| Finding a value in a array efficiently [message #48352] |
Thu, 13 April 2006 13:31 |
news.verizon.net
Messages: 47
Registered: August 2003
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I was asked an apparently simple question -- what is the most efficient
way in IDL to determine if a particular scalar value is in an array.
The loop method is straightforward:
found = 0
for i=0,n-1 do begin
if array[i] EQ value then begin
found = 1
goto, DONE
endif
endfor
DONE:
This ugly code will end the loop as soon as a match is found.
A first attempt at vectorized code might be
index = where(array EQ value, ng)
found = ng gt 0
But this code both creates an unnecessary index array, and requires
searching the entire array even if the match is found in the first
element. A more efficent (if less transparent) method might be
found = 1 - array_equal( array EQ value, 0)
where (array EQ value) will contain all zeros if there is no match.
The ARRAY_EQUAL(x,0) function returns a value of zero as soon as it
finds a non-zero value in x. So we are part way there but (array EQ
value) still requires testing every value of "array".
So what we need is a new ARRAY_OR(x,y) function which returns 1 as
soon as it finds any match between X and Y. Or am I missing another
method? --Wayne
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