4.7) where Function (cont'd)

When using WHERE, the count variable is very important. If WHERE does not find any elements which satisfy the specified criteria then it returns a value of -1. In the latest versions of IDL, if you use -1 as a reference to index an array, then this will return the last element of the array.

x = indgen(10)
y = WHERE(x gt 20, count)
print, x(y)

IDL will print "9" to the screen, which is clearly not what we want. Therefore, you must check that count is non-zero before using the result of a WHERE statement.

IF count ne 0 THEN print, x(y) ELSE print, 'WHERE found no match'