How do I create an element wise if-else code, and apply equations to inputs that meet certain criteria?

Here is my code, (sigma is a (74724,1) column vector)

Eya = 2.83*10^7;
Y = log10((28300.*sigma)/Eya)
X1 = (17.0181 - 19.8713*Y + 4.21366*Y.^2)./(1 - 0.1720606*Y - 0.633592*Y.^2)
X2 = 1./(-.331096 + (4.3261*log(Y))./Y.^2)

if 10.^Y >= 14.4;                    %**this is where i assume my issue lies**
    X = X1;
else
    X = X2;
end

N = 10.^X                         %Allowable number of cycles (should be very high)
nc = 45000;                       %Total number of actual cycles (constant)
D = nc./N                         %Damage ratio (should be very low for most values of sigma, and never exceeding 1)

I need a new vector, X, (74724,1) to be created from either X1 or X2 depending on Y meeting the criteria as seen in the if then statement

Any help is much appreciated, Thank you!

Answer 1

[Jan]

"Elementwise" does either mean a loop:

for k = 1:numel(sigma)
    Y = log10(28300 * sigma(k) / Eya);
    ...
end

Or you use logical indexing:

...
m     = 10.^Y >= 14.4;
X     = zeros(size(sigma));
X(m)  = X1(m);
X(~m) = X2(~m);
...    

or slightly easier:

...
m     = 10.^Y >= 14.4;
X     = X2;
X(m)  = X1(m);
...    

As usual I mention, that 2.83*10^7 is a multiplication and an expensive power operation, while 2.83e7 is a cheap constant.

[Nick Lavanture]

Thank you, unfortunately none of these solutions worked, here is my current script

clc,clear

Stress = readmatrix('Nonlinear 1 Stress 500psi.txt');

sigma = Stress(:,5);

n = 1;

for n = 1:74723; %this loop converts any sub 1000psi value of sigma to 1000psi, necessay to avoid a negative log and imaginary numbers for X2

if sigma(n) < 1000;

sigma(n) = 1000;

end

n = n+1;

end

%sigma = 13000;

%test input, will overwrite the above data, and output the correct values I am looking for

Eya = 2.83e7;

Y = log10((28300.*sigma)/Eya)

X1 = (17.0181 - 19.8713*Y + 4.21366*Y.^2)./(1 - 0.1720606*Y - 0.633592*Y.^2)

X2 = 1./(-.331096 + (4.3261*log(Y))./Y.^2)

%if 10.^Y >= 14.4;

%X = X1;

%else

%X = X2;

%end

%m = 10.^Y >= 14.4;

%X = X2;

%X(m) = X1(m);

m = 10.^Y >= 14.4;

X = zeros(size(sigma));

X(m) = X1(m);

X(~m) = X2(~m);

N = 10.^X

nc = 45000;

D = nc./N

[Jan]

This is not the way for loops work:

for n = 1:74723  % No ;
    if sigma(n) < 1000;
        sigma(n) = 1000;
    end
    n = n+1;  % Nope, omit this!
end

A nicer code to perform this without a loop:

sigma = min(sigma, 1000);

Please mention, what "none of these solutions work" mean. Do you get an error message? Does the result differ from your expectations? I do not have your input data, so I cannot run the code by my self. If you do not explain, what the problem is, I cannot guess it.