Added readdlm option to ignore empty columns.

[tanmaykm]

This adds a new option ignore_empty_columns for readdlm. Setting this to true results in adjoining column delimiters being squashed instead of resulting in a cell with empty string. This option can be used to read fixed width format data files.

e.g: with delimiter set to default (whitespace), and ignore_empty_columns set to false (default):

julia> iob = IOBuffer("  1  2  3  4\n 11 12 13 14");

julia> readdlm(iob)
2x9 Array{Any,2}:
 ""    " "   1.0    ""  2.0  ""  3.0  ""   4.0
 ""  11.0   12.0  13.0   ""  ""   ""  ""  14.0

With ignore_empty_columns set to true:

julia> iob = IOBuffer("  1  2  3  4\n 11 12 13 14");

julia> readdlm(iob; ignore_empty_columns=true)
2x4 Array{Float64,2}:
  1.0   2.0   3.0   4.0
 11.0  12.0  13.0  14.0

Also updated tests and docs for this option and added some new tests for utf8 strings.

[nalimilan]

How about calling this skip_repeated_separators or something like that? When you have such a file, you typically do not think of it as containing empty columns. That's the way you think about it because readlm() considered them as separate columns.

[JeffBezanson]

This is a great feature. I'm not sure what to call the option though; three words with underscores is pretty awkward.

[johnmyleswhite]

Sorry to be contrary, but I'm not sure this is such a great feature. Fixed-width files aren't that similar to delimited files, so I don't think readdlm should be made capable of reading them. After all, the defining characteristic of fixed width files is that they don't ever contain delimiters, not that they contain a haphazard number of consecutive delimiters.

readtable handles the specific example given in this pull request by treating whitespace separation as a highly idiosyncratic special case in which the delimiter is allowed to be repeated arbitrarily many times. This wouldn't make sense for files separated by commas, but it is how R processes whitespace separated files.

[JeffBezanson]

I'm sure you're right, but I think this feature has other uses. For example a delimited file might have empty columns that you'd rather throw out than get an Array{Any} back.

Whether we expose this flag or not, should we effectively set it to true when the delimiter(s) are whitespace?

[johnmyleswhite]

Based on the large number of files in the wild with erratic numbers of whitespace in them, something like this flag should probably be turned on by default for whitespace-separated files.

Regarding your Array{Any} point, throwing out an entire column definitely seems reasonable. But what if only one entry is missing?

[JeffBezanson]

True the Array{Any} argument is a bit weak; you also have the option of specifying the element type, allowing you to get NaNs instead of empty strings.

@tanmaykm why is the second entry above " " instead of ""?

Anyway, for now, let's turn this on by default for whitespace-separated files, and not yet expose the option in the interface.

[mschauer]

A bug in the old readdlm, one also gets

julia> iob = IOBuffer(",,,1,,,,2,3,,,4,\n,,,1,,,1\n");

julia>  readdlm(iob, ',', Any)
2x13 Array{Any,2}:
 ""  ","  ""  1.0  ""  ""   ""  2.0  3.0  ""  ""  4.0  ""
 ""  ""   ""  1.0  ""  ""  1.0   ""   ""  ""  ""   ""  ""

[tanmaykm]

Yes, that seems to be a bug. I shall update the PR with the bug fixed, this option turned on by default for whitespace separated files and not exposed in the API.

[tanmaykm]

Updated. The ignore_empty_columns option is not exposed in the API, but it's behavior is turned on implicitly for whitespace delimited data.

Here are the results with this change:

julia> iob = IOBuffer("  1  2  3  4\n 11 12 13 14");

julia> readdlm(iob)
2x4 Array{Float64,2}:
  1.0   2.0   3.0   4.0
 11.0  12.0  13.0  14.0

julia> iob = IOBuffer(",,,1,,,,2,3,,,4,\n,,,1,,,1\n");

julia> readdlm(iob, ',')
2x13 Array{Any,2}:
 ""  ""  ""  1.0  ""  ""   ""  2.0  3.0  ""  ""  4.0  ""
 ""  ""  ""  1.0  ""  ""  1.0   ""   ""  ""  ""   ""  ""

[kmsquire]

Can you clarify what happens with tab-delimited data (i.e., reading splits
on tabs, but not spaces)? I work frequently with tab-delimited data, where
two tabs implies the same behavior as two commas in csv files.

[tanmaykm]

They will be treated similar to space delimited data. That is:

readdlm(IOBuffer("1\t2\t\t3\n4\t5\t6\n"), '\t')
2x3 Array{Float64,2}:
 1.0  2.0  3.0
 4.0  5.0  6.0

Whereas the same data delimited with comma would yield:

julia> readdlm(IOBuffer("1,2,,3\n4,5,6\n"), ',')
2x4 Array{Any,2}:
 1.0  2.0   ""  3.0
 4.0  5.0  6.0   ""

Would it be preferable instead to turn this option on only when no delimiters are specified by the caller and default delimiters are used?

[kmsquire]

To me that seems reasonable. If others disagree, I would ask to special-case tab-separated files, since they are pretty common, but the fewer special cases, the better.

[johnmyleswhite]

I don't think tab-separated files need to be treated as a special case. In my experience, whitespace separated means "any whitespace character any number of times without need for consistency within a file". People and systems (e.g. Hive) who use tabs as separators usually mean something much stricter than this where only tabs count and repetition singles column changes.

[tanmaykm]

Agree. Ignoring adjoining delimiters only when no delimiter character is specified by the caller would appropriately handle both types of files - where tabs/spaces are significant and the ones with padded columns.

I shall update the PR with this change.

[tanmaykm]

Updated. Now:

julia> readdlm(IOBuffer("1\t2\t\t3\n4\t5\t6\n"), '\t')
2x4 Array{Any,2}:
 1.0  2.0   ""  3.0
 4.0  5.0  6.0   ""

julia> readdlm(IOBuffer("1\t2\t\t3\n4\t5\t6\n"))
2x3 Array{Float64,2}:
 1.0  2.0  3.0
 4.0  5.0  6.0

julia> readdlm(IOBuffer("  1  2  3  4\n 11 12 13 14"))
2x4 Array{Float64,2}:
  1.0   2.0   3.0   4.0
 11.0  12.0  13.0  14.0

[kmsquire]

That works for me.

On Saturday, January 18, 2014, Tanmay Mohapatra notifications@github.com
wrote:

Updated. Now:

julia> readdlm(IOBuffer("1\t2\t\t3\n4\t5\t6\n"), '\t')
2x4 Array{Any,2}:
1.0 2.0 "" 3.0
4.0 5.0 6.0 ""

julia> readdlm(IOBuffer("1\t2\t\t3\n4\t5\t6\n"))
2x3 Array{Float64,2}:
1.0 2.0 3.0
4.0 5.0 6.0

julia> readdlm(IOBuffer(" 1 2 3 4\n 11 12 13 14"))
2x4 Array{Float64,2}:
1.0 2.0 3.0 4.0
11.0 12.0 13.0 14.0

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