How to filter rows from Julia Array based on value of value in specified column?

Question

I have data like this in a text file:

CLASS     col2    col3    ...
1         ...     ...     ...
1         ...     ...     ...
2         ...     ...     ...
2         ...     ...     ...
2         ...     ...     ...

I load them using the following code:

data = readdlm("file.txt")[2:end, :] # without header line

And now I would like to get array with rows only from class 1.

(Data could be loaded using some other function if it would help.)

Answer 1

Logical indexing is the straight-forward way to do filtering on an array:

data[data[:,1] .== 1, :]

If, though, you read your file in as a data frame, you'll have more options available to you, and it'll keep track of your headers:

julia> using DataFrames
julia> df = readtable("file.txt", separator=' ')
5×4 DataFrames.DataFrame
│ Row │ CLASS │ col2  │ col3  │ _     │
├─────┼───────┼───────┼───────┼───────┤
│ 1   │ 1     │ "..." │ "..." │ "..." │
│ 2   │ 1     │ "..." │ "..." │ "..." │
│ 3   │ 2     │ "..." │ "..." │ "..." │
│ 4   │ 2     │ "..." │ "..." │ "..." │
│ 5   │ 2     │ "..." │ "..." │ "..." │

julia> df[df[:CLASS] .== 1, :] # Refer to the column by its header name
2×4 DataFrames.DataFrame
│ Row │ CLASS │ col2  │ col3  │ _     │
├─────┼───────┼───────┼───────┼───────┤
│ 1   │ 1     │ "..." │ "..." │ "..." │
│ 2   │ 1     │ "..." │ "..." │ "..." │

There are even more tools available with the DataFramesMeta package that aim to make this simpler (and other packages actively under development). You can use its @where macro to do SQL-style filtering:

julia> using DataFramesMeta
julia> @where(df, :CLASS .== 1)
2×4 DataFrames.DataFrame
│ Row │ CLASS │ col2  │ col3  │ _     │
├─────┼───────┼───────┼───────┼───────┤
│ 1   │ 1     │ "..." │ "..." │ "..." │
│ 2   │ 1     │ "..." │ "..." │ "..." │

Answer 2

data[find(x -> a[x,1] == 1, 1:size(data)[1]),:]