Question

This question is a continuation of a previous thread to compare two lists with the same length:

Is there any efficient easy way to compare two lists with the same length with Mathematica?

Given two lists A={a1,a2,a3,...an} and B={b1,b2,b3,...bn}, I would say A>=B if and only if all ai>=bi. Now we have k lists H={{a11,a12,a13,...a1n}, {a21,a22,a23,...a2n},...,{ak1,ak2,ak3,...akn}}, and want to find the maximum one if exist.

Here’s my code:

Do[If[NonNegative[Min[H[[i]] - h]], h = H[[i]], ## &[]], {i, h = H[[1]]; 1, Length[H]}];h

Any better trick to do this?

EDIT:

I want to define this as a function like:

maxList[H_]:=Do[If[NonNegative[Min[H[[i]] - h]], h = H[[i]], ## &[]], {i, h = H[[1]]; 1, Length[H]}];h

But the question is the code above cross two lines, any fix for this? Here is some code working but not that beautiful

maxList[H_] := Module[{h = H[[1]]}, Do[If[NonNegative[Min[H[[i]] - h]], h = H[[i]], ## &[]], {i, Length[H]}]; h]

or

maxList[H_]:=Last[Table[If[NonNegative[Min[H[[i]] - h]], h = H[[i]], ## &[]], {i, h = H[[1]]; 1, Length[H]}]]

Answer 1

A modification of Mr.Wizard’s approach works a few times faster.

maxListFast[list_List] := Module[{l}, 
                                 l = Max /@ Transpose@list; 
                                 If[MemberQ[list, l], l, {}]]

We test the both methods with

test  = RandomInteger[100, {500000, 10}];
test1 = Insert[test, Table[100, {10}], RandomInteger[{1, 500000}]]; 

and we get

In[5]:= maxList[test] // Timing
        maxListFast[test] // Timing

        Out[5]= {2.761, {}}
        Out[6]= {0.265, {}}

and

In[7]:= maxList[test1] // Timing
        maxListFast[test1] // Timing

Out[7]= {1.217, {{100, 100, 100, 100, 100, 100, 100, 100, 100, 100}}}
Out[8]= {0.14, {100, 100, 100, 100, 100, 100, 100, 100, 100, 100}}

EDIT

In general, to choose a method, we should know first what kind of data we are to deal with. (lenght of lists, their number, types of numbers ). While we have a large number of short lists
of integers maxListFast works even 10 times better (in case of 500000 lists of length 10) than maxList.
However for lists of real numbers it is only 3-4 times faster, and the more and the longer list we have the more it slows down, e.g. :

         A = RandomReal[1000, {3000, 3000}];
         First@AbsoluteTiming[maxListFast@A;]/ First@AbsoluteTiming[maxList@A;]

Out[19]= 2.040516    

however if we insert “the greatest element” :

In[21]:= IA = Insert[A, Table[1000, {3000}], RandomInteger[{1, 3000}]];
In[22]:= First@AbsoluteTiming[maxListFast@IA;]/ First@AbsoluteTiming[maxList@IA;]

Out[22]= 0.9781931

timings close up.