Iterator Filtering

Rust's iterator system provides powerful filtering methods that allow you to selectively process elements from a sequence. These methods are essential for data processing, validation, and transformation pipelines.

Core Filtering Methods

filter()

The filter() method retains only elements that satisfy a predicate:

let numbers = vec![1, 2, 3, 4, 5, 6];
 
// Keep only even numbers
let evens: Vec<i32> = numbers
    .iter()
    .filter(|&x| x % 2 == 0)
    .cloned()
    .collect();
assert_eq!(evens, vec![2, 4, 6]);
 
// Keep only numbers greater than 3
let large: Vec<i32> = numbers
    .iter()
    .filter(|&x| *x > 3)
    .cloned()
    .collect();
assert_eq!(large, vec![4, 5, 6]);

filter_map()

The filter_map() method combines filtering and mapping in one step. It applies a function that returns Option<T> and keeps only the Some values:

let strings = vec!["1", "two", "3", "four", "5"];
 
// Parse to integers, keeping only successful parses
let numbers: Vec<i32> = strings
    .iter()
    .filter_map(|s| s.parse::<i32>().ok())
    .collect();
assert_eq!(numbers, vec![1, 3, 5]);

take_while() and skip_while()

These methods process elements based on a condition at the beginning of the sequence:

let numbers = vec![1, 2, 3, 4, 5, 1, 2];
 
// Take elements while they're less than 4
// (stops at first failure)
let taken: Vec<i32> = numbers
    .iter()
    .take_while(|&&x| x < 4)
    .cloned()
    .collect();
assert_eq!(taken, vec![1, 2, 3]);
 
// Skip elements while they're less than 4
// (takes the rest after first failure)
let skipped: Vec<i32> = numbers
    .iter()
    .skip_while(|&&x| x < 4)
    .cloned()
    .collect();
assert_eq!(skipped, vec![4, 5, 1, 2]);

Note that take_while and skip_while only check the condition at the beginning - once the condition fails (for take_while) or succeeds (for skip_while), the behavior changes for all remaining elements.

Your Task

Implement the following functions that demonstrate iterator filtering patterns:

  1. filter_even(numbers: &[i32]) -> Vec<i32> - Return only even numbers
  2. filter_by_predicate<F>(numbers: &[i32], predicate: F) -> Vec<i32>
    • Return numbers matching a custom predicate (where F: Fn(&i32) -> bool)
  3. parse_valid_numbers(strings: &[&str]) -> Vec<i32>
    • Parse strings to integers, keeping only valid parses
  4. filter_map_with<T, U, F>(items: &[T], f: F) -> Vec<U>
    • Generic filter_map (where T: Clone, F: Fn(T) -> Option<U>)
  5. take_while_positive(numbers: &[i32]) -> Vec<i32>
    • Take numbers while they're positive (> 0)
  6. skip_while_negative(numbers: &[i32]) -> Vec<i32>
    • Skip numbers while they're negative (< 0)
  7. filter_in_range(numbers: &[i32], min: i32, max: i32) -> Vec<i32> - Keep numbers within range [min, max] inclusive
  8. first_matching<T: Clone, F>(items: &[T], predicate: F) -> Option<T>
    • Find first element matching predicate (where F: Fn(&T) -> bool)

Examples

 
// filter_even
assert_eq!(filter_even(&[1, 2, 3, 4, 5, 6]), vec![2, 4, 6]);
 
// filter_by_predicate
assert_eq!(
    filter_by_predicate(&[1, 2, 3, 4, 5], |&x| x > 3),
    vec![4, 5]
);
 
// parse_valid_numbers
assert_eq!(
    parse_valid_numbers(&["1", "hello", "3", "world"]),
    vec![1, 3]
);
 
// filter_map_with
let doubled: Vec<i32> = filter_map_with(
    &[1, 2, 3],
    |x| if x % 2 == 0 { Some(x * 2) } else { None }
);
// Only 2 is even, doubled to 4
assert_eq!(doubled, vec![4]);
 
// take_while_positive
assert_eq!(
    take_while_positive(&[3, 5, -1, 2, 4]),
    vec![3, 5]
);
 
// skip_while_negative
assert_eq!(
    skip_while_negative(&[-3, -1, 2, -4, 5]),
    vec![2, -4, 5]
);
 
// filter_in_range
assert_eq!(
    filter_in_range(&[1, 5, 10, 15, 20], 5, 15),
    vec![5, 10, 15]
);
 
// first_matching
assert_eq!(
    first_matching(&[1, 2, 3, 4, 5], |&x| x > 3),
    Some(4)
);
assert_eq!(
    first_matching(&[1, 2, 3], |&x| x > 10),
    None
);

Hints
Click here for hints
  • filter() takes a closure that receives &&T when iterating over &[T], so use |&&x| or |&x| patterns
  • filter_map() combines filter and map - return Some(value) to keep, None to skip
  • take_while() stops at the first element that doesn't match the predicate
  • skip_while() skips until predicate fails, then takes all remaining elements
  • Use .cloned() to convert from &T to T when collecting
  • For first_matching, consider using .find() or .filter().next()
  • Remember that parse::<i32>() returns a Result, use .ok() to convert to Option