Deleting or selecting rows of a struct with a condition

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im db d
im db d 2015-6-29
编辑： James Tursa 2015-6-29
Dear All, I would like to delete or select the rows of a struct by checking the value of a column. For example in the bellow, I say if the value was not 'p' and not 'h', then delete the row.
for i=1:numel(logfile)
if (logfile(i).code ~= 'p') && (logfile(i).code ~= 'H')
logfile(i)=[];
end
end
It only deletes one row, The error that i get is as follow:
Operands to the || and && operators must be convertible to logical scalar values.
Would you please give me some tips?
If i wanted to filter the rows with code value of 'p', then what should I do?
I tried this
logfile([logfile.code == 'p'])
but the error code is as follows :
Index exceeds matrix dimensions.
Thank you in advance.

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采纳的回答

James Tursa
James Tursa 2015-6-29
编辑：James Tursa 2015-6-29
The first part, assuming the .code fields are all 1 character:
logfile_code = [logfile.code]; % Get all the codes
g = logfile_code == 'p' | logfile_code == 'H'; % Find the p and H codes
logfile = logfile(g); % Select only the p and H codes, delete the rest
The second part:
g = logfile_code == 'p'; % Find the p codes
logfile_p = logfile(g); % Select only the p code rows
If the .code fields could be any length string and not single characters, then you could use a cell array of strings instead. E.g.,
logfile_code = {logfile.code};
g = strcmp(logfile_code,'p') | strcmp(logfile_code,'H');
logfile = logfile(g); % Select only the p and H codes, delete the rest
and
g = strcmp(logfile_code,'p');
logfile_p = logfile(g); % Select only the p code rows

更多回答（2 个）

Azzi Abdelmalek
Azzi Abdelmalek 2015-6-29
Don't use ==
for i=1:numel(logfile)
if ~isequal(logfile(i).code,'p') && ~isequal(logfile(i).code,'H')
logfile(i)=[];
end
end
Guillaume
Guillaume 2015-6-29
Most likely, you're getting the error with && because some of your code strings are more than one character. See the output of:
'xyz' ~= 'p'
As a rule, if you're going to compare strings always use strcmp (or its companions strcmpi, strncmp, strncmpi) which will always return a scalar regardless of the length of the string. So,
if ~strcmp(logfile(i).code,'p') && ~strcmp(logfile(i).code, 'H')
will fix your problem.
But you're using a loop, and that's not efficient. You were on the right track with your other expression. Again, you should be using strcmp and you should concatenate all the codes into a cell array rather than a vector, so:
logfile(~strcmp({logfile.code}, 'p') & ~strcmp({logfile.code}, 'H')) = [] %no need for loop
Finally, rather than doing two strcmp, you can use ismember (which uses strcmp internally), so it becomes:
logfile(~ismember({logfile.code}, {'p', 'H'}) = [];

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