How do I display individual values from an if statement in a messgae?

Ok bear with me.

I am trying to write a script that gives the root to a function using the bisection method. However it is not the traditional bisection using step halving, it is using a weighted version so it converges quicker. I have it working, however I need to get it to display specific values for my output. One of these is the final interval that the root falls into. Here is the part of my script that calculates each individual upper or lower interval value:

if fa*fc<0
    b=c;
else
    a=c;
end

What I want to do it have a message at the end of my command window that states:

"The final interval is 'a' and 'b'" but with a and b being replaced by the final interval, i.e. the final values of a and b that the root falls between. I can get it to print every value of a and b individually for every iteration, but not the single values for the final interval which is what I want.

I am reluctant to send my whole script as I suspect it isn't written 'perfectly' but I want to make it more efficient on my own, I just cannot work out how to do this part.

My script starts with :

function [a,b,it,itc] = WB(f,a,b,w,tol)

and it is the [a,b] from the left hand side of the equals that are the final interval values.

Thank you in advance!

Answer 1

[Mischa Kim]

Hi Matthew, I assume you are using a loop to do the iterations, most likely a while loop. If you say you can print only all the interval information this suggests you do the printing inside the loop. How about simply moving the print command right after the end command of the loop? Something like

 fprintf('Final interval: [%f, %f]\n',a,b)

Without access to the entire code it is challenging to impossible to point you in the right direction.

[Matthew Worker]

Thank you so much! I was trying all sorts of things along those lines but a lot further up my script, hence the issue!

Would you mind telling me what the %f actually does?

[Matthew Worker]

Would you mind giving me a hand with the other part I wasn't so sure about?

I also need to print how many iterations the script takes to get to the root, as well as the number of iterations where the root was in the smaller interval. Should I use a similar method to the interval above? I was trying that kind of language but I was clearing putting it in the wrong place or missing a component.

[Mischa Kim]

Same as above...

 fprintf('Final interval: [%f, %f] with %d iterations\n',a,b,it)

You can simply add info into the print command.

[Matthew Worker]

This tells me it has taken 1 iteration to find the root, which is wrong it take about 14 I think.

[Mischa Kim]

Again, without seeing your code I cannot help. I assume you are not incrementing it inside the loop.

it = it + 1;

[Matthew Worker]

I have managed to get something but I'm pretty sure that the last number of iterations, where the root was in the smaller interval should be different to the normal iterations...

[Image Analyst]

Sine it and itc both start at zero, and both get incremented once for each while loop iteration, why do you think they should be different?

[Mischa Kim]

Thanks for sharing. In your code you are updating it and itc simultaneously

 it  = it+1;
 itc = itc+1;

so the values of both variables always will be the same. In other words, you need to move the second command (updating itc) to the proper place in your code. I do not know what itc is supposed to count, so I cannot help you there.

Also, this thread has become somewhat long at this point. If your first couple of questions have been addressed successfully, please accept the answer and post a new question. This also helps other users find answers for their questions. Thanks.

[Matthew Worker]

Image Analyst due to the last comment please go to this link to continue with this question:

https://uk.mathworks.com/matlabcentral/answers/366444-how-do-i-count-how-many-iterations-my-root-is-in-a-smaller-interval