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MATCH

MATCH

## Purpose

Routine to match values in two vectors.

## Calling Sequence

match, a, b, suba, subb, [ COUNT =, /SORT, EPSILON = ]

## Inputs

a,b - two vectors to match elements, numeric or string data types

## Outputs

suba - subscripts of elements in vector a with a match
in vector b
subb - subscripts of the positions of the elements in
vector b with matchs in vector a.
suba and subb are ordered such that a[suba] equals b[subb]

## Optional Input Keyword

/SORT - By default, MATCH uses two different algorithm: (1) the
/REVERSE_INDICES keyword to HISTOGRAM is used for integer data,
while (2) a sorting algorithm is used for non-integer data. The
histogram algorithm is usually faster, except when the input
vectors are sparse and contain very large numbers, possibly
causing memory problems. Use the /SORT keyword to always use
the sort algorithm.
epsilon - if values are within epsilon, they are considered equal. Used only
only for non-integer matching. Note that input vectors should
be unique to within epsilon to provide one-to-one mapping..
Default=0.

## Optional Keyword Output

COUNT - set to the number of matches, integer scalar

## Side Effects

The obsolete system variable !ERR is set to the number of matches;
however, the use !ERR is deprecated in favor of the COUNT keyword

## Restrictions

The vectors a and b should not have duplicate values within them.
You can use rem_dup function to remove duplicate values
in a vector

## Example

If a = [3,5,7,9,11] & b = [5,6,7,8,9,10]
then
IDL> match, a, b, suba, subb, COUNT = count
will give suba = [1,2,3], subb = [0,2,4], COUNT = 3
and a[suba] = b[subb] = [5,7,9]

## Method

For non-integer data types, the two input vectors are combined and
sorted and the consecutive equal elements are identified. For integer
data types, the /REVERSE_INDICES keyword to HISTOGRAM of each array
is used to identify where the two arrays have elements in common.

## History

D. Lindler Mar. 1986.
Fixed "indgen" call for very large arrays W. Landsman Sep 1991
Added COUNT keyword W. Landsman Sep. 1992
Fixed case where single element array supplied W. Landsman Aug 95
Use a HISTOGRAM algorithm for integer vector inputs for improved
performance W. Landsman March 2000
Work again for strings W. Landsman April 2000
Use size(/type) W. Landsman December 2002
Work for scalar integer input W. Landsman June 2003
Assume since V5.4, use COMPLEMENT to WHERE() W. Landsman Apr 2006
Added epsilon keyword Kim Tolbert March 14, 2008

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