Rust - fn Example (Lookup Table)

Fn, function. In Rust the fn keyword can be used to declare a function. But we can also use fn in a type expression to store function pointers in a vector.

With a vec of function pointers, we can call functions based on an index. This eliminates the need for a match statement or if-else chain.

const fn

Example program. Consider this Rust example program—it has 4 functions declared at the top. In main, we create a Vec of function pointers.

And The "no_action" function is used for all 128 indexes. This function could be used to indicate no action is taken.

Vec push

Next The core, art and top functions are indexed by the first letters of their names.

Finally We loop over the bytes of the string "cat" and call the core, art and top functions by the byte values.

Loop, String Chars

fn no_action() -> &'static str {
    "no action"
}

fn core() -> &'static str {
    "core"
}

fn art() -> &'static str {
    "art"
}

fn top() -> &'static str {
    "top"
}

fn main() {
    // Create vector of function pointers.
    let mut lookup: Vec<fn() -> &'static str> = vec![];
    for _ in 0..128 {
        lookup.push(no_action);
    }
    // Store functions as function pointers at specific indexes.
    lookup[b'c' as usize] = core;
    lookup[b'a' as usize] = art;
    lookup[b't' as usize] = top;

    // Loop over this value and call function for each byte.
    let value = "cat";
    println!("VALUE: {}", value);
    for c in value.bytes() {
        println!("RESULT: {}, {}", c as char, lookup[c as usize]());
    }
}VALUE: cat
RESULT: c, core
RESULT: a, art
RESULT: t, top

Lookup table notes. It is possible to use a lookup table of function pointers to replace match or if-statements in Rust. This will affect performance and should be tested.

match

A summary. With a function pointer lookup table, we can develop a simple bytecode interpreter. Each byte can be handled with a function call based on the byte value.

Dot Net Perls is a collection of pages with code examples, which are updated to stay current. Programming is an art, and it can be learned from examples.

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This page was last updated on May 11, 2022 (edit link).

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