MATLAB How to make a while loop stop when reaching a certain value

[WhiteWolf98]

TL;DR

I basically want Q in the code to be 30,000 or something near to 30,000 (perhaps between two values, for example between 29,750 and 30,250). I don't know how to define the range though, and the code keeps going on infinitely

m_1 = 1;
C_p1 = 2219;
T_1in = 100 + 273.15;
T_1out = 75 + 273.15;

Q_1 = (m_1)*C_p1*(T_1in-T_1out);

while Q_1 ~= 30000
    T_1outnew = T_1out + 0.1;
    Q_1 = (m_1)*C_p1*(T_1in-T_1outnew);
    T_1out = T_1outnew;
end

T_1outnew keeps increasing, but the code never stops; and naturally, Q becomes negative

 

[Wrichik Basu]

If you want to define a range, use for loop.

To break out from a loop, use the break statement.

 

[WhiteWolf98]

I'm not sure how a for loop would help me here, I just want Q to be in a range. What if I defined another variable, which was a range, and used the while loop with that? For example:

m_1 = 1;
C_p1 = 2219;
T_1in = 100 + 273.15;
T_1out = 75 + 273.15;
Y = 29750:30250;

Q_1 = (m_1)*C_p1*(T_1in-T_1out);

while Q_1 ~= Y
    T_1outnew = T_1out + 0.1;
    Q_1 = (m_1)*C_p1*(T_1in-T_1outnew);
    T_1out = T_1outnew;
end

It still doesn't work, but I don't see how to use the for loop because I just want a check to see if something is between two numbers

 

[Wrichik Basu]

I'm not sure how a for loop would help me here, I just want Q to be in a range. What if I defined another variable, which was a range, and used the while loop with that? For example:

m_1 = 1;
C_p1 = 2219;
T_1in = 100 + 273.15;
T_1out = 75 + 273.15;
Y = 29750:30250;

Q_1 = (m_1)*C_p1*(T_1in-T_1out);

while Q_1 ~= Y
    T_1outnew = T_1out + 0.1;
    Q_1 = (m_1)*C_p1*(T_1in-T_1outnew);
    T_1out = T_1outnew;
end

It still doesn't work, but I don't see how to use the for loop because I just want a check to see if something is between two numbers

Let's say you want Q_1 to be within the two values lower_lim and upper_lim. You can change your code to this:

m_1 = 1;
C_p1 = 2219;
T_1in = 100 + 273.15;
T_1out = 75 + 273.15;
lower_lim = 29750;
upper_lim = 30250;

Q_1 = (m_1)*C_p1*(T_1in-T_1out);

while (lower_lim < Q && Q < upper_lim)
    T_1outnew = T_1out + 0.1;
    Q_1 = (m_1)*C_p1*(T_1in-T_1outnew);
    T_1out = T_1outnew;
end

 

[WhiteWolf98]

I'm unfamiliar with &&, so I searched it up quickly. Since lower_lim < Q is not true, won't that stop the code there? I don't get any output anymore

 

[WhiteWolf98]

Never mind, I apologise, while (lower_lim < Q && Q > upper_lim). One of the signs needed to be flipped.

Thanks a lot for your help, Wrichik

 

[Wrichik Basu]

I'm unfamiliar with &&

That is logical AND. If you are unfamiliar with logical operators, I would advise you to stop here and go look them up. They are fundamental to any programming language.

Never mind, I apologise, while (lower_lim < Q && Q > upper_lim). One of the signs needed to be flipped.

That is not the way to check whether Q_1 is in a certain range.

You want Q_1 to be between 29750 and 30250. More explicitly, you want it to be greater than 29750 AND less than 30250.

With the code you posted, you are checking whether Q_1 is greater than 29750 AND greater than 30250. Does this seem correct?

I sense some ambiguity building up here. Let's look at what you wrote in the first post of this thread:

Summary:: I basically want Q in the code to be 30,000 or something near to 30,000

30000 is the critical point. You want your loop to give you the value of Q_1 that is nearest to 30000.

In the code inside the while loop, the value of Q_1 will decrease. At one point, it will be just greater than 30000 (say that value is Q_1a), and then it becomes just less than 30000 (Q_1b). If Q_1a - 30000 is less than 30000 - Q_1b, it means Q_1a is closer to 30000 than Q_1b. Otherwise, it means that Q_1b is closer to 30000. We will choose the final value of Q_1 accordingly, and break out of the loop.

m_1 = 1;
C_p1 = 2219;
T_1in = 100 + 273.15;
T_1out = 75 + 273.15;

Q_1 = (m_1) * C_p1 * (T_1in-T_1out);

old_Q1 = Q_1;
criticalValue = 30000;

while true
    T_1out = T_1out + 0.1;
    Q_1 = (m_1) * C_p1 * (T_1in - T_1out);
    
    if (Q_1 == criticalValue)
        % Q_1 is as near 30000 as possible -- it is equal to 30000. So
        % break from loop.
        break;
    elseif (Q_1 > criticalValue)
        old_Q1 = Q_1;
    else
        if ((old_Q1 - criticalValue) < (criticalValue - Q_1))
            Q_1 = old_Q1;
        end
        break;
    end
end

The value of Q_1 comes out to be 29956.4999999942 using the above code. The code assumes that Q_1 is > 30000 in line 6.