SubIndex

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SubIndex(a, u, i)

Returns the value of index «i» for which array «a» (indexed by «i») is equal to «u». If «u» does not occur in «a», returns Null. If «u» occurs more than once in «a», it returns the last value of «i» that is equal to «u». If «a» has index(es) in addition to «i», or if the value «u» is an array with other indexes, those indexes also appear in the result.

SubIndex is the inverse of SubScript. While SubScript maps from the index value to the array element, SubIndex maps from the array element to the index value. SubIndex is not as efficient as SubScript -- while SubScript makes use of a hash-table for constant-time associative lookup, SubIndex must search the array each time for the indicated value, which takes linear time relative to the size of the array.

Array Abstraction

When «a» has more than one dimension, or «u» is an array, SubIndex array abstracts according to the standard rules of array abstraction. For example, when «a» contains an index other than «i», SubIndex is applied to each slice of that index, returning the position of «u» in the corresponding slice. When «u» contains an array of values, the position of each of those values in «a» is returned.

Examples

Let:

I → 'a' 'b' 'c' 'd' 'e' 'f'
A → 4 3 2 5 3 1
B I
'a' 'b' 'c' 'd' 'e' 'f'
J 1 3 5 2 4 5 3
2 2 1 5 2 3 4
K → 1 2 3 4 5
C → 4 2 5 1 3


Then: 

SubIndex(A,2,I) → 'c'
SubIndex(A,3,I) → 'e'
SubIndex(A,6,I) → «null»
SubIndex(A,1..5,I) → ['f', 'c', 'e', 'a', 'd']

SubIndex(A,C,I) →
K ▶
1 2 3 4 5
'a' 'c' 'd' 'f' 'e' 
SubIndex(B,2,I) →
J ▶
1 2
'c' 'd'

The following two examples can be found in the User Guide Examples.

Let:

Variable Car_prices :=

Years ▶
Car_type ▼ 2005 2006 2007 2008 2009
VW $16,000 $17,000 $18,000 $19,000 $20,000
Honda $18,000 $19,000 $20,000 $22,000 $24,000
BMW $25,000 $26,000 $28,000 $30,000 $32,000

Index Car_type := ['VW', 'Honda', 'BMW']

Then: 

SubIndex(Car_prices, 18K, Car_type) →
Years ▶
2005 2006 2007 2008 2009
Honda «null» VW «null» «null» 
SubIndex(Car_prices, 18K, Years) →
Car_type ▶
VW Honda BMW
2007 2005 «null»

If «u» is an array of values, an array of index values is returned: 

SubIndex(B,C,I) →
B K
1 2 3 4 5
J 1 'd' 'c' 'e' «null» 'f'
2 'f' 'd' 'c' 'd' 'e'

This example can also be found in the User Guide Examples; the variables are defined above. 

SubIndex(Car_prices, [18K, 19K] Car_type) →
Years ▶
Subindex ▼ 2005 2006 2007 2008 2009
18K Honda «null» VW «null» «null»
19K «null» Honda «null» VW «null»

See Also

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