Deleting column in Mathematica

Question

I have a huge list, ex. {a,b,c},{d,e,f} and I must have only {a,c},{d,f}. I using import from url. My sentence:

Drop[Import["url"],{2}]

And it doesn't work. Why?

why do you need the `Import[]`? You can use only the list as argument. Also, look at the manual for `Drop`: it says it deletes the first n elements. If you use `Drop[yourlist,2]` you will be deleting the first 2 elements of `yourlist`. Have a look into `Delete[]`, maybe this is what you want — Sos, Jul 24 '13 at 13:49
You could also try importing just the columns of interest. For example: `Import["URL", {"Data", {All}, {1, 3}}];` (See [here](http://mathematica.stackexchange.com/a/5921/106)) — 681234, Jul 26 '13 at 14:50
Please consider using the dedicated SE site for *Mathematica*: http://mathematica.stackexchange.com — Mr.Wizard, Jul 26 '13 at 21:53

score 10 · Accepted Answer · answered Jul 24 '13 at 19:51

10

Just use the third parameter of the Drop function, like so:

list = {{a, b, c}, {d, e, f}};
Drop[list, None, {2}]

This will return:

{{a, c}, {d, f}}

answered Jul 24 '13 at 19:51

Mr Alpha

2

Things you learn. I never noticed a third parameter in `Drop`, +1. – rcollyer Jul 25 '13 at 03:55
This is not only a third parameter: if your list is deeper, you can use as many parameters as dimensions in it. – ogerard Jul 25 '13 at 06:04

Chris Degnen · Answer 2 · 2013-07-24T16:09:47.060

2

You need to map drop over the list.

list = {{a, b, c}, {d, e, f}};
Map[Drop[#, {2}] &, list]

{{a, c}, {d, f}}

Alternatively, use transpose, but this is apparently less efficient because it makes copies of the list.

Transpose@Drop[Transpose@list, {2}]

edited Jul 24 '13 at 16:09

answered Jul 24 '13 at 16:04

Chris Degnen

2 Answers2