How to write this thetta fuction in Matlab? I have an example code in Python

How to write this thetta fuction in Matlab?

Someone wrote it in Python like this:

def thetta(r):
    thettar = np.zeros(r.size)
    h = (r_2 - r_1) / N
    for i in range(r.size):
        for j in range(i):
            thettar[i] += h * 0.5 * (1 / (r[j] * math.tan(betta(r[j]))) + 1 / (r[j+1] * math.tan(betta(r[j+1]))))
    return thettar

[Askic V]

Can you please show us your current attempt?

[Beket]

@Askic V but I think this is the wrong way(

thettar = zeros(size(r));
h = (r_2 - r_1) / N;
for i = 1:length(r)
    for j = 1:i-1
        thettar(i) = thettar(i) + h * 0.5 * (1 / (r(j) * tan(betta(r(j)))) + 1 / (r(j+1) * tan(betta(r(j+1)))));
    end
end

[Askic V]

How is betta defined? You should be able to execute the same code in Python and Matlab and compare results.

Matlab has some very good built in functions for integration. For numerical integration I use cumtrapz, but I guess your task is to rewrite the Python implementation to Matlab.

[Beket]

@Askic V Yes, I have to rewrite the Python implementation to Matlab. I have no idea how to write this integral. Because it shows that Conversion to function_handle from double is not possible.

[Dyuman Joshi]

Are you getting an error? If so, please mention the full error.

And/Or are you not getting the output you want? In that case, attach or provide the full code and data for r (a sample data would work as well) and the corresponding required output.

[Beket]

@Dyuman Joshi

betta = @(r) asin(c_rinf(r) / w(r) + z * s / (2 * pi * r));
N = 100;
thettar = @(r) zeros(size(r));
r = linspace(r_1, r_2, N);
h = (r_2 - r_1) / N;
for i = 1:length(r)
    for j = 1:i-1
        thettar(i) = thettar(i) + (h * 0.5 * ((1 / (r(j) * tan(betta(r(j))))) + (1 / (r(j+1) * tan(betta(r(j+1)))))));
    end
end
thettar;

Error:

Conversion to function_handle from double is not possible.

Error in characteristics_impeller (line 115)

thettar(i) = thettar(i) + (h * 0.5 * ((1 / (r(j) * tan(betta(r(j))))) + (1 / (r(j+1) * tan(betta(r(j+1)))))));

[Dyuman Joshi]

What are c_rinf, w, s and z in the definition of betta?

Are they defined before their use in betta?

[Beket]

@Dyuman Joshi Can I sent my Matlab code and Python code to you in direct? Can you help me with this code?

[Dyuman Joshi]

You can attach it here, use the paperclip icon.

Or you can copy paste releveant code.

[Beket]

@Dyuman Joshi but I'm afraid about security, because I really need this code

Answer 1

[Askic V]

Ok, without going into too much details, let's say you have the following Python functions that you need to rewrite in Matlab.

Python code:

import math
import numpy as np

def thetta(r, N):
    thettar = np.zeros(r.size)
    r_1 = r[0]
    r_2 = r[-1]
    h = (r_2 - r_1) / N
    for i in range(r.size):
        for j in range(i):
            thettar[i] += h * 0.5 * (1 / (r[j] * math.tan(betta(r[j]))) + 1 / (r[j+1] * math.tan(betta(r[j+1]))))
    return thettar

def betta(x):
    return 2*x
% Test results 
N = 10
r_1 = 1
r_2 = 5
h =  h = (r_2 - r_1) / N
r = np.arange(r_1 ,r_2 +h,h)
y = thetta(r, N)
print(y)

The Python code will produce teh following result:

[ 0.         -0.4933463  -0.66999674 -0.41547207 -0.42690584 -0.69679015 
 -0.82237975 -0.70521665 -0.7204894  -0.93958357 -1.06804844]

Now, the equivalent implementation in Matlab would be:

clear
clc
r_1 = 1;
r_2 = 5;
N = 10;
h = (r_2 - r_1) / N;

% betta = @(x) 2*x; you can use function handle instead of full function def
r = r_1:h:r_2; % to have the same number of elements compared to python
thettar = zeros(size(r));
for i = 1: size(r,2)-1 
    for j = 1:i 
        thettar(i+1) = thettar(i+1) + h * 0.5 * (1 / (r(j) * tan(betta(r(j)))) + 1 / (r(j+1) * tan(betta(r(j+1)))));
    end
end

thettar
%Some simple betta function, just to be used for test
function y = betta(x)
    y = 2*x;
end

and this code will produce the result:

thettar =

         0   -0.4933   -0.6700   -0.4155   -0.4269   -0.6968   
        -0.8224   -0.7052   -0.7205   -0.9396   -1.0680

So, basically that is all you need. The biggest hassle is to keep in mind that index strats with 0 in Python and with 1 in Matlab.

[Beket]

thank a lot! I think I make sure this issue, but I have another problem in code. I should send you all code. Error: Error: File: Copy_of_characteristics_impeller.m Line: 89 Column: 1

Function definitions in a script must appear at the end of the file.

Move all statements after the "c_rinf" function definition to before the first local function definition.

[Beket]

@Askic V Did you see what I sent?

[Askic V]

First of all, all function definitions should go at the end of the script or to save each function in the separate file 'function_name.m' (name of the file must be the same as the name of the function inside it).

Second, you use undefined variables, for example: Q_rk, how Matlab would know in the line:

c_rinf = Q_rk / (2 * pi * r * b(r));

what Q_rk is?

[Beket]

@Askic V I wrote how to define Q_rk in line 25

[Askic V]

You did, but it is not a global variable, and it is not visible inside the function:

function c_rinf = c_rinf(r)
    c_rinf = Q_rk / (2 * pi * r * b(r));
end

so you need to send it as an input argument, for example:

function c_rinf = c_rinf(Q_rk, r)
    c_rinf = Q_rk / (2 * pi * r * b(r));
end

You need to pay attention to this type of things.

Next you use a lot of bad practice, for example, variable name is the same as function etc...

Answer 2

[Dyuman Joshi]

@Beket, your code needs some changes

94th and 95th line of your code, you used b in c_rinf before defining it.

c_rinf = @(r) Q_rk ./ (2 .* pi .* r .* b(r));
b = @(r) b_1 - (b_1 - b_2) ./ (r_2 - r_1) .* (r - r_1);

Similarly, in lines 110th and 111th, you used r in thettar before defining it.

thettar = zeros(size(r));
r = linspace(r_1, r_2, N);

Correct the order and the error will be rectified.

Also, while defining a function handle, it's better to use element-wise operators .*, ./ and .^ , you can see that change above in c_rinf and b.

Your code still has some errors, mostly syntax errors (check line 121 and 150).

Additionally, what is the purpose of these lines?

bettar = betta(r);
thettar = thettar(r);

[Beket]

can you look at this code?

[Dyuman Joshi]

I already have your original code. Is this one different from that one?

[Beket]

@Dyuman Joshi No, I thought you could fix specific places

[Dyuman Joshi]

Did you make the changes I suggested in my answer and run the code?