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

Question

I have such data in a text file:

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

I download them using the following code:

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

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

(Data may be downloaded using some other function, if that helps.)

Answer 1

Logical indexing is a straightforward way to filter by array:

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

If, however, you read your file as a data frame, you will have more options available to you, and it will track 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     │ "..." │ "..." │ "..." │

, DataFramesMeta, ( ). @where SQL-:

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]),:]