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Update tables documentation (#15807)

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Added a case where a user might use mgetOrPut and create an accidental
copy of a seq.
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Thomas Tay 2020-11-02 01:33:16 -08:00 • committed by GitHub
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lib/pure/collections/tables.nim
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@ -460,6 +460,13 @@ proc mgetOrPut*[A, B](t: var Table[A, B], key: A, val: B): var B =
## Retrieves value at ``t[key]`` or puts ``val`` if not present, either way ## Retrieves value at ``t[key]`` or puts ``val`` if not present, either way
## returning a value which can be modified. ## returning a value which can be modified.
## ##
##
## Note that while the value returned is of type `var B`,
## it is easy to accidentally create an copy of the value at `t[key]`.
## Remember that seqs and strings are value types, and therefore
## cannot be copied into a separate variable for modification.
## See the example below.
##
## See also: ## See also:
## * `[] proc<#[],Table[A,B],A>`_ for retrieving a value of a key ## * `[] proc<#[],Table[A,B],A>`_ for retrieving a value of a key
## * `hasKey proc<#hasKey,Table[A,B],A>`_ ## * `hasKey proc<#hasKey,Table[A,B],A>`_
@ -474,6 +481,17 @@ proc mgetOrPut*[A, B](t: var Table[A, B], key: A, val: B): var B =
doAssert a.mgetOrPut('z', 99) == 99 doAssert a.mgetOrPut('z', 99) == 99
doAssert a == {'a': 5, 'b': 9, 'z': 99}.toTable doAssert a == {'a': 5, 'b': 9, 'z': 99}.toTable
# An example of accidentally creating a copy
var t = initTable[int, seq[int]]()
# In this example, we expect t[10] to be modified,
# but it is not.
var copiedSeq = t.mgetOrPut(10, @[10])
copiedSeq.add(20)
doAssert t[10] == @[10]
# Correct
t.mgetOrPut(25, @[25]).add(35)
doAssert t[25] == @[25, 35]
mgetOrPutImpl(enlarge) mgetOrPutImpl(enlarge)
proc len*[A, B](t: Table[A, B]): int = proc len*[A, B](t: Table[A, B]): int =
@ -944,6 +962,12 @@ proc mgetOrPut*[A, B](t: TableRef[A, B], key: A, val: B): var B =
## Retrieves value at ``t[key]`` or puts ``val`` if not present, either way ## Retrieves value at ``t[key]`` or puts ``val`` if not present, either way
## returning a value which can be modified. ## returning a value which can be modified.
## ##
## Note that while the value returned is of type `var B`,
## it is easy to accidentally create an copy of the value at `t[key]`.
## Remember that seqs and strings are value types, and therefore
## cannot be copied into a separate variable for modification.
## See the example below.
##
## See also: ## See also:
## * `[] proc<#[],TableRef[A,B],A>`_ for retrieving a value of a key ## * `[] proc<#[],TableRef[A,B],A>`_ for retrieving a value of a key
## * `hasKey proc<#hasKey,TableRef[A,B],A>`_ ## * `hasKey proc<#hasKey,TableRef[A,B],A>`_
@ -958,6 +982,16 @@ proc mgetOrPut*[A, B](t: TableRef[A, B], key: A, val: B): var B =
doAssert a.mgetOrPut('z', 99) == 99 doAssert a.mgetOrPut('z', 99) == 99
doAssert a == {'a': 5, 'b': 9, 'z': 99}.newTable doAssert a == {'a': 5, 'b': 9, 'z': 99}.newTable
# An example of accidentally creating a copy
var t = newTable[int, seq[int]]()
# In this example, we expect t[10] to be modified,
# but it is not.
var copiedSeq = t.mgetOrPut(10, @[10])
copiedSeq.add(20)
doAssert t[10] == @[10]
# Correct
t.mgetOrPut(25, @[25]).add(35)
doAssert t[25] == @[25, 35]
t[].mgetOrPut(key, val) t[].mgetOrPut(key, val)
proc len*[A, B](t: TableRef[A, B]): int = proc len*[A, B](t: TableRef[A, B]): int =