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How to extract last element from the lists

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Aswin Sandirakumaran
评论： Walter Roberson 2018-4-30
Suppose if my List has
list = {[1,2],2,[],[1,2,3]}
eg:
list{1} = [1,2] = %% print last element ==>> 2 list{2} = 2 list{3} = [] list{4} = [1,2,3] ==> 3 new_list = {2,2,[],3}
I need to extract last element from each list{i} Output should be like
new_list = {2,2,0,3}
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dpb
dpb 2018-4-29
That's not what the original said; no fair changing the problem and then acting like didn't... :)
Aswin Sandirakumaran
Aswin Sandirakumaran 2018-4-30
Oh common.. Yes I made a mistake to interpret ! But cuz of you.. I realised my mistake and changed it . And one more thing even before changing it someone understood my problem and he solved it.

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Walter Roberson
Walter Roberson 2018-4-29
Nth = @(L, N) L(N);
PadN = @(L,N) [L,zeros(1,N)];
list = {[1,2],2,[],[1,2,3]};
N = 2;
cellfun(@(L) Nth(PadN(L,N),N), list)
This is what you asked for, the nth element from the lists. But the example output you gave is asking for the last element of the list, not the n'th element.
Last = @(L) L(end);
NonEmpty = @(L) [L,zeros(1,1-length(L))];
cellfun(@(L) Last(NonEmpty(L)), list)
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Walter Roberson
Walter Roberson 2018-4-29
I need to run some errands for a while; perhaps someone else will have time to explain it.
Walter Roberson
Walter Roberson 2018-4-30
You have a list of values in a cell, so it is easiest to run a cellfun() operation to calculate the results.
Now, if you knew ahead of time that the entries were all non-empty then you could just code
cellfun(@(L) L(end), list)
to take the last entry of each of them. But you do have empty entries, and you have defined that those entries should return 0. So we need to detect empty entries and pad them with a 0 before taking the last element of them.
One of the ways to pad with 0 conditionally is to concatenate with zeros(1,1-length(L)) . When the entry is empty then length() is 0 and 1-length() is 1-0 so that becomes zeros(1,1) which is a single 0 concatenated on to the end of the emptiness, leaving a 0. When the entry is not empty then length() is 1 or more and 1-length() is 0 or negative, so that becomes zeros(1,0 or less), which is emptiness concatenated to the end of the non-empty vector, leaving the vector unchanged.
You would be tempted to put those two functionalities together in a single cellfun, like
cellfun(@(L) [L,zeros(1,1-length(L))](end), list) %WRONG
to try to do the concatenation and then take the last element of what results. However, MATLAB does not permit that syntax.
There is an actual function, subsref, that you can call to do indexing. But it is ugly to call and hard to understand. So it is easier to add in an auxillary function such as
Last = @(L) L(end)
and then you can
cellfun(@(L) Last([L,zeros(1,1-length(L))]), list)
The code I posted just broke this into one more level of operation to add a bit of clarity, doing the padding in another auxiliary function.

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