Calculation across rows without FOR LOOP

Question

William Ambrose 2020-10-22

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/622358-calculation-across-rows-without-for-loop

评论： KSSV 2020-10-22

Hi,

I have a table with 20 million records where I need to calculate a value which is dependent on a previous row's value and the current row's value. This new value is subsequently input to the next row. I have solved this with a FOR LOOP but with 20 million records this is very slow. I have previously got suggestions for use of cumprod and vectorized calculations which have been very useful, but due to a simplification of the original problem didn't really work. I have therefore redefind the issue and was hoping someone would have another potential solution without the FOR LOOP.

The calculation below is not a real issue, it is intended to show that I have a problem which needs to be solved sequentially with one value having been calculated is subsequently used as input into the next calculation.

Thank you for your time and help.

Kind regards,

William

B = table([1;1;1;1;1;2;2;2;3],[1;2;3;4;5;6;7;8;500]);
B.Var3 = zeros(height(B),1);
i = [false; B.Var1(1:end-1) == B.Var1(2:end)];
j = find(~i);
B.Var3(j) = B.Var2(j);
for n = 1:height(B)
    if  i(n) == 1
        B.Var3(n) = power((sin(B.Var3(n-1)) + sqrt(B.Var2(n))) / 2,3);
    end
end
>> B
B =
  9×3 table
    Var1    Var2     Var3 
    ____    ____    ______
     1        1          1
     1        2     1.4346
     1        3     2.5232
     1        4      2.146
     1        5     3.6351
     2        6          6
     2        7     1.6563
     2        8      6.994
     3      500        500

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Answer 1

KSSV 2020-10-22

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/622358-calculation-across-rows-without-for-loop#answer_521223

编辑：KSSV 2020-10-22

在 MATLAB Online 中打开

B = table([1;1;1;1;1;2;2;2;3],[1;2;3;4;5;6;7;8;500]);
B.Var3 = zeros(height(B),1);
i = [false; B.Var1(1:end-1) == B.Var1(2:end)];
j = find(~i);
B.Var3(j) = B.Var2(j);
idx = find(i == 1) ;
T = sin(B.Var3) ;
B.Var3(idx) = power((T(idx-1) + sqrt(B.Var2(idx))) / 2,3);

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William Ambrose 2020-10-22

在 MATLAB Online 中打开

Thank you for this. Although a nice way of doing this I think the challenge is that the calculation is all done in parallel and therefor doesn't capture how a calculated value is input into the next calculation.

Values for Var3 in the original problem:

B =
  9×3 table
    Var1    Var2          Var3      
    ____    ____    ________________
     1        1                    1
     1        2     1.43464717050245
     1        3     2.52322347665458
     1        4     2.14595795245583
     1        5     3.63511709803536
     2        6                    6
     2        7      1.6563005366775
     2        8     6.99402697270611
     3      500                  500

When running your suggestion

B =
  9×3 table
    Var1    Var2          Var3       
    ____    ____    _________________
     1        1                     1
     1        2      1.43464717050245
     1        3     0.649519052838329
     1        4                     1
     1        5      1.39754248593737
     2        6                     6
     2        7       1.6563005366775
     2        8      2.82842712474619
     3      500                   500

If one takes a look at B.Var3(3) that is the same as power(sqrt(3)/2,3) while what I wanted was power((sin(B.Var3(2)) + sqrt(B.Var2(3)))/2,3) which was a different number.

Do you see what I mean

William Ambrose 2020-10-22

Thank you, but I still get the same answer, the issue is that I need to calculate a number and then in the next instance use the newly calculated number as input to the next calculation. Hence I see a need for sequential calculations whilst I think your approach doesn't do this, unless I am missing something.