Convert a fixed width char array into a column vector

2020 7 28
3 个回答
回答已采纳
更新时间：2020 7 29
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Hello! I have troubles converting numbers from a char array to double format. The char array always has the dimensions 1x48. Every 8 characters represent an integer. Sometimes there are spaces between numbers and sscanf fails to give the results I need. For example:
aa = ' 1703434 42 1012275140184521401845314018473';
sscanf(aa,'%8d')
ans =
1703434
42
10122751
40184521
40184531
4018473
should return in fact
ans =
1703434
42
10122751
14018452
14018453
14018473
The reason why I don't use str2double for each 8 characters in 'aa' is that it gets very slow for many conversions. The original file I read also contains other words, and this array 'aa' is only a part of each line I read. So far sscanf was the fastest method and it worked well until this line was encountered.
Could anyone please explain me why sscanf behaves like this? Is there any quick alternative?

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 采纳的回答

David Hill
David Hill 2020-7-28

1 个投票

str2num(reshape(aa,8,[])');

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Adri
Adri 2020-7-28
编辑：Adri 2020-7-28
I still used sscanf using the reshape idea of yours. It's a little bit faster:
aa = ' 1703434 1012275140184521401845314018473';
tic
for ii = 1:500000
colvec = sscanf([reshape(aa,8,[])', repmat(' ',length(aa)/8,1)]','%8f ');
end
toc
tic
for ii = 1:500000
colvec2 = str2num(reshape(aa,8,[])');
end
toc
isequal(colvec,colvec2)
Elapsed time is 5.666218 seconds.
Elapsed time is 9.619199 seconds.
ans =
logical
1
Thank you for your suggestion!

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更多回答（2 个）

Stephen23
Stephen23 2020-7-28
编辑：Stephen23 2020-7-28

1 个投票

This will be quite efficient:
>> aa = ' 1703434 42 1012275140184521401845314018473';
>> vec = sscanf(sprintf('%c%c%c%c%c%c%c%c,',aa),'%f%*[ ,]')
vec =
1703434
42
1012275
14018452
14018453
14018473

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Stephen23
Stephen23 2020-7-28
Timing for 1e4 iterations:
Elapsed time is 2.80135 seconds. % reshape, repmat, sscanf
Elapsed time is 7.16556 seconds. % reshape, str2num
Elapsed time is 0.92013 seconds. % this answer
Adri
Adri 2020-7-29
I tried this alternative both in my main code and in this example and it seems that on my machine it's slower (weird or not?):
aa = ' 1703434 1012275140184521401845314018473';
tic
for ii = 1:500000
colvec = sscanf([reshape(aa,8,[])', repmat(' ',length(aa)/8,1)]','%8f ');
end
toc
tic
for ii = 1:500000
colvec2 = str2num(reshape(aa,8,[])');
end
toc
tic
for ii = 1:500000
colvec3 = sscanf(sprintf('%c%c%c%c%c%c%c%c,',aa),'%f%*[ ,]');
end
toc
Elapsed time is 5.695740 seconds.
Elapsed time is 9.565251 seconds.
Elapsed time is 15.707137 seconds.
Anyway, thank you very much for your suggestion!

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