Comment by nanairo | Hacker News Reader

Comment by nanairo | Hacker News Reader

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I may be wrong but I think that:

list => are basically hash, or an array that can have mixed objects inside

vector, matrix, array => are all the same thing. They are what in most computer languages are called arrays, and can have only one type. The difference between those three is just the number of dimensions (vector:1, matrix:2, array:3+).

dataframe I will concede is a little more complex, and I still have some problem with it. But I basically think of it as a table, where a row represents a value (say temperature) and the column different measuremnts. So, for example:

rows=> temperature, humidity, hours of light, peak UV columns=> Day1, day2, day3, day4, ...

Hope that helps.

Lists are hashes on acid. The default return value of indexing in is NULL. That's a weird semantic for a list, but maybe not so much a hash. The weird part is that if you explicitly assign null to an element of the list, it deletes the element. That's particularly weird because the list has knows all the elements in it, so it's not like it can't tell the difference between a value that has been set to NULL and a value that has never been set. See http://gist.github.com/578110 for a little transcript.

Vectors are not the same as 1d matrices/arrays because a vector does not have direction (i.e. it's not a row vector or a column vector, but will work as either depending on circumstance).

  > all.equal(1:10, matrix(1:10, ncol = 1))
  [1] "Attributes: < target is NULL, current is list >"
  [2] "target is numeric, current is matrix"   
  > all.equal(matrix(1:10, ncol = 1), array(1:10, c(10, 1)))
  [1] TRUE

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