# How do I avoid using if and else statements in this code

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SA on 1 May 2020
Commented: SA on 7 May 2020
I want to rewrite this code but without if and else statements. I just want to enhance convergence rate
for i=1:N-1
for j=1:M-1
A(i+(M-1-j)*(N-1),i+(M-1-j)*(N-1))=-2/dx^2-2/dy^2;
B(i+(M-1-j)*(N-1))=B(i+(M-1-j)*(N-1))-(cos(x(i)+y(j))+cos(x(i)-y(j)));
if i==1
B(i+(M-1-j)*(N-1))=B(i+(M-1-j)*(N-1))-cos(y(j))/dx^2;
else
A(i+(M-1-j)*(N-1),i-1+(M-1-j)*(N-1))=1/dx^2;
end
end
end
Ameer Hamza on 1 May 2020
Can you write down the equation you are trying to implement with these for loops?

Vimal Rathod on 5 May 2020
Hi,
If your main goal is to remove the if else statements from the code, you can write the following code first and then write the main for loop with i starting from 2 instead on one and remove the if statement and keep the statement in the else condition.
i = 1;
for j=1:M-1
A(i+(M-1-j)*(N-1),i+(M-1-j)*(N-1))=-2/dx^2-2/dy^2;
B(i+(M-1-j)*(N-1))=B(i+(M-1-j)*(N-1))-cos(y(j))/dx^2;
end
for i=2:N-1
for j=1:M-1
A(i+(M-1-j)*(N-1),i+(M-1-j)*(N-1))=-2/dx^2-2/dy^2;
B(i+(M-1-j)*(N-1))=B(i+(M-1-j)*(N-1))-(cos(x(i)+y(j))+cos(x(i)-y(j)));
A(i+(M-1-j)*(N-1),i-1+(M-1-j)*(N-1))=1/dx^2;
end
end
if you want to further optimize you can vectorize the first loop.
Hope this helps!
SA on 7 May 2020
Hi Rik,
Yes, I just want to avoid the if/else block. I want to solve the pde another way (without using the if/else block).
I am not using an toolboxes in matlab. I just want to solve the pde using the difference algorithm.