How to make my code run faster

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beginner94 2018-4-24

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评论： beginner94 2018-5-12

采纳的回答： Guillaume

modified.csv

在 MATLAB Online 中打开

Good Morning,

this line apparently takes 21s to run:

s = size(xlsread(filename, 1, 'F:F'))

It is supposed to count the number of rows of an Excel sheet. It is actually a .csv data file, but the values are not separated by commas but by semicolons, so I cannot use the Matlab function ''csvread''. Is there maybe a faster or more elegant way to do this? And is it the ''xlsread'' oder the ''size'' that takes so long...?

Thank you in advance

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Guillaume 2018-4-24

编辑：Guillaume 2018-4-24

xlsread needs to

start excel
get excel to read the file
parse the text to convert it into numbers
read the data from the worksheet

The first three operations in particular are going to take some time. Excel is not a lightweight program and reading data from disk always takes time.

All that just to get the number of rows in a text file?

beginner94 2018-4-27

I needed the number of rows to know how many loops were necessary to get through the file and then I want to transmit the content to a matrix

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Guillaume 2018-4-24

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If all you want to do is count the number of lines in the file, then parsing it is a waste of time:

numlines = sum(fileread('modified.csv') == char(10))

is probably the fastest way to get the number of lines.

If you do want to actually access the content of the file, then readtable is probably the easiest way to get it. This does not involve transitioning through excel so should be much faster than xlsread. readtable can usually detect the file format on its own. If not you can give it hints.

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Walter Roberson 2018-5-7

在 MATLAB Online 中打开

ds = strcat(column{2}, {' '}, column{3})

and then you can use datenum() on the date string to decode into serial date numbers.

beginner94 2018-5-11

编辑：beginner94 2018-5-11

@Walter Roberson

This doesn't solve the problem unfortunately because I don't know yet how to specify the time/date format. For now I only get columns with "NaN"... Is there maybe a way to search for the colons in the time column with "strfind" and then return the hours, minutes and seconds seperately and put them together afterwards?

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Walter Roberson 2018-5-11

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S = fileread('modified.csv');
S(S==',') = '.';   %file uses comma for decimal point
fmt = ['%f%s%s', repmat('%f', 1, 8)];
datacell = textscan(S, fmt, 'HeaderLines', 3, 'Delimiter', ';', 'MultipleDelim', true, 'CollectOutput', true);
ds = strcat(datacell{2}(:,1), {' '}, datacell{2}(:,2));  
dates = datenum(ds, 'dd.mm.yyyy HH:MM:SS');
all_data = [datacell{1}, dates, datacell{3}];