[][src]Crate cruncher

Cruncher, a crate for dynamic evaluation of mathematical expressions.

This crate provide run-time evaluation of mathematical expressions, embedded in strings. The easiest way to use this crate is with the eval function:

assert_eq!(cruncher::eval("3 + 5 * 2", None), Ok(13.0));

The second argument to eval is a HashMap, that can define variables:

use hashbrown::HashMap;

let mut context :HashMap<String,f64> = HashMap::new();
context.insert("a".into(), 3.5);
assert_eq!(cruncher::eval("2 * a", &context), Ok(7.0));

It is also possible to separate the parsing from the evaluation of an expression with the Expr type. This allow to reuse the same expression with different values for variables.

use hashbrown::HashMap;
use cruncher::{Expr};

let expr = Expr::parse("3 + 5 * 2").unwrap();
assert_eq!(expr.eval(None), Ok(13.0));

let expr = Expr::parse("3 / c + b").unwrap();
let mut context :HashMap<String,f64> = HashMap::new();
context.insert("c".into(), 1.0);
context.insert("b".into(), 5.0);
assert_eq!(expr.eval(&context), Ok(8.0));

context.insert("b".into(), 10.0);
assert_eq!(expr.eval(&context), Ok(13.0));

Language definition

The language implemented by cruncher can contain the following elements:

  • float literal values: -12.456, +0.0045e78, ...;
  • left and right parenthesis;
  • mathematical operators: + for addition, - for subtraction, * for multiplication, / for division and ^ for exponentiation (std::f64::powf);
  • variables. Variables names are ASCII only, and can start by a letter or _, and can contain letters, digits, ., _, [ or ].
  • function call: sin(a), atan(22.0). The following function are accessible, with the same meaning as the corresponding std::f64 function: sqrt, cbrt, sin, cos, tan, asin, acos, atan, sinh, cosh, tanh, asinh, acosh, atanh, floor, ceil, abs, exp, ln, log2, log10.

Any other symbol is forbidden in the input.

The mathematical operators obey the usual relations of associativity and precedence, but still carry the floating point properties: addition is not commutative, NaN and infinities exist, ...

Technical details

cruncher is based on an AST interpreter, and uses a simple Shuntting-Yard algorithm for parsing the expressions. It works only with f64 data, and perform a simple constant propagation to optimize the expressions.


pub use expr::eval;
pub use expr::Expr;





Error type for the caldyn crate



Check if ident is a valid variable name