The str2double function is taking too long?

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？

[KSSV]

Why and how you have an array of number of dimension 46259*503 into a string?

[Joel Lynch]

If you are actually extracting that much data, then if it is formatted in a regular pattern, you should be able to speed up reading the data using sscanf(), which works because time is not spent interpreting the strings

[xingxingcui]

At present, a better implementation should be as follows.

IS = "_040825_1735_IS.log";
lines = strip(readlines(IS));
lines = lines(strlength(lines)>0);
lg = startsWith(lines,"%");
data = split(lines(~lg)); % 46259*503 size , string array
myNumerticData = double(data);

@KSSV Because I need to import experimental datasets _040825_1735_IS.log linked to the source "http://eia.udg.es/~dribas/" for my research, the data is log files recorded by the instrument,the pure data has a size of 46259*503 size.

@KSSV，@Walter Roberson thanks very much!

[Walter Roberson]

T3 = readmatrix('_040825_1735_IS.log', 'delimiter',' ');

That gives 46264 rows, 502 variables, everything already numeric.

Answer 1

[Walter Roberson]

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

[Stephen23]

Why on earth would you put poor SSCANF inside ARRAYFUN, unless you intentionally want to slow it down?

Here is much more efficient use of SSCANF, with conversion speed on the same order as DOUBLE(S):

M = sscanf(sprintf(' %s',S.'), '%f', [503,Inf]).'

This is faster than your incorrectly named TIME_FOR_SSCANF (which should be named TIME_FOR_ARRAYFUN, as this is what 99.8% of its time is measuring).

[Walter Roberson]

Why? Because sscanf() does not operate on string arrays. I temporarily had a more complicated arrangement with nested calls and cell arrays, but realized that I was operating on the wrong datatype and simplified the code drastically.

[Stephen23]

"Because sscanf() does not operate on string arrays"

True, but using ARRAYFUN is an inefficient workaround.

The variable name is misleading, because 99.8% of that time is ARRAYFUN.

[Walter Roberson]

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.