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Weight-balanced trees support operations that view the tree as sorted sequence of associations. Elements of the sequence can be accessed by position, and the position of an element in the sequence can be determined, both in logarthmic time.

- procedure:
**wt-tree/index***wt-tree index*¶ - procedure:
**wt-tree/index-datum***wt-tree index*¶ - procedure:
**wt-tree/index-pair***wt-tree index*¶ Returns the 0-based

`index`th association of`wt-tree`in the sorted sequence under the tree’s ordering relation on the keys.`wt-tree/index`

returns the`index`th key,`wt-tree/index-datum`

returns the datum associated with the`index`th key and`wt-tree/index-pair`

returns a new pair`(`

which is the`key`.`datum`)`cons`

of the`index`th key and its datum. The average and worst-case times required by this operation are proportional to the logarithm of the number of associations in the tree.These operations signal a condition of type

`condition-type:bad-range-argument`

if`index``<0`

or if`index`is greater than or equal to the number of associations in the tree. If the tree is empty, they signal an anonymous error.Indexing can be used to find the median and maximum keys in the tree as follows:

median: (wt-tree/index

`wt-tree`(quotient (wt-tree/size`wt-tree`) 2)) maximum: (wt-tree/index`wt-tree`(- (wt-tree/size`wt-tree`) 1))

- procedure:
**wt-tree/rank***wt-tree key*¶ Determines the 0-based position of

`key`in the sorted sequence of the keys under the tree’s ordering relation, or`#f`

if the tree has no association with for`key`. This procedure returns either an exact non-negative integer or`#f`

. The average and worst-case times required by this operation are proportional to the logarithm of the number of associations in the tree.

- procedure:
**wt-tree/min***wt-tree*¶ - procedure:
**wt-tree/min-datum***wt-tree*¶ - procedure:
**wt-tree/min-pair***wt-tree*¶ Returns the association of

`wt-tree`that has the least key under the tree’s ordering relation.`wt-tree/min`

returns the least key,`wt-tree/min-datum`

returns the datum associated with the least key and`wt-tree/min-pair`

returns a new pair`(key . datum)`

which is the`cons`

of the minimum key and its datum. The average and worst-case times required by this operation are proportional to the logarithm of the number of associations in the tree.These operations signal an error if the tree is empty. They could have been written

(define (wt-tree/min tree) (wt-tree/index tree 0)) (define (wt-tree/min-datum tree) (wt-tree/index-datum tree 0)) (define (wt-tree/min-pair tree) (wt-tree/index-pair tree 0))

- procedure:
**wt-tree/delete-min***wt-tree*¶ Returns a new tree containing all of the associations in

`wt-tree`except the association with the least key under the`wt-tree`’s ordering relation. An error is signalled if the tree is empty. The average and worst-case times required by this operation are proportional to the logarithm of the number of associations in the tree. This operation is equivalent to(wt-tree/delete

`wt-tree`(wt-tree/min`wt-tree`))

- procedure:
**wt-tree/delete-min!***wt-tree*¶ Removes the association with the least key under the

`wt-tree`’s ordering relation. An error is signalled if the tree is empty. The average and worst-case times required by this operation are proportional to the logarithm of the number of associations in the tree. This operation is equivalent to(wt-tree/delete!

`wt-tree`(wt-tree/min`wt-tree`))

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