How to tranfer the data types of the selected columns of the table from cell string to numeric

5 vues (au cours des 30 derniers jours)
I want to tranfer the data type of the selected columns of the table from cellstring to numeric.
However, I am stuck by this error: cannot convert from double to cell
selected_cols={'Dailypeakwindspeed', 'Dailyprecip', ...
'Dailysnowdepth', 'Dailysnowfall'};
% tb_daily is my table
tb_daily(:,selected_cols)= varfun( @str2double, tb_daily,...
'InputVariables', selected_cols); % this line errors
I found out that tb.Var is not equivalent with tb{:,'Var'}, which is shown by the example below.
tb=table({'1';'2'});
tb(:,'Var1')= table([1;2]) % this gives error: cannot convert from double to cell
tb{:,'Var1'}= [1;2] % this gives error: cannot convert from double to cell
tb.Var1= [1;2] % this is successful! Weird!!!
Too weird!
One solution would be to use the dot syntax, but that means I have to repeat this for every selected column, therefore not satisfied.

Réponse acceptée

Peter Perkins
Peter Perkins le 7 Avr 2017
What you're seeing is because parenthesis and brace assignments are into, while dot assignments replace. To see why, consider tbl(2:end-1,'Var1') = value -- you wouldn't want that to be able to change Var1's type.
There are two options:
1) Write a loop like this:
for i = 1:length(selectedVars)
varName = selectedVars{i};
tbl.(varName) = str2double(t.(varName));
end
If you want to convert all the cellstrs to numeric, you could also write the loop like this:
for i = 1:width(tbl)
var = tbl.(i);
if iscellstr(var)
tbl.(i) = str2double(var);
end
end
2) Horzontally concatenate the output from varfun with the remaining part of tbl, and rearrange the variables afterwards. Something like this:
origVarOrder = tbl.Properties.VariableNames;
selectedVars = varfun(@isnumeric,tbl,'UniformOutput',true);
tbl1 = tbl(~selectedVars);
tbl2 = varfun(@str2double,tbl,'InputVariables',selectedVars);
tbl = [tbl1 tbl2];
tbl = tbl(:,origVarOrder);
Hope this helps.
  3 commentaires
Peter Perkins
Peter Perkins le 11 Avr 2017
Apologies from the mistakes, was typing without a MATLAB in front of me.
Your real problem is how those variables in the table got to be strings to begin with. This seems like an import issue that should be solved at its source. You didn't provide any information on why that is, so can't really help you there.
If a numeric array is what you want, from a table that's all cellstrs, this is how to do it:
>> t = table({'1';'2';'3'},{'4';'5';'6'})
t =
3×2 table
Var1 Var2
____ ____
'1' '4'
'2' '5'
'3' '6'
>> str2double(table2array(t))
ans =
1 4
2 5
3 6
If you want to hold a mixture of numeric and text in one variable in a table, here's how you do it:
>> t = table([1;2;3],{'4';5;'6'},{'7';'8';'9'})
t =
3×3 table
Var1 Var2 Var3
____ ____ ____
1 '4' '7'
2 [5] '8'
3 '6' '9'
Why you would want that, I don't know, it seems like something you'd want to fix.
Liutong
Liutong le 12 Avr 2017
Thank you for the reply.
I should have provided a more detailed background for the problem. I describe it below.
Background: I have a messy data.csv file. Some of the columns contain records like this
%data.csv file
col1,col2
1.0,2ss
1.2s,3.
...,...
To the best of my knowledge, I can only readtable by setting the VariableTypes as string/cellstr first and then process the strings to cut the tail. For example regexprep(col1,'^(\d+)s+','$1') gives ['1';'1.2'].
After I cut the tail comprising 's's, I convert column type from cellctr to numeric using str2double. And that was where I encountered the problem, which has been solved using your solution 2.

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