cursor is at 'f' let num_bytes = cursor.read_line(&mut buf) .expect("reading from cursor won't fail"); assert_eq!(num_bytes, 4); assert_eq!(buf, "foo\n"); buf.clear(); // cursor is at 'b' let num_bytes = cursor.read_line(&mut buf) .expect("reading from cursor won't fail"); assert_eq!(num_bytes, 3); assert_eq!(buf, "bar"); buf.clear(); // cursor is at EOF let num_bytes = cursor.read_line(&mut buf) .expect("reading from cursor won't fail"); assert_eq!(num_bytes, 0); assert_eq!(buf, "");

Thank You so much for taking the time. I'm new to Rust and now the program works perfectly! On to the next chapter....

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