Question

I’m new to APL. How can I remove unwanted elements from an array?

For example:

 X←1 2 3 4 5 6 7 8 9

Now I want to remove elements of X that equal either 4 or 6. I tried

X←4↓X

to drop 4 from X, but it didn’t work. Thanks in advance.

Answer 1

Try this, assuming x = 1 2 3 4 5 6 7 8 9

x←(~x∊4 6)/x

Explaination

Find 4 and 6 in the array x with x∊4 6
which outputs: 0 0 0 1 0 1 0 0 0 (the locations of 4 and 6)

Now, Negate this array with ~ which outputs 1 1 1 0 1 0 1 1 1 (the opposite of the above)

Apply this to the array x with /x which leaves you with 1 2 3 5 7 8 9

Finally, assign this to x with x← leaving x with your desired output.