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The str2double function is taking too long?

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cui
cui on 10 Jun 2021
Commented: Walter Roberson on 10 Jun 2021
The built-in function "str2double" is very time consuming when I want to convert an array of string type to an array of numertic type, especially when I have an array of string type with many elements(46259*503 size). Is there any way to improve the performance?
my os: win10
matlab 2021a
当我准备把一个string类型的数组(46259*503 大小)转换为numertic类型数组时候,此内置函数“str2double”非常耗时,特别是我的string类型数组较多元素的时候。请问有什么办法可以提高性能?
  4 Comments
Walter Roberson
Walter Roberson on 10 Jun 2021
T3 = readmatrix('_040825_1735_IS.log', 'delimiter',' ');
That gives 46264 rows, 502 variables, everything already numeric.

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Accepted Answer

Walter Roberson
Walter Roberson on 10 Jun 2021
There have been some test results posted showing that double() of a string() object is even faster than str2double()
format long g
S = compose("%.16g", randn(1000,50));
S(1:3,1:3)
ans = 3×3 string array
"0.6965617957186385" "0.7061472333823291" "-0.7023246730823328" "0.02816411095732173" "1.507324316664719" "-1.236968837728482" "-0.834231469338338" "-0.8834500860277891" "-1.431844364400984"
time_for_double = timeit(@()double(S), 0)
time_for_double =
0.020335829
time_for_str2double = timeit(@()str2double(S), 0)
time_for_str2double =
0.574124829
time_for_sscanf = timeit(@()arrayfun(@(V)sscanf(V, '%f'),S))
time_for_sscanf =
3.451887829
t1 = tic;
arrayfun(@str2double,S);
time_for_str2double = toc(t1)
time_for_str2double =
5.699698
  4 Comments
Walter Roberson
Walter Roberson on 10 Jun 2021
format long g
S = compose("%.16g", randn(1000,50));
S(1:3,1:3)
ans = 3×3 string array
"-0.87290576061866" "0.04846286053980744" "-2.710234520504661" "-1.026614391407934" "-0.0983918203860604" "-0.1155795524871058" "1.340938807862134" "0.2622135116577263" "0.7506333968994494"
t = tic;
double(S);
time_for_double = toc(t)
time_for_double =
0.029172
t0 = tic;
str2double(S);
time_for_str2double = toc(t0)
time_for_str2double =
0.624089
t1 = tic;
arrayfun(@(V) sscanf(V, '%f'),S);
time_for_sscanf = toc(t1)
time_for_sscanf =
3.423017
t2 = tic;
arrayfun(@(V) 1, S);
time_for_arrayfun = toc(t2)
time_for_arrayfun =
2.218439
t3 = tic;
arrayfun(@str2double,S);
time_for_str2double = toc(t3)
time_for_str2double =
5.802806
t4 = tic;
sscanf(sprintf(' %s',S.'), '%f', [size(S,2),Inf]).';
time_for_stephen_sscanf = toc(t4)
time_for_stephen_sscanf =
0.068427
t5 = tic;
reshape(str2num(strjoin(S)),size(S));
time_for_str2num = toc(t5)
time_for_str2num =
0.06454
The last of those is marginally better than your sscanf/sprintf approach... on this run.

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