How to convert a Vec<Vec<f64>> into a string

Question

I am new to Rust, and I am struggling with a simple task. I'd like to convert a matrix into a string, with the fields separated by tabs. I think this is possible by using the map function or something similar, but right now whatever I try gives me an error.

This is what I have, and I'd like to convert the col part into function, which returns a tab separated string, which I can print. In Python this is something like row.join("\t"). Is there something similar in Rust?

fn print_matrix(vec: &Vec<Vec<f64>>) { for row in vec.iter() { for col in row.iter() { print!("\t{:?}",col); } println!("\n"); } }

What part of this doesn't work? Can you provide a minimal, complete, example on the Playpen? There doesn't seem to be anything wrong with this code. — Simon Whitehead
– Simon Whitehead, Commented Mar 20, 2016 at 8:56
it should print string<tab>string<tab>string but it prints a leading tab. there must be a more elegant solution than that, without fiddling — Mandragor
– Mandragor, Commented Mar 20, 2016 at 9:09
Look at the examples under doc.rust-lang.org/std/fmt/#format_args — starblue
– starblue, Commented Mar 20, 2016 at 9:28

Lukas Kalbertodt · Accepted Answer · 2016-03-20 10:31:44Z

There is indeed a join in the standard library, but it's not super useful (often additional allocation is required). But you can see a solution here:

fn print_matrix(vec: &Vec<Vec<f64>>) { for row in vec { let cols_str: Vec<_> = row.iter().map(ToString::to_string).collect(); let line = cols_str.join("\t"); println!("{}", line); } }

The problem is that this join works with slices and not with iterators. We have to convert all elements into a string first, collect the result in a new vector and can use join then.

The crate itertools defines a join method for iterators and can be applied like so:

for row in vec { let line = row.iter().join("\t"); println!("{}", line); }

And to avoid using any of the named functionality, you can of course do it manually:

for row in vec { if let Some(first) = row.get(0) { print!("{}", first); } for col in row.iter().skip(1) { print!("\t{}", col); } println!(""); }

swizard · Accepted Answer · 2016-03-20 10:34:30Z

Besides join from itertools you could always use fold on iterator (which is really useful), like this:

row.iter().fold("", |tab, col| { print!("{}{:?}", tab, col); "\t" });

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