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Can I create a Do loop where instead of varying only 1 variable, I vary multiple sets of variable ?

For example (the syntax is invented and only serves as a purpose to show what I mean):

list = {}
Do[
list = AppendTo[list,{a,b}],
{a,b,{{1,2},{3,4}}];

should give me

list

{{1,2},{3,4}}

I really need to loop over the variables.

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    $\begingroup$ Why not something like Table[g[a, b] /. Thread[{a, b} -> list], {list, {{1, 2}, {3, 4}}}]? $\endgroup$ – J. M.'s ennui♦ Mar 30 '18 at 11:52
  • $\begingroup$ @J.M. Thank you for your comment. I cannot use such structure because the variables are parameters inside a large function. In each loop I want to update specific parameters to specific values, while keeping other parameters the same. $\endgroup$ – james Mar 30 '18 at 11:55
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    $\begingroup$ Based on this and your other question, I have the nagging suspicion that you've chosen an architecture that is making simple substitutions and looping more complicated than it has to be. $\endgroup$ – J. M.'s ennui♦ Mar 30 '18 at 11:59
  • $\begingroup$ @J.M. here is my code: dropbox.com/s/bfbfstrsbmorjvy/… $\endgroup$ – james Mar 30 '18 at 12:08
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    $\begingroup$ What about list = {};Do[list = AppendTo[list,x],{x,{{1,2},{3,4}}];? Note however that using Append in loops will slow things down considerably as it involves a lot of copy operations. Solutions based on Sow and Reap (or even better: Internal`Bag) should yield better performance. $\endgroup$ – Henrik Schumacher Mar 30 '18 at 12:14
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  1. Pass a list of parameter-tuples to Table
  2. Unpack the tuples into the variables you want

Example:

Table[
 Block[{a, b},
  {a, b} = pair;
  a^2 + b^2
 ], {pair, {{1, 2}, {3, 4}, {5, 6}}}
]
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