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Toggle between temperature units for unit conversion

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Fazlollah
Fazlollah 2023-2-14
评论： Image Analyst 2023-2-23
Hi,
I am trying to make a simple temeprature unit conversion app in MATLAB app maker with a dropdown menu to toggle between temperature units: C, F, and K. I used switch case function to convert degree c to F or K; however, when I want to return back to c or toggle between the units the program gives wrong values. Please help me complete the code to be able to toggle between temperature units. Thanks
function DropDownValueChanged(app, event)
Value=app.TemperatureEditField.Value
switch app.DropDown.Value
case 'C'
app.TemperatureEditField.Value=Value;
case 'F'
app.TemperatureEditField.Value=Value*1.8+32;
case 'K'
app.TemperatureEditField.Value=Value+273.15;
end
end
  1 个评论
Fangjun Jiang
Fangjun Jiang 2023-2-14
Of course it has issues. If you continue selecting "K", the program flow will continu adding 273.15 to the current value. You need to add something to recognize the current unit.

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采纳的回答

Voss
Voss 2023-2-17
You need to take into account what the old unit was before it was changed. Fortunately, the ValueChangedFcn of a uidropdown provides an event structure which contains that information. See the attached app.
  2 个评论
Jon
Jon 2023-2-17
Very nice!
Gives the OP what he wanted with an interface with just one box. Good to know that the event structure returns the previous value.
Fazlollah
Fazlollah 2023-2-23
编辑：Fazlollah 2023-2-23
Thank you Voss, this is working great.
With this conversion, I have another question. Assume I have an equation for density of water as a function of temperature in degree C. For example,
Rho_H2O=999.6+2.0438e-1*Tc-6.174e-2*Tc^1.5
If my temperature input is in F, then how can I tell the program to put the temperatuer in degree C into the density equation? Can you help me with this one too?

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更多回答（1 个）

Jon
Jon 2023-2-14
编辑：Jon 2023-2-14
I think this does what you want.
I have fixed a few errors in your original code, I think you were confused about the value of the drop down selection and the value of the temperature, which was in the edit field.
Also I added another edit field for the resulting output in the new units. Otherwise, as mentioned by @Fangjun Jiang, with repeated operation the value stored in the original edit field no longer has units of C, so the conversion which you have which convert from C to F or K would no longer be correct. Easier to keep the original value as is and then just output to a different edit field.
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Fazlollah
Fazlollah 2023-2-23
编辑：Fazlollah 2023-2-23
Thank you Jon for the answer, yours also works perfectly. However, Voss answer is easier for me to understand.
With this conversion, I have another question. Assume I have an equation for density of water as a function of temperature in degree C. For example,
Rho_H2O=999.6+2.0438e-1*Tc-6.174e-2*Tc^1.5
If my temperature input is in F, then how can I tell the program to put the temperatuer in degree C into the density equation? Can you help me with this one too?
Image Analyst
Image Analyst 2023-2-23
Not sure what you mean. Isn't it the obvious:
Tc = (Tf - 32) * 5/9;
Rho_H2O = 999.6 + 2.0438e-1 * Tc - 6.174e-2 * Tc ^ 1.5

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