Rust: Folding a set of tuples

Question

I'm working on a pet project and I've found myself with a HashSet of tuples. Specifically

HashSet<(String,generic_array::GenericArray<u8,generic_array::typenum::U32>, u64)>

I'd like to sum the u64 element and can do so with a for loop without issue:

for element in hashset{
        sum = sum+element.2;
    }

However, I've come across the fold function for sets and I've written:

let y = hashset.fold(0, |sum x| sum+x)

Which works, but I'm unclear on the syntax of the |sum x|. I can infer that I'm simply naming variables, but I don't understand how I would expand on this. Also how does fold know which element of the tuple to operate on?

`|sum x|` is not valid Rust syntax. I invite you to write a proper [MCVE] of what you have. — E_net4, Nov 24 '17 at 11:17
Well... it compiles and runs so... ya. As for writing a proper example... I'm asking a question about that every segment of code which I don't understand. Rewriting something that I don't understand probably won't result in a better code segment. — Darakian, Nov 24 '17 at 11:25
That's not how it works. It's the asker's job to provide a minimal, yet reproducible example of what you are experiencing. I'm telling you that what you put in the question should not compile, so what you have in the question right now _is insufficient_ for us to understand the problem. Whether you don't understand it either is usually not relevant, working on a [MCVE] is part of the asking process. Also, please include your compiler's version and toolchain. — E_net4, Nov 24 '17 at 11:29

score 6 · Accepted Answer · edited Nov 24 '17 at 13:13

You really should show a minimal, complete, and verifiable example.

First, I'd suggest you read the Rust book. It is really good and will show you the basics of Rust, especially iterators, which seems to be your problem here.

Also, the documentation of the fold function might help you a lot.

I simplified your example from above, because I do not know about the generic_array::GenericArray<u8, generic_array::typenum::U32> type, so I removed it, assuming you want to summarize the u64 from the tuple:

use std::collections::HashSet;

#[allow(unused_variables)]
fn main() {
    let values: HashSet<(String, u64)> = HashSet::new();
    let sum = values.iter().fold(0, |acc, x| acc + x.1);
}

The x.1 syntax accesses the second element (counting starts at zero) of the tuple. That'd be x.2 in your case, because you have three elements. The fold method takes an initial value (0 in my example`) and a closure.

The closure in your example is not valid Rust code (it is missing the comma between the arguments) and it therefor won't compile. Go ahead and read how the Iterator::fold function works, you'll understand then how this works.

Got it. I was unsure if x was itself an element of the tuple or if it was the tuple itself. Thank you! — Darakian, Nov 24 '17 at 12:50

Rust: Folding a set of tuples

1 Answers1