Loop iteration not working

Hi there! I am really confused why this is not working. Please Help

so I want to iterate 1 2 3 4 5 6 7 8 i.e. the length of array waves

when I run this loop it simply assigns q to 1x1 double with the value 8...

when I run the exact same command in the command window it gives me what i am looking for

q = [1 2 3 4 ... etc ]

i must be missing something

waves=[.21 .22 .23 .5 .81 .82 .83 .84];
for q = 1:length(waves)
    %Find intitial values 
    Lo(q) = (g/(2*pi))*(periods(q)^2);
    k(q) = (2*pi)/Lo(q);  
    c(q) =((g/k(q)*tanh(k(q)*Depth(q))))^(1/2);
    c_g(q) = c(q)*(.5 + ((k(q)*Depth(q))/sinh(2*k(q)*Depth(q))));
    Ew(q) = .125*rho*g*Hrms(q)^2;
end 
% oddly the same style of code works in this script: 
for y = 2:length(profile_u)
    h(y) = profile_u(y)+eta(y-1);  % removed eta for now as that was causing the problem
    if h(y) > dmin
    L(y) = Lo*(tanh((4*(pi^2)*h(y)/(g*period^2).^(3/4)))).^(2/3); % fix equation as above
    k(y) = (2*pi)/L(y);
    c(y) = ((g/k(y)*tanh(k(y)*h(y))))^(1/2);
    c_g(y) = c(y)*(.5 + ((k(y)*h(y))/sinh(2*k(y)*h(y))));
    theta(y)= asin((sin(theta(y-1)*c(y)/c(y-1))));
    
    Eb(y) = EbCoefficient* rho*g*B^3*freq_init.*(Hrms(y-1)^5)./((gamma^2)*(h(y).^3)).*(1-(1./(1+(Hrms(y-1)./(gamma*h(y)).^2)).^(5/2)));
    Ew(y) = (Ew(y-1)*c_g(y-1)*cos(theta(y-1))-Eb(y)* deltax )/(c_g(y)*cos(theta(y)));
    Hrms(y) = sqrt(8*(Ew(y)/(rho*g)));  % put rho and g in ()
    %%%%%setup
    %radiation stress 
    Sxx(y) = Ew(y)*((cos(theta(y))^2+1)*c_g(y)/c(y) - .5); % take () around c_g/c - .5
    
    %Setup 
    dSxxdx(y) = (Sxx(y-1)-Sxx(y))/deltax;
    deltaEta(y) = dSxxdx(y)/(rho*(g*h(y)));
    eta(y) = deltaEta(y)*deltax+eta(y-1);  % added deltax
    if y == 2
       Hrms_b(1)=Hrms(1);
       Ew_b(1) = Ew(1);
       Eb_b(1) =0;
    end 
    [Hrms_b,Ew_b,Eb_b,Hb]=waves_baldock(Lo,freq_init,Hrms_b,y,Hb,Eb_b,Ew_b,rho,g,c_g,deltax,h);
    if y==2 
        Hrms_f(1)=Hrms(1);
        Ew_f(1)=Ew(1);
        epsilon_f(1)=0;
        gamma_f=.37;
    end
    [Hrms_f,Ew_f,epsilon_f]=waves_falk(rho,g,B,freq_init,Hrms_f,y,gamma_f,Ew_f,deltax,c_g,h,epsilon_f);
    if y==2 
        Hrms_r(1)=Hrms(1);
        Ew_r(1)=Ew(1);
        epsilon_r(1)=0;        
        gamma_r(1)=.76*k(1)*h(1)+.29;
        Hbr(1)=.88/k(1)*tanh(gamma_r(1)*k(1)*h(1)/.88);
    end        
    [Hrms_r,Ew_r,epsilon_r,Hbr]=waves_ruessink(k,h,y,rho,g,freq_init,Hrms_r,Ew_r,epsilon_r,c_g,deltax,Hbr);
    %setup
    else
        Hrms(y)=NaN; Hrms_b(y)=NaN; Hrms_f(y)=NaN; Hrms_r(y)=NaN;        
        eta(y)=NaN; eta_RK(y)=NaN; Sxx(y)=NaN; L(y)=NaN; c(y)=NaN;
        k(y)=NaN;
    end
end 

[Stephen23]

"when I run this loop it simply assigns q to 1x1 double with the value 8... "

When the loop has finished, then the iterator retains its last value. But of course it iterated through all of the prior values first. That is how loops work.

"when I run the exact same command in the command window it gives me what i am looking for q = [1 2 3 4 ... etc ] "

I doubt that.

[Theodore Anderson]

why then is k(q) for example all the same value?

[Stephen23]

"why then is k(q) for example all the same value? "

As you did not provide g or periods or Depth I cannot run your code. Have you checked the values in periods?

[Theodore Anderson]

g is a constant, periods an 1x8 array, and depth is a 1x8 array

[Theodore Anderson]

check this out: the indexed variables in the loop change every time. This is because i hard coded a 1:8 array called array rather than using the length of an array. This will be a problem though because I will have to create another nested loop which will be 1:length(profile) which is 8x435

[Theodore Anderson]

Im not crazy i swear! look at my command window:

[Stephen23]

@Theodore Anderson: the code you showed in your question is different to that shown in your screenshots.

Note that in your actual code (i.e. screenshots) the FOR loop is called with a column vector for its VALUES input:

Careful reading of the FOR documentation tells us that it actually loops over the columns of the VALUES array.

This tells us that your code will iterate exactly once, with the loop iterator equal to that entire column vector that you provided as for its VALUES array. This is easy to confirm:

A = 1:8

A = 1×8

1 2 3 4 5 6 7 8

for q = A(:) % what you did (column vector -> iterates once)
    display(q)
end

q = 8×1

1 2 3 4 5 6 7 8

for q = A % probably what you should have done (row vector -> iterates 8 times)
    display(q)
end

q = 1

q = 2

q = 3

q = 4

q = 5

q = 6

q = 7

q = 8

"Im not crazy i swear! look at my command window:"

Sure.. but so far everything you have described is consistent with the documentation. Apparently what you expect to occur is something else... but due to your inconsistent code (i.e. difference between FOR loop given in your question vs screenshot) and the fact that your original complaint "when I run this loop it simply assigns q to 1x1 double with the value 8... " describes the expected behavior of a FOR loop after it has completed (and is also inconsistent with your screenshots), I am still trying to figure out what the problem is.

Changing the information details makes it harder to pinpoint problems that are due to details in the code.

Assume that I am stupid and that I cannot see your computer monitor. Explain what you expect. Explain what occurs.

PS: as DGM wrote, the loop is most likely superfluous anyway.

Answer 1

[DGM]

Consider:

x = 1:10

x = 1×10

1 2 3 4 5 6 7 8 9 10

This expression by itself results in x being stored as a vector

for y = 1:10
    y
end

y = 1

y = 2

y = 3

y = 4

y = 5

y = 6

y = 7

y = 8

y = 9

y = 10

y

y = 10

In a loop statement, the same expression results in y being assigned the same values one at a time in sequence. In this case, y is a scalar and when the loop exits, y retains the last value.

Ultimately, it's kind of moot here anyway. That whole loop isn't needed.

waves=[.21 .22 .23 .5 .81 .82 .83 .84];
periods = rand(size(waves)); % dummy values
Depth = rand(size(waves));
Hrms = rand(size(waves));
Lo = rand(size(waves));
c = rand(size(waves));
g = 1;
rho = 1;

% use the loop
for q = 1:length(waves)
    %Find intitial values 
    Lo1(q) = (g/(2*pi))*(periods(q)^2);
    k1(q) = (2*pi)/Lo1(q);  
    c1(q) =((g/k1(q)*tanh(k1(q)*Depth(q))))^(1/2);
    c_g1(q) = c1(q)*(.5 + ((k1(q)*Depth(q))/sinh(2*k1(q)*Depth(q))));
    Ew1(q) = .125*rho*g*Hrms(q)^2;
end 

% do it without a loop
Lo = (g/(2*pi))*(periods.^2);
k = (2*pi)./Lo;  
c = sqrt((g./k.*tanh(k.*Depth)));
c_g = c.*(.5 + ((k.*Depth)./sinh(2*k.*Depth)));
Ew = .125*rho*g*Hrms.^2;

% show that the results are identical
Lo-Lo1

ans = 1×8

0 0 0 0 0 0 0 0

k-k1

ans = 1×8

0 0 0 0 0 0 0 0

c-c1

ans = 1×8

0 0 0 0 0 0 0 0

c_g-c_g1

ans = 1×8

0 0 0 0 0 0 0 0

Ew-Ew1

ans = 1×8

0 0 0 0 0 0 0 0