[ferret_users] listing single values

[igaratza@xxxxxxxxxxxxxxxxxxxx]

I want to list a variable, but no the complete grid but only below certain
conditions. For example I want to have the values only when a/b is between 3
and 4. I defined another variable

yes? let c=a/b
yes? LET a_b_gt_3_lt_4 = IF c GT 3 and c LT 4 THEN c

Now, I want to know the coordinates when this is happening. I try to list this
new variable, a_b_gt_3_lt_4, only when is has a value.

If I simply do
yes? list a_b_gt_3_lt_4

it writes all the grid, and since is listing every 3th point I don't get all the
points. When I plot them in a map I can see all of them.
How can I do to list just this few points and to know the coordinates where the
relation a/b is between 3 and 4?