how do I write column oriented data to a CSV format?

[hanusek]

Hello.
I would like to use the library to create a CSV file from HashMap<String, Vec<i64>>. Where String is the header name and Vec is a vector with values in a column.

[BurntSushi]

CSV data is written one row at a time. That's a fundamental fact about the CSV format and there is no way to change it. Thus, if your data is arranged by column and you need to format it as CSV data, then the only choice available to you is to convert your data such that it is arranged by row in some way.

Here's one somewhat simplistic example that will write a HashMap<String, Vec<i64>> as CSV data:

use std::collections::HashMap;

fn main() -> anyhow::Result<()> {
    let mut data = HashMap::new();
    data.insert("field1".to_string(), vec![1, 2, 3, 4, 5]);
    data.insert("field2".to_string(), vec![10, 20, 30, 40, 50]);
    data.insert("field3".to_string(), vec![100, 200, 300, 400, 500]);
    write_column_data(&data)?;
    Ok(())
}

fn write_column_data(
    columns: &HashMap<String, Vec<i64>>,
) -> anyhow::Result<()> {
    let mut fields: Vec<&str> = columns.keys().map(|k| k.as_str()).collect();
    fields.sort();
    let mut iters: Vec<std::slice::Iter<'_, i64>> =
        fields.iter().map(|f| columns[*f].iter()).collect();

    let mut wtr = csv::Writer::from_writer(std::io::stdout().lock());
    wtr.write_record(fields.iter())?;
    'OUTER: loop {
        for it in iters.iter_mut() {
            let Some(value) = it.next() else { break 'OUTER };
            wtr.write_field(value.to_string())?;
        }
        wtr.write_record(None::<&[u8]>)?;
    }
    wtr.flush()?;
    Ok(())
}

And the Cargo.toml:

[package]
publish = false
name = "d340"
version = "0.1.0"
edition = "2021"

[dependencies]
anyhow = "1.0.75"
csv = "1.3.0"

[[bin]]
name = "d340"
path = "main.rs"

And running the program and its output:

$ cargo run --quiet
field1,field2,field3
1,10,100
2,20,200
3,30,300
4,40,400
5,50,500

Note that this program assumes every column has the same length. If that isn't true the code above might not work as you expect it will.