Filter items from a list

(define-read-only (filter-item (l (list 100 uint)) (remove uint))
  (get newList (fold remove-value l { compareTo: remove, newList: (list) }))
)

(define-private (remove-value (listValue uint) (trackerTuple { compareTo: uint, newList: (list 100 uint) }))
  (merge trackerTuple {newList:
    (if (is-eq listValue (get compareTo trackerTuple))
      (get newList trackerTuple)
      (unwrap-panic (as-max-len? (append (get newList trackerTuple) listValue) u100))
    )
  })
)

;; Example usage
(filter-item (list u1 u2 u3 u2 u4) u2) ;; Returns (u1 u3 u4)

Use cases

Removing blacklisted addresses from access lists
Filtering out completed tasks from todo lists
Excluding specific tokens from portfolios
Data cleanup in smart contracts

Key concepts

This pattern uses fold to iterate through a list and build a new list:

Accumulator: Tracks the value to remove and builds the new list
Conditional append: Only adds items that don't match the filter
Type safety: Maintains list type and maximum length

Filter multiple values

(define-read-only (filter-multiple (l (list 100 uint)) (removeList (list 10 uint)))
  (fold filter-item removeList l)
)

;; Example: Remove multiple values
(filter-multiple (list u1 u2 u3 u4 u5) (list u2 u4))
;; Returns (u1 u3 u5)

Filter with predicate function

(define-read-only (filter-greater-than (l (list 100 uint)) (threshold uint))
  (get newList 
    (fold filter-by-threshold l { threshold: threshold, newList: (list) })
  )
)

(define-private (filter-by-threshold (value uint) (tracker { threshold: uint, newList: (list 100 uint) }))
  (merge tracker {
    newList: (if (> value (get threshold tracker))
      (unwrap-panic (as-max-len? (append (get newList tracker) value) u100))
      (get newList tracker)
    )
  })
)

;; Keep only values greater than 10
(filter-greater-than (list u5 u15 u8 u20 u3) u10)
;; Returns (u15 u20)

Advanced filtering patterns

;; Filter principals from a list
(define-read-only (filter-principal 
  (principals (list 100 principal)) 
  (remove principal))
  (get newList 
    (fold remove-principal principals { 
      compareTo: remove, 
      newList: (list as principal) 
    })
  )
)

;; Filter with custom comparison
(define-read-only (filter-custom 
  (items (list 100 { id: uint, active: bool })) )
  (get newList 
    (fold filter-inactive items { newList: (list) })
  )
)

(define-private (filter-inactive 
  (item { id: uint, active: bool }) 
  (tracker { newList: (list 100 { id: uint, active: bool }) }))
  (merge tracker {
    newList: (if (get active item)
      (unwrap-panic (as-max-len? (append (get newList tracker) item) u100))
      (get newList tracker)
    )
  })
)

Performance note

Filtering creates a new list, so be mindful of gas costs with large lists. Consider storing data in maps if you need frequent filtering operations.

Filter items from a list

Use cases

Key concepts

Filter multiple values

Filter with predicate function

Advanced filtering patterns

Related snippets