Import grid data and surface fit

2018 9 2
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更新时间：2018 9 5
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Hi,
Looking for help on what is probably an easy problem. I can't even get data into Matlab correctly to start!
I have MS Excel grid - temperature (orange), %alcohol (blue), density (green). This is excerpt:
I would like to create a function/formula: input density and temperature, output %alcohol. This function should interpolate/extrapolate, so for example could enter temperatures like 34.6, 40.7
So guess I would like to fit a surface and get function equation, and then reorganize so %alcohol is on LHS.
Questions:
1. First problem how to import data into grid format I can use? I have tried to bring in as table, but treats every column as different variable.
2. How to best fit a function to this data and "solve" for %alcohol.
Any suggestions, similar examples, outline workflows appreciated.
Thanks, Michael

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Walter Roberson
Walter Roberson 2018-9-4

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data = xlsread('YourFile.xlsx');
alch = data(2:end,1);
temperature = data(1,2:end);
density = data(2:end,2:end);
surf(temperature, alch, density, 'edgecolor','none')
xlabel('temperature'); ylabel('% alcohol')

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Michael Cody
Michael Cody 2018-9-5
Hi Walter, This is nice succinct answer to my question on how to order data for 3D surface. Obvious of course, when someone shows you!
I think will close this question now and split my second question into a new thread after I do some more research.
Thanks a lot. Michael

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ahmed nebli
ahmed nebli 2018-9-2

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1- u need to use the function xlsread , see this link : https://www.mathworks.com/help/matlab/ref/xlsread.html

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Michael Cody
Michael Cody 2018-9-3
Hi Ahmed,
Thanks for answer. With this function can import row, columns, ranges etc. But can't figure out how to organize variables so could use surf function, for example. Data dimensions must agree.
Can you give me any further suggestions?
Thanks, Michael
ahmed nebli
ahmed nebli 2018-9-3
i think once you imported the xls sheet, u need to store it in a .mat file , then you can treat the double sttored in the .mat like you want
Michael Cody
Michael Cody 2018-9-4
Loaded data, each set into own variable matrix. My main problem was how to organize data for 3D plot and curve fitting.
I eventually found meshgrid which seemed to help me.
[abv_x,temperature_y] = meshgrid(abv,temperature)
surf (abv_x,temperature_y,density)
Thanks for tip on xlsread.
On question 2, if I fit with interpolation curve then seems cannot evaluate for points outside original data range e.g my max temperature is 40, if try feval on fit function get NaN. Is this limitation of interpolation do you know?
Thanks, Michael

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