Iterator Pipelines

Fold Count

Accumulate a Derived Value

fold carries an accumulator through every item that reaches the end of an iterator pipeline.

Program

Play the program to choose whether zero readings should count as included readings.

fold_count.rs 
fn main() {
    let include_zero = ;
    let readings = [3, 0, 4];
    let count = readings.iter().copied().filter(|value| include_zero || *value != 0).fold(0, |acc, _value| acc + 1);
    println!("count={count}");
}

fn main() {
    let include_zero = ;
    let readings = [3, 0, 4];
    let count = readings.iter().copied().filter(|value| include_zero || *value != 0).fold(0, |acc, _value| acc + 1);
    println!("count={count}");
}
lazy pipeline The filter and fold stages run only when the final count is requested.
fold `fold(0, |acc, _value| acc + 1)` increments the accumulator for each included item.
predicate `include_zero || *value != 0` changes which values reach the fold.