Adding a new column of differences to a table

Question

Stef1998 2021-11-21

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回答： Peter Perkins 2021-11-23

Hi I have a table T of values - 17520x3, the first column is date/time data (A), the second (B) and third (C) are numbers.

I would like to create 2 new columns that show the difference between each consecutive row in column B and C.

I keep getting an error because the number of rows in the table do not match now. The diffB and diffC columns will always have 1 less.

Is there a way to fix this error?

I would like the output to look like this:

A B C diffB diffC

10/10/2001 3 5 0 0

10/10/2001 5 30 2 25

10/10/2001 10 50 5 20

T = readtable(datafile.csv)
array2table(T, 'VariableNames', {'A', 'B', 'C'})
[T array2table(diff(T{:,{'B','C'}}),'VariableNames',{'diffB','diffC'})];

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Answer 1

David Hill 2021-11-21

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[T array2table([0,0;diff(T{:,{'B','C'}})],'VariableNames',{'diffB','diffC'})];

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Stef1998 2021-11-22

thank you so much!

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Answer 2

Peter Perkins 2021-11-23

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https://ww2.mathworks.cn/matlabcentral/answers/1592309-adding-a-new-column-of-differences-to-a-table#answer_838764

在 MATLAB Online 中打开

Even simpler:

T.diffB = [0;diff(T.B)];
T.diffC = [0;diff(T.C)];

David's soln has the advantage of being easier to write when there are a zillion variables. Here's a slightly more succinct version of it:

>> T = array2table([3 5; 5 30; 10 50],"VariableNames",["B" "C"])
T =
  3×2 table
    B     C 
    __    __
     3     5
     5    30
    10    50
>> T{2:end,["diffB" "diffC"]} = diff(T{:,1:2})
T =
  3×4 table
    B     C     diffB    diffC
    __    __    _____    _____
     3     5      0        0  
     5    30      2       25  
    10    50      5       20  