Rust で JSON ファイルを読む

フォルダー構造

$ tree -L 2
.
├── Cargo.lock
├── Cargo.toml
├── cities.json
├── src
│   └── main.rs
└── target
    └── debug

Cargo.toml

[package]
name = "json_read"
version = "0.1.0"
edition = "2018"

#

[dependencies]
serde = { version = "1.0", features = ["derive"] }
serde_json = "1.0"

src/main.rs

// --------------------------------------------------------------------
/*
	json_read.rs

						Jul/15/2020
*/
// --------------------------------------------------------------------
use std::env;
use std::fs::File;
use std::io::BufReader;

// --------------------------------------------------------------------
fn main() -> std::io::Result<()> {
	eprintln!("*** 開始 ***");

	let args: Vec<_> = env::args().collect();
    
	let ref fname_in = args[1];

	let file = File::open(fname_in).unwrap();
	let reader = BufReader::new(file);

	let json: serde_json::Value = serde_json::from_reader(reader)?;

	let obj = json.as_object().unwrap();

	for (key,value) in obj.iter() {
		println!("{}\t{}\t{}\t{}",key,value["name"],
			value["population"],value["date_mod"]);
		}

	eprintln!("*** 終了 ***");

	Ok(())
}

// --------------------------------------------------------------------

cities.json

{
  "t0921": {
    "name": "宇都宮",
    "population": 41295,
    "date_mod": "2003-8-12"
  },
  "t0922": {
    "name": "小山",
    "population": 38756,
    "date_mod": "2003-5-15"
  },
  "t0923": {
    "name": "佐野",
    "population": 71294,
    "date_mod": "2003-6-8"
  },
  "t0924": {
    "name": "足利",
    "population": 27138,
    "date_mod": "2003-7-21"
  },
  "t0925": {
    "name": "日光",
    "population": 74682,
    "date_mod": "2003-4-19"
  },
  "t0926": {
    "name": "下野",
    "population": 82951,
    "date_mod": "2003-10-14"
  }
}

コンパイル

cargo build

実行

$ cargo run cities.json 
    Finished dev [unoptimized + debuginfo] target(s) in 0.03s
     Running `target/debug/read cities.json`
*** 開始 ***
t0921	"宇都宮"	41295	"2003-8-12"
t0922	"小山"	38756	"2003-5-15"
t0923	"佐野"	71294	"2003-6-8"
t0924	"足利"	27138	"2003-7-21"
t0925	"日光"	74682	"2003-4-19"
t0926	"下野"	82951	"2003-10-14"
*** 終了 ***

確認したバージョン

$ rustc --version
rustc 1.75.0 (82e1608df 2023-12-21) (built from a source tarball)