Re: [ferret_users] masking using and/or

Hi -
I agree that adding masks is the easiest way to combine several conditions, especially if there are more than two.

Paul, to answer your question, what's implied if you don't give an "else" is the missing-data flag. Compare:

use etopo60
let mask1 = if rose gt 0 then 1
shade mask1*rose

let mask2 = if rose gt 0 then 1 else 0
shade mask2* rose

The missing-value form of this is I think more general, so you aren't potentially confusing the mask's zero with valid zero values. (See the discussion under the IFV keyword in the documentation.)

Ansley

On 2/3/2012 12:59 PM, Paul Young wrote:

Hi Carol,

I can get the desired result by adding "else 0" to the mask1 and mask2 definitions (which I thought was implied):

let mask1 = if y[gy=sst] lt 20 then 1 else 0

let mask2 = if y[gy=sst] gt 0 then 1 else 0

let mask3 = if (mask1 and mask2) then 1 !funnily enough, no need for "else 0" here

fill/l=1 sst*mask3 !Just Eq to 20N

I can also get it by making the mask in one shot:

let mask = if y[gy=sst] gt 0 and y[gy=sst] lt 20 then 1 else 0

Hope this helps.....but perhaps a ferret expert can clarify when the "else 0" is needed?

Thanks!

Paul

On Feb 3, 2012, at 12:56 PM, Carol Ladd wrote:

I am trying to create a lat/lon mask using if statements. I create two overlapping masks and then want to combine them. I would assume I should use “or” to combine the two regions. However, using either “and” or “or” in the masking definition, I get the same result. Is this a bug or am I doing something wrong? I am using Ferret v6.65.

See the following script for a quick example:

use coads_climatology

sh d

let mask1=if y[gy=sst] lt 20 then 1

let mask2=if y[gy=sst] gt 0 then 1

let mask3 = if (mask1 and mask2) then 1

let mask4 = if (mask1 or mask2) then 1

fill/l=1 sst*mask1

pause

fill/l=1 sst*mask2

pause

fill/l=1 sst*mask3

pause

fill/l=1 sst*mask4

q

Thanks.

Carol