I would like to have a function to select terms containing q[i] terms in expressions, for example, for an expression
a q[i]+b +c q[j]+d
I would like the function to return
a q[i]+c q[j]
as result. Also at the same time, for a bare
q[i]
the same function would return
q[i]
How to realize such function? Thanks
Answer
A pattern-based approach.
fn[x_. y_q + z_.] := x y + fn[z]
_fn = 0;
a q[i] + b + c q[j] + d // fn
q[i] // fn
a q[i] + c q[j]
q[i]
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