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1

Table[10i+j,{i,4},{j,3}] will generate a table that looks like this: {{11,12,13},{21,22,23},{31,32,33},{41,42,43}}

I would like to have a table with elements like {10i+j, i, j}, I mean {{11, 1, 1}, {12, 1, 2}, ..., {43, 4, 3}}

How do I make such a table?

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1  
Flatten[Table[{10i + j, i, j}, {i, 1, 4}, {j, 1, 3}], 1] – ssch Nov 6 '12 at 20:28
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@ssch Please post that as an answer instead of a comment. – Brett Champion Nov 6 '12 at 21:50

5 Answers

up vote 8 down vote accepted

There is one way to produce the expected result with Table pointed out by ssch in the comments, here are another ones, with Array :

Array[{10 #1 + #2, #1, #2} &, {4, 3}] // Flatten[#, 1] &

or with a powerful function like Outer :

Outer[{10 #1 + #2, #1, #2} &, Range[4], Range[3]] // Flatten[#, 1] &

{{11, 1, 1}, {12, 1, 2}, {13, 1, 3},
 {21, 2, 1}, {22, 2, 2}, {23, 2, 3},
 {31, 3, 1}, {32, 3, 2}, {33, 3, 3},
 {41, 4, 1}, {42, 4, 2}, {43, 4, 3} }
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1  
Shorter: Join @@ Array[{10 # + #2, #, #2} &, {4, 3}] – Mr.Wizard♦ Nov 21 '12 at 19:59
@Mr.Wizard Nicer but a bit slower. – Artes Nov 22 '12 at 13:51
up vote 8 down vote

If you create a table like:

Table[{10i + j, i, j}, {i, 1, 4}, {j, 1, 3}]

that nearly gives you what you want, but you'll notice they are grouped together, but that can be sorted by simply flattening that result appropriately

Flatten[Table[{10i + j, i, j}, {i, 1, 4}, {j, 1, 3}], 1]
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up vote 8 down vote

An alternative

{FromDigits@{##} , ##} & @@@ Tuples@Range@{4, 3}
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+1 for being clever -- btw {10 # + #2, ##} & @@@ Tuples@Range@{4, 3} is shorter. – Mr.Wizard♦ Nov 21 '12 at 19:58
@Mr.Wizard Thanks. I prefer the longer but higher level and more extensible version – Rojo Nov 21 '12 at 20:02
up vote 5 down vote

Though not nearly as general as the method using Table here is a faster method for this specific case:

Join[#.{{10}, {1}}, #, 2] & @ Tuples @ Range @ {4, 3}

{{11, 1, 1}, {12, 1, 2}, {13, 1, 3}, {21, 2, 1}, {22, 2, 2}, {23, 2, 3},
 {31, 3, 1}, {32, 3, 2}, {33, 3, 3}, {41, 4, 1}, {42, 4, 2}, {43, 4, 3}}

Comparative timings:

SetAttributes[timeAvg, HoldFirst]

timeAvg[func_] := 
  Do[If[# > 0.3, Return[#/5^i]] & @@ Timing@Do[func, {5^i}], {i, 0, 15}]

{x, y} = {100, 50};

{FromDigits@{##}, ##} & @@@ Tuples @ Range @ {x, y}   // timeAvg

Table[{10 i + j, i, j}, {i, x}, {j, y}] ~Flatten~ 1   // timeAvg

Array[{10 # + #2, #, #2} &, {x, y}] ~Flatten~ 1       // timeAvg

Join[#.{{10}, {1}}, #, 2] & @ Tuples @ Range @ {x, y} // timeAvg

0.004248

0.002496

0.00026976

0.00017984

FromDigits is clever but slow, Array auto-complies and is an order of magnitude faster than Table, and Dot is faster still.

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Nice analysis, ++ . Why do you use timeAvg instead of e.g. AbsoluteTiming ? – Artes Nov 21 '12 at 20:31
@Artes thanks. I use it for consistency as the time taken is too short to be accurate. I would have to create a huge array to get an accurate timing in one pass and I don't have enough RAM for that. – Mr.Wizard♦ Nov 21 '12 at 20:33
@Artes on second thought I could have used a larger array dimension but I would still favor timeAvg out of habit. – Mr.Wizard♦ Nov 21 '12 at 20:35
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Dot is great and often underused, +1. I expected FromDigits to be slow and it clearly lived up to my expectations. – Rojo Nov 21 '12 at 20:48
up vote 1 down vote 
Join @@ Table[{10 i + j, i, j}, {i, 4}, {j, 3}]
Sequence @@@ Table[{10 i + j, i, j}, {i, 4}, {j, 3}]
Tuples[Range@{4, 3}] /. {x_, y_} -> {10 x + y, x, y}
Level[#, {2}] &@Table[{10 i + j, i, j}, {i, 4}, {j, 3}]
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