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// usage: cargo run --release --example slab
use std::collections::BTreeMap;
use fasteval::Evaler; // use this trait so we can call eval().
fn main() -> Result<(), fasteval::Error> {
let parser = fasteval::Parser::new();
let mut slab = fasteval::Slab::new();
// See the `parse` documentation to understand why we use `from` like this:
let expr_ref = parser.parse("x + 1", &mut slab.ps)?.from(&slab.ps);
// Let's evaluate the expression a couple times with different 'x' values:
let mut map : BTreeMap<String,f64> = BTreeMap::new();
map.insert("x".to_string(), 1.0);
let val = expr_ref.eval(&slab, &mut map)?;
assert_eq!(val, 2.0);
map.insert("x".to_string(), 2.5);
let val = expr_ref.eval(&slab, &mut map)?;
assert_eq!(val, 3.5);
// Now, let's re-use the Slab for a new expression.
// (This is much cheaper than allocating a new Slab.)
// The Slab gets cleared by 'parse()', so you must avoid using
// the old expr_ref after parsing the new expression.
// One simple way to avoid this problem is to shadow the old variable:
let expr_ref = parser.parse("x * 10", &mut slab.ps)?.from(&slab.ps);
let val = expr_ref.eval(&slab, &mut map)?;
assert_eq!(val, 25.0);
Ok(())
}
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