Re: How to use Thread when second argument to function is
- To: mathgroup at smc.vnet.net
- Subject: [mg86405] Re: [mg86372] How to use Thread when second argument to function is
- From: Bob Hanlon <hanlonr at cox.net>
- Date: Mon, 10 Mar 2008 02:06:27 -0500 (EST)
- Reply-to: hanlonr at cox.net
p = {{1, 0, 0, 0, 0}, {0, 0.2, 0.8, 0, 0},
{0, 0.7, 0.3, 0, 0}, {0.1, 0, 0.1, 0.4, 0.4},
{0, 0.1, 0.3, 0.2, 0.4}};
Select[#, # != 0 &] & /@ p
{{1},{0.2,0.8},{0.7,0.3},{0.1,0.1,0.4,0.4},{0.1,0.3,0.2,0.4}}
Cases[#, _?(# != 0 &)] & /@ p
{{1},{0.2,0.8},{0.7,0.3},{0.1,0.1,0.4,0.4},{0.1,0.3,0.2,0.4}}
p /. 0 -> Sequence[] (* space required after /. *)
{{1},{0.2,0.8},{0.7,0.3},{0.1,0.1,0.4,0.4},{0.1,0.3,0.2,0.4}}
% == %% == %%%
True
Bob Hanlon
---- Nasser Abbasi <nma at 12000.org> wrote:
> Hello;
>
> I have a function which I want to call repeatedly for all rows of a matrix
> to find non-zero elements in each row.
> This function takes as input the row number and the matrix itself and
> returns a list that contain the non-zero elements in the that row.
>
> i.e. function is f[ list , matrix]:=Module[{},.....]
>
> I want to use Thread to call this function 'f' over all the rows of the
> matrix. Something like
>
> Thread[ f[{1,2,3}, matrix] ]
>
> But it is not working. I guess becuase matrix is a list of lists and it is
> of different dimension than the list of the row numbers?
> I can use MapThread, but then I have to write
>
> MapThread[ f {{1,2,3}, {matrix,matrix,matrix}} ]
>
> and the above works but I have to duplicate 'matrix' to make it work, which
> I do not like.
>
> Here is the code that does not work
>
> ------------------- Thread not working, why? -----------------
>
> findNonZeroElements[row_, p_] := Module[{}, Select[p[[row,All]], #1 != 0
> & ]]
>
> mchain[p_] := Module[{lhs, rhs},
> {nRow, nCol} = Dimensions[p];
> lhs = Range[nRow];
> Thread[findNonZeroElements[lhs, p]] (* NOT WORKING *)
> ]
>
> p = {{1, 0, 0, 0, 0}, {0, 0.2, 0.8, 0, 0}, {0, 0.7, 0.3, 0, 0},
> {0.1, 0, 0.1, 0.4, 0.4}, {0, 0.1, 0.3, 0.2, 0.4}};
>
> mchain[p]
> ---------------------------------------------------------
>
> Here is the same as above but using MapThread and this works
>
> In[172]:= findNonZeroElements[row_, p_] := Module[{}, Select[p[[row,All]],
> #1 != 0 & ]]
>
> mchain[p_] := Module[{lhs, rhs},
> {nRow, nCol} = Dimensions[p];
> lhs = Range[nRow];
> MapThread[findNonZeroElements, {lhs, Table[p, {nRow}]}] (*OK*)
> ]
>
> p = {{1, 0, 0, 0, 0}, {0, 0.2, 0.8, 0, 0}, {0, 0.7, 0.3, 0, 0},
> {0.1, 0, 0.1, 0.4, 0.4}, {0, 0.1, 0.3, 0.2, 0.4}};
> mchain[p]
>
> Out[175]= {{1}, {0.2, 0.8}, {0.7, 0.3}, {0.1, 0.1, 0.4, 0.4}, {0.1, 0.3,
> 0.2, 0.4}}
>
> thanks,
> Nasser
>
>