a toy example of CSV wordcount.

Cargo.toml

[package]
name = "hello"
version = "0.1.0"
authors = ["P. Oscar Boykin <boykin@pobox.com>"]
edition = "2018"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
csv = "1.1"
serde = { version = "1.0.114", features = ["derive"] }
~

rust_wordcount.rs

use std::error::Error;
use std::io;
use std::process;
use std::collections::HashMap;

use serde::Deserialize;

#[derive(Debug, Deserialize)]
struct Record {
    key: String,
    value: Option<i64>,
}

fn example() -> Result<(), Box<dyn Error>> {
    let mut map: HashMap<String, i64> = HashMap::new();
    let mut rdr = csv::Reader::from_reader(io::stdin());
    for result in rdr.deserialize() {
        // Notice that we need to provide a type hint for automatic
        // deserialization.
        let record: Record = result?;
        if let Some(v) = record.value {
            match map.get_mut(&record.key) {
              Some(v0) => {
                *v0 += v;
              },
              None => {
                map.insert(record.key, v);
              }
            }
        }
    }

    println!("{:?}", map);
    Ok(())
}

fn main() {
    if let Err(err) = example() {
        println!("error running example: {}", err);
        process::exit(1);
    }
}