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-- Status: rewritten July 2018
-- Author: Lily Silverstein
doc///
Key
position
(position, ZZ, Function)
(position, VisibleList, Function)
(position, VisibleList, VisibleList, Function)
[position, Reverse]
Headline
the first element of a list satisfying a condition
Usage
position(A, f)
position(A, B, f)
position(A, f, Reverse => true)
position(n, f)
Inputs
A: VisibleList
B: VisibleList
n: ZZ
f: Function
Outputs
p: ZZ
the first index to satisfy the boolean function {\tt f}
Description
Text
{\tt position(A, f)} returns the smallest index {\tt i} such that {\tt f(A#i)}
is true. If no element satisfies the condition, @TO null@ is returned.
Example
position((10,20,43,105,6,93), odd)
position((10,20,43,105,6,93), i -> i<0)
Text
Use {\tt position(A, B, f)} to return the smallest index {\tt i} such that {\tt f(A#i, B#i)} is true.
Example
position((10,20,43,105,6,93),(18,82,12,7,35,92), (a,b) -> a>b)
Text
The {\tt Reverse} option will return the largest index instead.
Example
position((10,20,43,105,6,93), odd, Reverse => true)
position((10,20,43,105,6,93),(18,82,12,7,35,92), (a,b) -> a>b, Reverse => true)
Text
Use {\tt position(n, f)} to return the smallest index {\tt i} such that {\tt f(i)} is true.
Example
position(17, i -> i^2 % 17 == 13)
Text
To find all indices of elements satisfying the condition, see @TO positions@. To return the
elements, rather than their indices, see @TO select@. The function @TO number@ counts the
number of elements satisfying the condition.
Example
positions((10,20,43,105,6,93), odd)
select((10,20,43,105,6,93), odd)
number((10,20,43,105,6,93), odd)
SeeAlso
minPosition
maxPosition
number
positions
select
take
"lists and sequences"
///
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