Skip to main content

evaluation - direct assignment to the element in a Table



I have already asked a related question here why set values in this way doesn't work? But I think I have to write the question which I encountered explicitly?


tmp = {1, 2, 3, 4, 5}
label[i_, p_] := i + p
test[p_] := Table[tmp[[label[i, p]]], {i, 1, 2}]

Here I defined a function test and the thing I want to is that change the values of list tmp via setting values to function test. While simply write


test[1]={111,222}

and expecting tmp will change to {1,111,222,3,4,5} is not possible.


So how to achive this? I have tried many Hold things, but failed.



Inspired by andre. I know that Set has attribute HoldFirst. So I tried Evaluate


 Evaluate[test[1]]={111,222}

this won't work! I guess the problem is that I must properly Hold label[i,p] and tmp[[]] in the table. But I can't work it out. Anyone help?



Answer



This seems rather convoluted, and there is almost certainly an easier way to approach whatever it is you are wishing to do, but I like answering questions like this as it allows working with more unusual aspects of the language.


We can do this:


Block[{tmp, Part}, Hold @@ {test[2]}] /. _[x_] :> (x = {111, 222});

As with my previous answer we may wish to wrap this into a more convenient syntax, but there is a problem; how do we know that tmp is the symbol to be blocked? We can give it explicitly, if that is acceptable.



func_[boost2[expr_, sym__], arg___] ^:= 
  Block[{sym, Part}, Hold @@ {expr}] /. _[x_] :> func[x, arg]

Use syntax:


boost2[test[1], tmp] = {111, 222};

I'm not certain if there is any point in keeping the generalized form of this function in analogy to the original bump function, but it was easy for me to write it this way. If you wish only to have this operation apply to Set it may be simpler to write a mySet analog, like this:


SetAttributes[mySet, {HoldAll, SequenceHold}]

mySet[expr_, sym__, value_] := 

  Block[{sym}, Hold @@ {expr} // Quiet] /. _[x_] :> (x = value)

mySet[test[1], tmp, {111, 222}]

To me this syntax is not as elegant as the form above.


Comments

Post a Comment

Popular posts from this blog

front end - keyboard shortcut to invoke Insert new matrix

I frequently need to type in some matrices, and the menu command Insert > Table/Matrix > New... allows matrices with lines drawn between columns and rows, which is very helpful. I would like to make a keyboard shortcut for it, but cannot find the relevant frontend token command (4209405) for it. Since the FullForm[] and InputForm[] of matrices with lines drawn between rows and columns is the same as those without lines, it's hard to do this via 3rd party system-wide text expanders (e.g. autohotkey or atext on mac). How does one assign a keyboard shortcut for the menu item Insert > Table/Matrix > New... , preferably using only mathematica? Thanks! Answer In the MenuSetup.tr (for linux located in the $InstallationDirectory/SystemFiles/FrontEnd/TextResources/X/ directory), I changed the line MenuItem["&New...", "CreateGridBoxDialog"] to read MenuItem["&New...", "CreateGridBoxDialog", MenuKey["m", Modifiers-...
Post a Comment
Read more

How to thread a list

I have data in format data = {{a1, a2}, {b1, b2}, {c1, c2}, {d1, d2}} Tableform: I want to thread it to : tdata = {{{a1, b1}, {a2, b2}}, {{a1, c1}, {a2, c2}}, {{a1, d1}, {a2, d2}}} Tableform: And I would like to do better then pseudofunction[n_] := Transpose[{data2[[1]], data2[[n]]}]; SetAttributes[pseudofunction, Listable]; Range[2, 4] // pseudofunction Here is my benchmark data, where data3 is normal sample of real data. data3 = Drop[ExcelWorkBook[[Column1 ;; Column4]], None, 1]; data2 = {a #, b #, c #, d #} & /@ Range[1, 10^5]; data = RandomReal[{0, 1}, {10^6, 4}]; Here is my benchmark code kptnw[list_] := Transpose[{Table[First@#, {Length@# - 1}], Rest@#}, {3, 1, 2}] &@list kptnw2[list_] := Transpose[{ConstantArray[First@#, Length@# - 1], Rest@#}, {3, 1, 2}] &@list OleksandrR[list_] := Flatten[Outer[List, List@First[list], Rest[list], 1], {{2}, {1, 4}}] paradox2[list_] := Partition[Riffle[list[[1]], #], 2] & /@ Drop[list, 1] RM[list_] := FoldList[Transpose[{First@li...
Post a Comment
Read more

plotting - How to draw lines between specified dots on ListPlot?

I would like to create a plot where I have unconnected dots and some connected. So far, I have figured out how to draw the dots. My code is the following: ListPlot[{{1, 1}, {2, 2}, {3, 3}, {4, 4}, {1, 4}, {2, 5}, {3, 6}, {4, 7}, {1, 7}, {2, 8}, {3, 9}, {4, 10}, {1, 10}, {2, 11}, {3, 12}, {4,13}, {2.5, 7}}, Ticks -> {{1, 2, 3, 4}, None}, AxesStyle -> Thin, TicksStyle -> Directive[Black, Bold, 12], Mesh -> Full] I have thought using ListLinePlot command, but I don't know how to specify to the command to draw only selected lines between the dots. Do have any suggestions/hints on how to do that? Thank you. Answer One possibility would be to use Epilog with Line : ListPlot[ {{1, 1}, {2, 2}, {3, 3}, {4, 4}, {1, 4}, {2, 5}, {3, 6}, {4, 7}, {1, 7}, {2, 8}, {3, 9}, {4, 10}, {1, 10}, {2, 11}, {3, 12}, {4, 13}, {2.5, 7}}, Ticks -> {{1, 2, 3, 4}, None}, AxesStyle -> Thin, TicksStyle -> Directive[Black, Bold, 12], Mesh -> Full, Epilog -> { Line[ ...
Post a Comment
Read more