# Command like “find” in Matlab [closed]

Is there a command working exactly like Matlab's find?

I know that Pick might work, but I just want to give a logic condition to select the elements of a list instead of a particular value.

For instance:

T={a, b, c, d, e}
Ctl={2.3, 0, 5, 0, 0}


I'd like to pick the elements of T for which the corresponding element of ctl is greater than 0.

## closed as off-topic by MarcoB, user9660, Jens, dr.blochwave, LLlAMnYPNov 9 '15 at 8:35

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• Yes, Pick[] is entirely appropriate: Pick[T, Positive[Ctl]]. – J. M. will be back soon Nov 8 '15 at 17:08
• Thanks! And if I wanted to to know the position of the element of Ctl >2? – pugliam Nov 8 '15 at 17:13
• Replace T with Range[Length[T]], or something like that... – J. M. will be back soon Nov 8 '15 at 17:14
• Now that I think about it, Position[] is useful, too: Position[Ctl, x_ /; x > 2]. – J. M. will be back soon Nov 8 '15 at 17:31

You can find the position/index within an array of all values in a specified interval, (like Matlab's find) using Position. For instance:

pos = Position[a, _?((0.3 < # < 0.7) &)]


or

pos = Position[a, x_ /; (0.3 < x < 0.7)]


These find the indices of all elements in a with values between 0.3 and 0.7. The elements can be extracted from a using

Extract[a, pos]


For the requested example:

t = {a, b, c, d, e};
ctl = {2.3, 0, 5, 0, 0};
pos = Position[ctl, _?((# > 2) &)]
Extract[t, pos]

{{1}, {3}}
{a, c}


As suggested by J.M. in the comments, you can also use Pick:

Pick[t, Thread[ctl > 2]]
{a, c}


You use a PositionIndex with KeySelect for this, which is a good approach if you need to use multiple tests against the same array.

In[1]:= T = {a, b, c, d, e};
Ctl = {2.3, 0, 5, 0, 0};

In[2]:= index = PositionIndex[Ctl]
Out[2]= <|2.3 -> {1}, 0 -> {2, 4, 5}, 5 -> {3}|>

In[3]:= Extract[T, Values@KeySelect[index, Positive]]
Out[3]= {a, c}

• Didn't know PositionIndex! – Silvia Nov 9 '15 at 2:35
• @Pillsy thank you for introducing PositionIndex to me +1 :) – ubpdqn Nov 9 '15 at 8:33

Just for some ridiculousness (covering some of the give answers):

t = {a, b, c, d, e};
ctl = {2.3, 0, 5, 0, 0};


Various:

Pick[t, Positive@ctl] (* JM*)
Extract[t, Position[ctl, _?Positive]] (* bill s *)
Cases[Transpose[{t, ctl}], {x_, _?Positive} :> x]
First /@ Select[Transpose[{t, ctl}], #[[2]] > 0 &]
Last@Reap[Sow @@@ (Transpose@{t, ctl}), _?Positive, Sequence @@ #2 &]
True /. GroupBy[Transpose[{t, ctl}], Positive[#[[2]]] & -> First]