Tag: find

In MATLAB, you can find the indices of array entries that match a user-supplied condition with the function find()

Here’s a similar function in Maxima.  This makes liberal use of the functions sublist_indices(), lambda(),  and several functions from the stringproc package: sequal(), eval_string(), and simplode().  Oh, and also the shameless hack of using ascii(32) as the space character 🙂

Here are some examples.  The code is included at the bottom.

find(exp):=block(
 oo:op(exp),
 if sequal(oo,"=") or sequal(oo,">") or sequal(oo,">=") or sequal(oo,"<") or sequal(oo,"<=") or sequal(oo,"#") then 
   sublist_indices(first(exp),lambda([x1],eval_string(simplode([x1,op(exp),last(exp)]))))
 else 
   if sequal(oo,"and") or sequal(oo,"or") then(
     e1:first(exp),
     e2:second(exp),
     sublist_indices(first(e1),lambda([x1],
       eval_string(simplode([x1,op(e1),last(e1),ascii(32),oo,ascii(32),x1,op(e2),last(e2)]))))
   )
)$