This is my first attempt at some moderately complex Rust code. Unfortunately, it feels very clunky to me, and I feel as if I am not using all of the features properly:

use std::collections::HashMap;

fn main() {
    let mut table: Vec<HashMap<&str, Vec<&str>>> = Vec::new();
    table[0].insert(" ", vec!["F"]);
    table[0].insert("(", vec!["-1"]);
    table[0].insert(")", vec!["-1"]);
    table[0].insert("+", vec!["-1"]);
    table[0].insert("a", vec!["a"]);

    table[1].insert(" ", vec!["S"]);
    table[1].insert("(", vec!["(", "S", "+", "F", ")"]);
    table[1].insert(")", vec!["-1"]);
    table[1].insert("+", vec!["-1"]);
    table[1].insert("a", vec!["F"]);

    let mut input = vec!["(", "a", "+", "a", ")"];

    parse_func(&mut table, &mut input);

fn parse_func(table: &mut Vec<HashMap<&str, Vec<&str>>>, input: &mut Vec<&str>) {
    let mut stack: Vec<&str> = Vec::new();
    for c in input.iter() {
        loop {
            println!("{:?}", stack);
            let rulenum: usize = match *c {
                "$" => return,
                sval if (sval == *stack.last().unwrap()) => {stack.pop(); break;},
                _ => evaluate_table(table, c, vec![stack.last().unwrap()]).expect("Bad input"),
            for item in table.get(rulenum).unwrap().get(c).unwrap().iter().rev() {

fn evaluate_table(table: &Vec<HashMap<&str, Vec<&str>>>, c: &str, nterminal: Vec<&str>) -> Option<usize> {
    for (i, row) in table.iter().enumerate() {
        if *row.get(" ").unwrap() == nterminal {
            return match row.get(c).unwrap() {
                k if (*k == vec!["-1"]) => None,
                _ => Some(i),
  • 1
    \$\begingroup\$ Welcome to Code Review. Please tell us about the language being parsed. \$\endgroup\$ Feb 12 '18 at 2:26
  • \$\begingroup\$ Its just context free grammar. the specific example was taken from the wikipedia page on ll parsing: en.wikipedia.org/wiki/LL_parser#Parsing_procedure \$\endgroup\$
    – hLk
    Feb 12 '18 at 2:47

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