How to do iteration until certain tolerance is achieve?

Hello! i wanted to iterate a function with an initial value to get a new value and then keep subtituting the new value back to the function, this process will continue until when the tolerance condition is achieved. i tried using "while loop" but it shows no iteration. No any new value obtained. please help me to check the code.

This is the equation: (psic/psit) + 1/(2*sigma) = 1

clear, clc;
psit=0.1;   % initial 
tol=1.0e-6;
sigma=0.8;
psic=-0.2079;
k=0;   % iteration
while abs((psic/psit)+(1/(2*sigma)))<tol
    psit=((psic/psit)+(1/(2*sigma)))-1
    k=k+1;
end
disp(psit)

Answer 1

[Walter Roberson]

while abs((psic/psit)+(1/(2*sigma)))<tol

That tells us that you are looking for (psic/psit)+(1/(2*sigma)) to become as close to 0 as possible, which is the same as solving

    psic/psit + 1/(2*sigma) == 0
    psic + psit/(2*sigma) == 0*psit
    psit/(2*sigma) == -psic
    psit == -psic*2*sigma    %solution

but...

    psit=((psic/psit)+(1/(2*sigma)))-1

notice the -1 . That -1 tells us that you want to solve for psic/psit + 1/(2*sigma) == 1 instead of psic/psit + 1/(2*sigma) == 0

The contradiction between those leads the equation to loop endlessly (once you change the < tol to > tol )

[Abba Alhaji Bala]

yeah, that is right. thanks