Changing part of a list of unequal arrays

Question

I have {x, y, f(x,y)} data in multiple (n x 3) arrays with unequal length, which I read in from different files in order to later plot them using ListPlot3D. I wanted to automate the process and save the data into an indexed array. As the length is unequal, a standard 3D would not easily work (I would need to cut the zeros all the time for the plots).

I considered using variable names created by Symbol["x" <> ToString@i], but dealing with this is quite circumstantial.

I saw that people use indexed variables, so that is what I ended up with:

Do[x[k]=ReadList["data_"<>k<>".txt", {Number, Number, Number}], {k, 1, ndata}]

However, I learned that only symbols allow part assignments, so I e.g., cannot flip the sign using x[1][[All, 3]] *= -1. I could copy x[1] to an symbol, do the change there, and copy back, but that becomes circumstantial, again.

Is there a way around this?

Any thoughts how to better handle a list of unequal arrays is also appreciated.

Thank you!

Answer 1

One solution that is quite close to your approach, is to use an Association instead of an indexed variable:

x = <||>;
Do[x[k]=ReadList["data_"<>k<>".txt", {Number, Number, Number}], {k, 1, ndata}]

Now you can do:

x[[Key[1], All, 3]] *= -1

If you use string keys, you can also get rid of the Key wrapper on the lhs.

Answer 2

Here is an example how you can store rows of different length in an anrray using e.g. Table:

d = Table[{a[i, j, 1], a[i, j, 1], a[i, j, 3]}, {i, 4}, {j, 
   RandomInteger[{2, 5}]}];
d // TableForm