Julia Row-wise operation of DataFrames

New to Julia
,
1 
    function substring_(x::String,i::Int32)
        if i==1
            return string(x[1])
        elseif i==2
            return x[2:end]
        else
            return "NA"
        end
    end

I would like to apply the function above on a column of dataframe:
i.e. substring_.(train[:col],2).
But it returns an array of single character e.g. [‘a’,‘b’,‘c’…] instead of [“apo”,“bksk”,“cssa”…].

In python, there is an apply + lambda method to do what I want. May I know how Julia can implement this? Thanks.

2

Make sure you are actually applying the function you have pasted (you might have changed the definition of substring_ and not reloaded it), as:

  • the method you describe does not return Char values;
  • the method will not get called by substring_.(train[:col],2), unless you use 32-bit Julia;

Now regarding your function I would recommend to rewrite it as:

function substring_(x::String,i::Int) # changed Int32 to Int so that we use the default integer type on your machine
        if i==1
            return first(x, 1) # this gives you a string containing the first character of x
        elseif i==2
            return chop(x, head=1, tail=0) # this is a safe way to remove the first character from x; your code would fail on non-ASCII characters
        else
            return "NA"
        end
    end

Then:

substring_.(train.col,2) # note that in DataFrames.jl you can access columns by name as a property

will work.

Also in Julia a standard way to indicate a missing value is missing, so you might consider using this value instead of "NA" in your code (“NA” might be OK for your purposes, so I did not change this in the code. However, note that if you write substring_("aNA", 2) you also get "NA" returned and then you are not able to distinguish these cases).

If some additional explanations of the above would be helpful please let me know.

3

Thanks for your useful advice. I think my function was not reloaded well. Your suggestions help improve my Julia programming skills.