Skip to main content

list manipulation - Create a table with inline conditions


I am trying to create a table with conditionals inline. For example, I'd like to create a 2 dimensional table like this:


Table[{i, j} -> 1, {i, 1, 3}, {j, 1, 3}]


Now, I'd like Table to generate values only if i != 1. It should be easy, but I'm lost. I've tried several approaches, like the following, but I don't get what I want in a neat way:


Table[If[i != 1, {i, j} -> 1], {i, 1, 3}, {j, 1, 3}]


{{Null, Null, Null}, {{2, 1} -> 1, {2, 2} -> 1, {2, 3} -> 1}, {{3, 1} -> 1, {3, 2} -> 1, {3, 3} -> 1}}



I know, I could delete cases, but there must be a clean and simple way!



Answer



To my mind, it would be better to control the values i is allowed take in the second argument to Table rather than in the first. For your particular example that means writing the very simple and efficient


Table[{i, j} -> 1, {i, 2, 3}, {j, 1, 3}]



{{{2, 1} -> 1, {2, 2} -> 1, {2, 3} -> 1},
{{3, 1} -> 1, {3, 2} -> 1, {3, 3} -> 1}}



This approach can be quit general. For example


Table[{i, j} -> 1, {i, #^2 & /@ Range[5]}, {j, 1, 3}]


{{{1, 1} -> 1, {1, 2} -> 1, {1, 3} -> 1},

{{4, 1} -> 1, {4, 2} -> 1, {4, 3} -> 1},
{{9, 1} -> 1, {9, 2} -> 1, {9, 3} -> 1},
{{16, 1} -> 1, {16, 2} -> 1, {16, 3} -> 1},
{{25, 1} -> 1}, {25, 2} -> 1, {25, 3} -> 1}}



Edit


Adding this to cover the case raised in senseiwa's comment:



I am not sure how I can use your solution for, say, i != K, given a K > 0.




There are many possibilities. Here is one.


With[{k = 4}, Table[{i, j} -> 1, {i, Delete[Range[5], k]}, {j, 1, 3}]]


{{{1, 1} -> 1, {1, 2} -> 1, {1, 3} -> 1},
{{2, 1} -> 1, {2, 2} -> 1, {2, 3} -> 1},
{{3, 1} -> 1, {3, 2} -> 1, {3, 3} -> 1},
{{5, 1} -> 1, {5, 2} -> 1, {5, 3} -> 1}}



Perhaps I should remark that the index specifier for i (or any index) in a Table expression can be a list specifying the exactly those indexes that i should obtain. By creating such a list, either within the Table expression (as I have done here) or external to it, it possible to select any subset of an index range.



Comments

Popular posts from this blog

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[ ...

dynamic - How can I make a clickable ArrayPlot that returns input?

I would like to create a dynamic ArrayPlot so that the rectangles, when clicked, provide the input. Can I use ArrayPlot for this? Or is there something else I should have to use? Answer ArrayPlot is much more than just a simple array like Grid : it represents a ranged 2D dataset, and its visualization can be finetuned by options like DataReversed and DataRange . These features make it quite complicated to reproduce the same layout and order with Grid . Here I offer AnnotatedArrayPlot which comes in handy when your dataset is more than just a flat 2D array. The dynamic interface allows highlighting individual cells and possibly interacting with them. AnnotatedArrayPlot works the same way as ArrayPlot and accepts the same options plus Enabled , HighlightCoordinates , HighlightStyle and HighlightElementFunction . data = {{Missing["HasSomeMoreData"], GrayLevel[ 1], {RGBColor[0, 1, 1], RGBColor[0, 0, 1], GrayLevel[1]}, RGBColor[0, 1, 0]}, {GrayLevel[0], GrayLevel...

list manipulation - Selecting multiple columns from a matrix?

Sample data: data = { {{2013, 1, 1}, 24.13, 167.67, 231.82}, {{2013, 1, 2}, 32.15, 170.92, 225.99}, {{2013, 1, 3}, 35.43, 172.68, 221.67}, {{2013, 1, 4}, 36.73, 173.05, 218.32}, {{2013, 1, 5}, 58.19, 165.96, 197.05}, {{2013, 1, 6}, 69.99, 163.50, 187.52}, {{2013, 1, 7}, 71.37, 154.21, 175.58}, {{2013, 1, 8}, 72.51, 149.66, 163.25}}; I want a DateListPlot with three graphs, so for a matrix formed by columns 1 and 2, one for columns 1 and 3, and 1 for columns 1 and 4. At the moment I'm using this code: data2 = Transpose[{data[[All, 1]], data[[All, 2]]}]; data3 = Transpose[{data[[All, 1]], data[[All, 3]]}]; data4 = Transpose[{data[[All, 1]], data[[All, 4]]}]; DateListPlot[{data2, data3, data4}, Joined -> True, Filling -> {3 -> {1}}] but I have a hunch that this can be done more efficiently. I don't like the Transpose s in particular. Any ideas? edit (for extra credit) What if I need to multiply the second column by 2, which in my solution is simp...