Why du/dx obtained from excel and Matlab gives different results?

Hi

I have csv file with some columns and I want to find the du/dx from two column named as u and x. I did it in excell as well as matlab but both the results are different. Can anyone please help to know why they are diffrent? Am I doing any mistake while calciulating the derivative in matlab. I want to do this for all the csv files but I wanted to varify first for one file. Here is my code that I used in matlab and the csv file named as 0001_derivative in which I have obtained du/dx i.e. (u2-u1)/(x2-x1)and named as "s"

Folder='G:\testcodeforcsvcolumn'

Data = readtable('0001.csv');

U = Data.u;

X = Data.x;

dudx = gradient(U)./gradient(X);

Thank you very much for helping me to learn.

[Raushan]

Hey guys,

I figured that out. I am writting here for those who needs it. Actually you don't need to use gradient in order to find the derivative. Gradient works diffrently than diff. If you use diff(U)./diff(X) you will get the same value of du/dx that you got in excel.

[Raushan]

I want to use the following codes to apply for all the above five csv files. I tried to use the for loop for that but getting concatation error. Can anyone help me out please.

Folder='G:\testcodeforcsvcolumn'

Data = readtable('0001.csv');

U = Data.u;

X = Data.x;

dudx = diff(U)./diff(X);

[Torsten]

It's nonsense to calculate dudx this way since your X data are not in increasing order.

You must first order X and then arrange U according to the X-ordering.

[Raushan]

Thanks for the suggestion. You are right we can arrange them either in increasing or decreasing order, but that will be manipulation with the data. We expect data to vary like this. Code is working for 1 file but not for all 5 files. Can you please help me to implement a for loop for 5 files, so that I will have 5 different dudx, I am trying but getting dimension related error.

Answer 1

[Torsten]

If the X grid is equidistant,

dudx = gradient(U,X(2)-X(1))

[Raushan]

The difference is 5 between the x values but us not in increasing or decreasing oredr. Sometimes it is in increasing and some times it is in decreasing order.

[Raushan]

I tried this but still the same value as before. Its not matching with ''s'' column of 0001_derivative.