WHERE Function

Returns a longword vector containing the one-dimensional subscripts of the nonzero elements of the input array.

result = WHERE(array_expr [, count ])

array_expr The array to be searched. May be of any data type. Both the real and imaginary parts of complex numbers must be zero for the number to be considered zero.

count (optional) If present, count is converted into a longword integer containing the number of nonzero elements found in array_expr. Must be a named variable.

result A longword vector containing the one-dimensional subscripts of the nonzero elements of array_expr. The length is equal to the number of nonzero elements in array_expr.

None.

Frequently the result of WHERE is used as a vector subscript to select elements of an array using given criteria.

As a side effect, the system variable !Err is set to the number of nonzero elements. This is for compatibility with previous versions of PV-WAVE. Therefore, it is recommended that the Count keyword be used in all new programs, rather than !Err.

The WHERE function can be used to select a range of values in an array. For example:

index = WHERE ((array GT 50) AND (array LT 100))

; Get the subscripts of those elements greater than 50 and less
; than 100. 

result = array(index)

; Put the selected values into result.

If all the elements of array_expr are zero, WHERE returns a scalar integer with a value of -1. If you attempt to use this result as an index into another array, you will get an error message about the subscripts being out of bounds. In some situations, code similar to the following can be used to avoid errors:

index = WHERE(array, count)

; Use count to get the number of nonzero elements.

IF (count GT 0) THEN result = array(index)

; Only subscript the array if it's safe to do so.

You can use WHERE to determine where an array of strings matches a specific pattern, or where the array of strings is non-NULL.

s = ['this', 'is', 'an', 'array', 'of', 'strings']
PRINT, WHERE(s eq 'an')

s = ['this', '', '', 'array', '', 'strings'] 
PRINT, WHERE(s)

0       3       5

QUERY_TABLE, SORT, WHEREIN

System Variables: !Err

For more information, see Chapter 5, Using Subscripts and Matrices, in the PV-WAVE Programmer's Guide.