Conditionally Altering Array Elements

The WHERE function can be used to select array elements that meet certain conditions. For example, the statement:

data[WHERE(data LT 0)] = -1

sets all negative elements of data to -1 without changing the positive elements. The result of the function, WHERE(data LT 0), is a vector composed of the subscripts of the negative elements of data. Using this vector as a subscript changes only the negative elements.

Similarly, the WHERE function can be used to select elements of an array using expressions similar to A[WHERE(A GT 0)], which results in a vector composed only of the elements of A that are greater than 0.

The following statements create and display a 5x5 identity matrix, which consists of ones along a diagonal, and zeros everywhere else:

A = FLTARR(5, 5) 
A[INDGEN(5) * 6] = 1 
PRINT, A

The following statement sets elements of A with values of zero or less to -1:

A[WHERE(A LE 0)] = -1 
PRINT, A

In this example, assume that the vector data contains data elements and that a data drop-out is denoted by a negative value. In addition, assume that there are never two or more adjacent drop-outs. The following statements replace all drop-outs with the average of the two adjacent good points:

; Subscript vector of drop-outs. 
bad = WHERE(data LT 0) 
 
; Replace drop-outs with average of previous and next point. 
data[bad] = (data[bad - 1] + data[bad + 1]) / 2

In this example, the following actions are performed:

We use the LT (less than) operator to create an array, with the same dimensions as data, that contains a 1 for every element of data that is less than zero and a zero for every element of data that is zero or greater. We use this "drop-out array" as a parameter for the WHERE function, which generates a vector that contains the one-dimensional subscripts of the elements of the drop-out array that are nonzero. The resulting vector, stored in the variable bad, contains the subscripts of the elements of data that are less than zero.

The expression data[bad - 1] is a vector that contains the subscripts of the points immediately preceding the drop-outs; while similarly, the expression
data[bad + 1] is a vector containing the subscripts of the points immediately after the drop-outs.

The average of these two vectors is stored in data[bad], the points that originally contained drop-outs.

Note
Also see Example—Using Array Operators and WHERE for an additional example.