If statement in loop not recognized

Hi

The following code:

clear all;
close all;
clc;

W = 16;                     %Collected weight in
g = 9.81;                   %gravity
rho = 1.225;                %density of air
T_per_rotor = W/2 * g;      %Thurst 
N_b = 2;                    %Number of blades
C_d0 = 0.05;                %Drag coeficcient of air foil of blade
C_l0 = 0.05;                %Lift coeficcient of air foil of blade
SolOne = 10000;
kMax = 200;

PlotR = zeros(1,kMax);
PlotOmega = zeros(1,kMax);
PlotPower = zeros(1,kMax);

for k = 1 : kMax
    j = k + 121;
    for n = 0.15 : 0.001 : 1
        A = pi * n ^2;
        c = n/10;
        P_0 = 1/8 * rho * N_b * (j)^3 * c * C_d0*(n)^4;
        P_hover = T_per_rotor * sqrt(T_per_rotor/(2*rho*A));
        Sol = P_0 + P_hover;
        Lift = (1/2)*(1/3)*rho*N_b*c*j^2*n^3*C_l0;
        if T_per_rotor - 5 < Lift < 5 + T_per_rotor 
            if Sol < SolOne
                SolR = n        %missing ; to see n
                SolOmega = j    %missing ; to see what j is
                k               %missing ; to see k ** 
                SolPower = Sol/0.8;
                SolOne = Sol;
                PlotR(k) = SolR;
                PlotOmega(k) = SolOmega;
                PlotPower(k) = SolPower;
            end
        end
    end
end 

The code runs and outputs

k = 200
n = 1 % the two for loops run till the upper limit.

SolR = 0.5120
SolOmega = 122
k = 1 % inside the if statement

With the above results, one would think that the only time the if statements are valid, would be when k = 1. However if you change the limit for k to 20:kMax, SolR and SolOmega changes as well as k in the if statement, meaning that other values of j and n also are valid in both if statements. Question is: why is SolR and SolOmega stopping at 0.5120 and 122 and not at higher numbers?

I'm using the online matlab compiler:

https://matlab.mathworks.com/

Cheers

/Erik

[ananya gupta]

Hi, Thanks to sharing code because i also faced issue while using codenut now I think I can resolve this. I also learn this from CETPA INFOTECH.

Answer 1

[Stephen23]

The problem is likely to be this invented syntax:

T_per_rotor - 5 < Lift < 5 + T_per_rotor 

MATLAB does not have ternary logical comparisons like X<Y<Z. The MATLAB documentation shows that all of the logical comparisons are binary functions like X<Y. So you will need to do join them together, e.g.:

(T_per_rotor-5)<Lift && Lift<(5+T_per_rotor) 

[Erik Lenstrup]

Hi, thanks for your answer

I have done the following, spilting up the first if statement.

clear all;
close all;
clc;

W = 16;                     %Collected weight in
g = 9.81;                   %gravity
rho = 1.225;                %density of air
T_per_rotor = W/2 * g;      %Thurst 
N_b = 2;                    %Number of blades
C_d0 = 0.05;                %Drag coeficcient of air foil of blade
C_l0 = 0.05;                %Lift coeficcient of air foil of blade
SolOne = 10000;
kMax = 200;

PlotR = zeros(1,kMax);
PlotOmega = zeros(1,kMax);
PlotPower = zeros(1,kMax);

for k = 20 : kMax
    j = k + 121;
    for n = 0.15 : 0.001 : 1
        A = pi * n ^2;
        c = n/10;
        P_0 = 1/8 * rho * N_b * (j)^3 * c * C_d0*(n)^4;
        P_hover = T_per_rotor * sqrt(T_per_rotor/(2*rho*A));
        Sol = P_0 + P_hover;
        Lift = (1/2)*(1/3)*rho*N_b*c*j^2*n^3*C_l0
        if (T_per_rotor-2)<Lift         %1st if statement 
           if Lift<(2+T_per_rotor)      %2nd if statement
                if Sol < SolOne         %3rd if statement
                    SolR = n
                    SolOmega = j
                    k
                    SolPower = Sol/0.8;
                    SolOne = Sol;
                    PlotR(k) = SolR;
                    PlotOmega(k) = SolOmega;
                    PlotPower(k) = SolPower;
                end
            end
        end
    end
end 

However now the 1st if-statement is not entered even though Sol is smaller than SolOne 10000 (which is the initial limit for SolOne). Do you know what is going on, can you produce results?

[Stephen23]

"However now the 1st if-statement is not entered even though Sol is smaller than SolOne"

That is the 3rd if-statement, not the 1st. Before it gets to the 3rd if-statement, it has to pass the 1st and 2nd. At the start of the code Sol might be less than SolOne, but the 1st if condition is not met:

>> T_per_rotor-2
ans =  76.480
>> Lift
Lift =  0.020549

It is not until many iterations later

>> k
k =  73
>> n
n =  0.99900

that the condition for the 1st if-statement is first met:

>> 2+T_per_rotor
ans =  80.480
>> Lift
Lift =  76.533

and luckily also the condition for the 2nd if. But not the 3rd one:

>> Sol
Sol =  11375.30525
>> SolOne
SolOne =  10000

Whether you like it or not, there is never an iteration where the conditions for all three if-statements are true. Perhaps there is a bug in the code, perhaps the algorithm has not been implemented correctly, perhaps the code is correct and those particular data simply do not produce those outputs, I have no idea. But for some reason you keep ignoring the conditions for the 1st and 2nd if-statements, which have to be true before you can even start to look at that 3rd if statement.