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I defined a function row2col[] to transform row into column, Where am I wrong?

 row2col[args__] := 
    Module[
    {argList = {args}, n = Length[{args}], i},  
    col = {};
    For[i = 1, i <= n, i++,
      midd = Map[List, argList[[i]]];
      col = Join[col, midd];
      ];
     col
      ];
 col = row2col[{1, 2, 3}, {4, 5, 6}, {7, 8, 9}];
 MatrixForm[col]
 Quit[];

The expexted result is 

(*{{{1},{2},{3}},{{4},{5},{6}},{{7},{8},{9}}}*)

I know other simple command can return the result i need, I want to know what's wrong with my definition.

share|improve this question
What result are you expecting from this? – RunnyKine Mar 15 at 5:07
1  
You have not included the semi-colon after the For loop, though it's not entirely clear what you are trying to do. – Jonathan Shock Mar 15 at 5:08
In general if you wish to change rows to columns you can use Tranpose[] – Jonathan Shock Mar 15 at 5:09
1  
@JonathanShock What trips up beginners is that you cannot transpose a simple list. It has to be a transposable list of depth 3... such as {{1,2,3}} – rm -rf♦ Mar 15 at 5:29
Please see mathematica.stackexchange.com/q/18393/193 for more avoidable pitfalls – belisarius Mar 15 at 7:52

closed as too localized by Yves Klett, m_goldberg, whuber, Sjoerd C. de Vries, Artes Mar 15 at 20:55

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2 Answers

up vote 2 down vote

Using your function definition, since you insist on it, adding Partition will give you what you want.

row2col[args__] := 
  Module[{argList = {args}, n = Length[{args}], i}, col = {}; 
   For[i = 1, i <= n, i++, midd = Map[List, argList[[i]]]; 
    col = Join[col, midd];]; Partition[col, 3]];

Then 

row2col[{1, 2, 3}, {4, 5, 6}, {7, 8, 9}]

returns

(* {{{1}, {2}, {3}}, {{4}, {5}, {6}}, {{7}, {8}, {9}}} *)

But if you care to learn Mathematica then think about using Transpose to write a one-liner for this that'll also be faster.

share|improve this answer
RunnyKine, You are right, but Transpose doesn't support single row vector, is it? e.g. Transpose[{1,2,3}] error occurs. – novice Mar 15 at 5:34
@user5463, you need an additional bracket when using Transpose. So it's Transpose[{{1,2,3}}]. – RunnyKine Mar 15 at 5:51
RunnyKine,what if I don't use Partition[]. why using Join[col, midd, 2] is wrong. – novice Mar 15 at 6:33
@user5463 It's not entirely clear what you want but please read these threads: (189), (SO 7537401), (14494) – Mr.Wizard♦ Mar 15 at 6:44
up vote 1 down vote

For loops are not usually the way to use Mathematica. See:
Alternatives to procedural loops and iterating over lists in Mathematica

Based on your given input and output you might use:

row2col[a__List] := Map[List, {a}, {2}]

row2col[{1, 2, 3}, {4, 5, 6}, {7, 8, 9}]

{{{1}, {2}, {3}}, {{4}, {5}, {6}}, {{7}, {8}, {9}}}

This also handles variable length inputs, e.g.:

row2col[{1, 2}, {4, 5, 6}, {7}]

{{{1}, {2}}, {{4}, {5}, {6}}, {{7}}}

If your input is rectangular you can use Transpose for greater efficiency.
Here it is better to give input as an array rather that a series of arguments:

Clear[row2col]

row2col[a_?MatrixQ] := Transpose[{a}, {3, 1, 2}]

row2col[{{1, 2, 3}, {4, 5, 6}, {7, 8, 9}}]

{{{1}, {2}, {3}}, {{4}, {5}, {6}}, {{7}, {8}, {9}}}
share|improve this answer
Mr Wizard, Wonderful, I didn't notice Map[] has an option to select the layer – novice Mar 15 at 7:34

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