7
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Consider the following tables:

ival = 10^6;
Tab1 = Table[{x, y}, {x, 1, ival, 1}, {y, 1, 6, 1}];
Tab2 = Table[{x, y1}, {x, 1, ival, 1}, {y1, 7, 8, 1}];

The values of columns are chosen as a toy example, there is no similar pattern in the actual tables that I use.

These tables have itot "blocks" with correspondingly 6 (Tab1) and 2 (Tab2) rows. I would like to join blocks from Tab1, Tab2 in one block, i.e. to produce the table TabTot with blocks containing 8 rows. The brute-force way is:

TabTot = Table[{Tab1[[i]][[1]], Tab1[[i]][[2]], Tab1[[i]][[3]], 
    Tab1[[i]][[4]], Tab1[[i]][[5]], Tab1[[i]][[6]], Tab2[[i]][[1]], 
    Tab2[[i]][[2]]}, {i, 1, Length[Tab1], 1}];

However, 1) it seems that this way is slower than it may be as it calls tables many times, 2) it requires tons of tedious repetitions of code, such as "Tab1[[i]][[1]]", which will be annoying in the case of many rows in blocks for instance.

Could you please tell me whether there is a more efficient way to join the tables?

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1
8
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You're looking for

TabTot = Transpose@Join[Transpose@Tab1, Transpose@Tab2]

More generally, Transpose is very useful when manipulating tables and higher-rank tensors.

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10
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One way is to use ArrayFlatten:

ArrayFlatten[{{Tab1, Tab2}}] === TabTot

(* True *)
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4
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I have chosen a smaller example for demo:

ival = 10^1;
Tab1 = Table[{x, y}, {x, 1, ival, 1}, {y, 1, 6, 1}]
Tab2 = Table[{x, y1}, {x, 1, ival, 1}, {y1, 7, 8, 1}]

tabRes = Join[Tab1[[#]], Tab2[[#]]] & /@ Range[Length@Tab1]

$$\left( \begin{array}{cccccccc} \{1,1\} & \{1,2\} & \{1,3\} & \{1,4\} & \{1,5\} & \{1,6\} & \{1,7\} & \{1,8\} \\ \{2,1\} & \{2,2\} & \{2,3\} & \{2,4\} & \{2,5\} & \{2,6\} & \{2,7\} & \{2,8\} \\ \{3,1\} & \{3,2\} & \{3,3\} & \{3,4\} & \{3,5\} & \{3,6\} & \{3,7\} & \{3,8\} \\ \{4,1\} & \{4,2\} & \{4,3\} & \{4,4\} & \{4,5\} & \{4,6\} & \{4,7\} & \{4,8\} \\ \{5,1\} & \{5,2\} & \{5,3\} & \{5,4\} & \{5,5\} & \{5,6\} & \{5,7\} & \{5,8\} \\ \{6,1\} & \{6,2\} & \{6,3\} & \{6,4\} & \{6,5\} & \{6,6\} & \{6,7\} & \{6,8\} \\ \{7,1\} & \{7,2\} & \{7,3\} & \{7,4\} & \{7,5\} & \{7,6\} & \{7,7\} & \{7,8\} \\ \{8,1\} & \{8,2\} & \{8,3\} & \{8,4\} & \{8,5\} & \{8,6\} & \{8,7\} & \{8,8\} \\ \{9,1\} & \{9,2\} & \{9,3\} & \{9,4\} & \{9,5\} & \{9,6\} & \{9,7\} & \{9,8\} \\ \{10,1\} & \{10,2\} & \{10,3\} & \{10,4\} & \{10,5\} & \{10,6\} & \{10,7\} & \{10,8\} \\ \end{array} \right)$$

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4
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Join with level specification 2 is as fast as ArrayFlatten which is faster than alternatives posted so far:

tabTot1 = Join[Tab1, Tab2, 2]; // RepeatedTiming // First
0.068
tabTot2 = ArrayFlatten[{{Tab1, Tab2}}]; // RepeatedTiming // First
0.071
tabTot3 = Transpose@Join[Transpose@Tab1, Transpose@Tab2]; // 
  RepeatedTiming // First
0.21
tabTot4 = Join[Tab1[[#]], Tab2[[#]]] & /@ Range[Length@Tab1]; // 
  RepeatedTiming // First
0.58
TabTot = Table[{Tab1[[i]][[1]], Tab1[[i]][[2]], Tab1[[i]][[3]], 
      Tab1[[i]][[4]], Tab1[[i]][[5]], Tab1[[i]][[6]], Tab2[[i]][[1]], 
      Tab2[[i]][[2]]}, {i, 1, Length[Tab1], 1}]; // 
  RepeatedTiming // First
1.68
tabTot1 == tabTot2 == tabTot3 == tabTot4 == TabTot
True
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