How to convert strings in a table to numbers?

I have a large table (280,000 x 40) in which some of the columns are numeric but stored as strings, for example:

Block   Lot     District
'121'  '35'  '13'
'121'  '36'  '13'
'121'  '37'  '13'

Don't ask me why they are stored this way - the original data is in a .txt file. Is there either

• a) a way to import this file so that the numbers don't show up as strings (currently I'm using readtable) or
• b) a way to quickly convert all these entries into numbers. Right now I'm using the following code, which seems to work, but takes a long time:

 for i=1:40
    b1(:,i)=table2array(bk13(:,i));
    b2(:,i)=str2double(b1(:,i));
    b3(:,i)=array2table(b2(:,i));
    bk13{:,i}=table2cell(b3(:,i));
end

Answer 1

[Walter Roberson]

It might help to specify the TreatAsEmpty option to readtable, and perhaps the Format option as well.

Using detectImportOptions() and passing the result to readtable() might help.

Answer 2

[Jan]

A simplification of your code omitting unnecessary steps:

for i=1:40
  bk13.(i) = str2double(bk13{:,i});
end

[Matthew Gordon]

Thanks, this worked for b, but the answer to a worked even better.