How to use Thread when second argument to function is a list of lists?

*To*: mathgroup at smc.vnet.net*Subject*: [mg86372] How to use Thread when second argument to function is a list of lists?*From*: "Nasser Abbasi" <nma at 12000.org>*Date*: Sun, 9 Mar 2008 05:06:48 -0500 (EST)*Reply-to*: "Nasser Abbasi" <nma at 12000.org>

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