why does it get stuck in loop if?

clear all
close all
L=40;
d1=6;
d2=6;
H1=4;
H2=3;
d=3;
k1=0.004;
k2=0.002;
avg=0.5*(k1+k2);
Nx =L+2;
Ny = d1+d2+1 ;
%sheet pile will be at Nx=0.5*L+1
% initialize coefficient matrix and constant vector
A = zeros(Nx*Ny);
X = zeros(Nx*Ny,1);
% initial head distribution
H(1:Nx*Ny,1) = 0;
% coefficient matrix and constant vector
% Corners
%bottom left
i=1;
A(i,Nx+i) = 1;
A(i,2) = 1;
A(i,1) = -4;
%bottom right
i = Nx;
A(i,i+Nx) = 1;
A(i,i-1) = 1;
A(i,i) = -4;
%top left
i = (Ny-1)*Nx + 1;
A(i,i+1) = 1;
A(i,i) = -4;
A(i,i-Nx) = 1;
X(i) = -(H1);
%top right
i = Nx*Ny;
A(i,i-1) = 1;
A(i,i) = -4;
A(i,i-Nx) = 1;
X(i) = -(H2);
% Boundary nodes
% bottom
for m = 2:(Nx-1)
    %n = 1
    i = m;
    A(i,i+Nx) = 2;
    A(i,i+1) = 1;
    A(i,i-1) = 1;
    A(i,i) = -4;
end
%top
for m = 2:((Nx)*0.5)
    i = (Ny-1)*Nx + m;
    A(i,i-Nx) = 1;
    A(i,i+1) = 1;
    A(i,i-1) = 1;
    A(i,i) = -4;
    X(i)=-H1;
end
for m = ((0.5*Nx)+1):(Nx-1)
    i = (Ny-1)*Nx + m;
    A(i,i-Nx) = 1;
    A(i,i+1) = 1;
    A(i,i-1) = 1;
    A(i,i) = -4;
    X(i)=-H2;
end
%left
for m=2:(Ny-1)
    %m = 1
    i = (m-1)*Nx + 1;
    A(i,i+Nx) = 1;
    A(i,i+1) = 2;
    A(i,i-Nx) = 1;
    A(i,i) = -4;
end
%right
for m=2:(Ny-1)
    %m = Nx
    i = (m-1)*Nx + Nx;
    A(i,i+Nx) = 1;
    A(i,i-1) = 2;
    A(i,i-Nx) = 1;
    A(i,i) = -4;
end
%intersection
for m=((d2*Nx) +1):((d2+1)*Nx)
    i=m;
    A(i,i+Nx) = k1;
    A(i,i-Nx) = k1;
    A(i,i+1) = avg;
    A(i,i-1) = avg;
    A(i,i) = -2*(avg+k1);
end
%inner nodes below intersection
for n = 2:d2
    for m = 2:(Nx-1)
        i = (n-1)*Nx + m;
        A(i,i+Nx) = 1;
        A(i,i-Nx) = 1;
        A(i,i+1) = 1;
        A(i,i-1) = 1;
        A(i,i) = -4;
    end
end
%inner nodes above intersection
for n = (d2+2):(Ny-1)
    for m = 2:(Nx-1)
        while((Ny-d)<n<=Ny)
            %left of sheet pile
            if m==(0.5*Nx)
                i=(n-1)*Nx+m;
                A(i,i+Nx) = 1;
                A(i,i-1) = 2;
                A(i,i-Nx) = 1;
                A(i,i) = -4;
            end
            %right of sheet pile
            if m==(Nx*0.5)+1
                i=(n-1)*Nx+m;
                A(i,i+Nx) = 1;
                A(i,i+1) = 2;
                A(i,i-Nx) = 1;
                A(i,i) = -4;
            end
            %bottom of sheet pile
            if n== Ny-d
                if m==(0.5*Nx)
                    i=(n-1)*Nx+m;
                    A(i,i-Nx) = 1;
                    A(i,i+1) = 1;
                    A(i,i-1) = 1;
                    A(i,i) = -4;
                    A(i,i+Nx)=0.5;
                    A(i,i+Nx+1)=0.5;
                end
            end
            i = (n-1)*Nx + m;
            A(i,i+Nx) = 1;
            A(i,i-Nx) = 1;
            A(i,i+1) = 1;
            A(i,i-1) = 1;
            A(i,i) = -4;
        end
        
    end
end
% Direct Method
H =(inv(A) * X);
for n=1:Ny
    for m=1:Nx
        i = (n-1)*Nx + m;
        while(n< (Ny-d))
            if  m==(0.5*Nx)
                H(i+1)=H(i);
            end
        end
        Direct(n,m) = H(i);
    end
    
end

[SL: formatted as code and Smart Indented it in the Editor to make the start and end of for, if, while statements align. Piyush Kelkar, in future questions please use the first button in the Code section on the toolbar to format your code as code when you enter it. Thanks.]

Answer 1

[Steven Lord]

In the part of your code that starts with the comment %inner nodes above intersection you have a while statement:

while((Ny-d)<n<=Ny)

This doesn't do what you think it does. It is the equivalent of:

while ((Ny-d) < n) <= Ny

(Ny-d) < n is either false (treated as 0) or true (treated as 1). So this is one of:

while 0 <= Ny
    
end
% or
while 1 <= Ny
    
end

To do what you want:

while ((Ny-d) < n) & (n <= Ny)
    
end

But note that none of Ny, d, or n change inside the while statement. So if MATLAB enters that statement because the condition is satisfied, it can never leave that statement because the condition will always be satisfied whenever it checks. You have this same problem with the while in the code started with the % Direct Method comment.

[Eshwaree Shingwekar]

clear all

close all

L=40;

d1=6;

d2=6;

H1=4;

H2=3;

d=3;

k1=0.004;

k2=0.002;

avg=0.5*(k1+k2);

Nx =L+2;

Ny = d1+d2+1 ;

%sheet pile will be at Nx=0.5*L+1

% initialize coefficient matrix and constant vector

A = zeros(Nx*Ny);

X = zeros(Nx*Ny,1);

% initial head distribution

H(1:Nx*Ny,1) = 0;

% coefficient matrix and constant vector

% Corners

%bottom left

i=1;

A(i,Nx+i) = 1;

A(i,2) = 1;

A(i,1) = -4;

%bottom right

i = Nx;

A(i,i+Nx) = 1;

A(i,i-1) = 1;

A(i,i) = -4;

%top left

i = (Ny-1)*Nx + 1;

A(i,i+1) = 1;

A(i,i) = -4;

A(i,i-Nx) = 1;

X(i) = -(H1);

%top right

i = Nx*Ny;

A(i,i-1) = 1;

A(i,i) = -4;

A(i,i-Nx) = 1;

X(i) = -(H2);

% Boundary nodes

% bottom

for m = 2:(Nx-1)

%n = 1

i = m;

A(i,i+Nx) = 2;

A(i,i+1) = 1;

A(i,i-1) = 1;

A(i,i) = -4;

end

%top

for m = 2:((Nx)*0.5)

i = (Ny-1)*Nx + m;

A(i,i-Nx) = 1;

A(i,i+1) = 1;

A(i,i-1) = 1;

A(i,i) = -4;

X(i)=-H1;

end

for m = ((0.5*Nx)+1):(Nx-1)

i = (Ny-1)*Nx + m;

A(i,i-Nx) = 1;

A(i,i+1) = 1;

A(i,i-1) = 1;

A(i,i) = -4;

X(i)=-H2;

end

%left

for m=2:(Ny-1)

%m = 1

i = (m-1)*Nx + 1;

A(i,i+Nx) = 1;

A(i,i+1) = 2;

A(i,i-Nx) = 1;

A(i,i) = -4;

end

%right

for m=2:(Ny-1)

%m = Nx

i = (m-1)*Nx + Nx;

A(i,i+Nx) = 1;

A(i,i-1) = 2;

A(i,i-Nx) = 1;

A(i,i) = -4;

end

%intersection

for m=((d2*Nx) +1):((d2+1)*Nx)

i=m;

A(i,i+Nx) = k1;

A(i,i-Nx) = k1;

A(i,i+1) = avg;

A(i,i-1) = avg;

A(i,i) = -2*(avg+k1);

end

%inner nodes below intersection

for n = 2:d2

for m = 2:(Nx-1)

i = (n-1)*Nx + m;

A(i,i+Nx) = 1;

A(i,i-Nx) = 1;

A(i,i+1) = 1;

A(i,i-1) = 1;

A(i,i) = -4;

end

end

%inner nodes above intersection

for n = (d2+2):(Ny-1)

for m = 2:(Nx-1)

%bottom of sheet pile

if n==( Ny-d)

if m==(0.5*Nx)

i=(n-1)*Nx+m;

A(i,i-Nx) = 1;

A(i,i+1) = 1;

A(i,i-1) = 1;

A(i,i) = -4;

A(i,i+Nx)=0.5;

A(i,i+Nx+1)=0.5;

end

end

while ((Ny-d) < n & n <= Ny)

%left of sheet pile

if m==(0.5*Nx)

i=(n-1)*Nx+m;

A(i,i+Nx) = 1;

A(i,i-1) = 2;

A(i,i-Nx) = 1;

A(i,i) = -4;

end

%right of sheet pile

if m==(Nx*0.5)+1

i=(n-1)*Nx+m;

A(i,i+Nx) = 1;

A(i,i+1) = 2;

A(i,i-Nx) = 1;

A(i,i) = -4;

end

n=n+1;

m=m+1;

end

end

i = (n-1)*Nx + m;

A(i,i+Nx) = 1;

A(i,i-Nx) = 1;

A(i,i+1) = 1;

A(i,i-1) = 1;

A(i,i) = -4;

end

% Direct Method

H =(inv(A) * X);

for n=1:Ny

for m=1:Nx

i = (n-1)*Nx + m;

while(n< (Ny-d))

if m==(0.5*Nx)

H(i+1)=H(i);

end

end

Direct(n,m) = H(i);

end

end

[Eshwaree Shingwekar]

how does the size of matrix A change when it was defined previously?