Defining and calling functions in MATLAB

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Spaceman 2024-2-27

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https://ww2.mathworks.cn/matlabcentral/answers/2087318-defining-and-calling-functions-in-matlab

评论： Spaceman 2024-3-2

采纳的回答： Voss

在 MATLAB Online 中打开

Given: Recall that Fahrenheit and Celsius are related by the following equation:

Find: Create a function called tempC2F that will accept an input argument of a single temperature in degrees Celsius and return the equivalent temperature in degrees Fahrenheit. I should be able to call your function using the following function call: TF=tempC2F(TC)

My Solution: This is what I have, but I am not getting the desired result, I'm not sure how to call the function to run...

function 
Invalid expression. Check for missing or extra characters.
tempC2F=(1.8*tC)+32
end
% You can test your function using the following function call.  Recall that 212F is the boiling point of water at sea level.  
% This corresponds to 100C.  If you click "Run", the following code will call your function.  You should get an output of tF=212. 
% Remember that we are "Calling" the function below.  As such, the variable names do not have to match the variables used in the function above.
% Those variables are specific to the function itself.
tC = 100;
tF = tempC2F(tC)

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Answer 1

Voss 2024-2-27

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Refer to the following documentation page for a description and examples of how to define and call a function:

https://www.mathworks.com/help/matlab/ref/function.html

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Voss 2024-2-27

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The example function, called calculateAverage, takes a numeric input and returns the average of its elements. In the definition of the function, the input is called x and the output is called ave.

function ave = calculateAverage(x) % this line defines the function name (calculateAverage) and its input (x) and output (ave)
ave = sum(x(:))/numel(x);          % the stuff inside the function (between "function" and "end") does the calculation
end                                % this line says the function definition is done

Your task is to write a function called tempC2F that takes a numeric input, temperature in degrees Celsius, converts it to degrees Fahrenheit, and returns the converted value. The conversion calculation you have written is OK. So all that's left to do is to name your function and define its input and output. The task tells you what the name of the function must be (tempC2F), so really you just need to pick names for the input and output variables. You are using tC as the Celsius temperature, so why not use tF as the calculated Fahrenheit temperature?

Using the calculateAverage example as a guide, you could say:

function tF = tempC2F(tC)
tF=(1.8*tC)+32;
end

That's it. That's your function.

Steven Lord 2024-2-28

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I think this has to do with the fact that I have to name the function itself 'tempC2F'... Even though it states in my practice example that the variable names do not have to match the variables used in the function.

These are two different things.

In general, the name of the main function in a function file should have the same name as the file. If they don't match, you will need to use the name of the file to call the function, not the name of the first function. So if you wrote the code function tF = tempC2F(tC) in a file named myTemperatureConverter.m you'd need to call it as tF = myTemperatureConverter(100) rather than tF = tempC2F(100).

The second sentence in the part of your message that I quoted deals with variables, not functions. In general (leaving out things like nested functions or global variables) a function call's workspace is completely independent of what's outside the workspace. If your function defines a variable named x, for example, that's not going to affect or be affected by a variable named x in the workspace in which you call your function. The main ways that data flows into or out of a function are through the input and output arguments.

X = pi
X = 3.1416
callFunctionThatUsesXInternally(42)
X % This didn't change even when the function assigned 42 to X inside its workspace
X = 3.1416
function callFunctionThatUsesXInternally(value)
X = value; % This is independent of the variable on the 1st line of code segment
end

Spaceman 2024-2-29

编辑：Spaceman 2024-3-2

Thank you, my lord. This clears things up. I think the main takeaway here is that a function call's workspace is completely independent of what's outside the workspace.

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Answer 2

Walter Roberson 2024-2-27

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在 MATLAB Online 中打开

Create a function called tempC2F

function tempC2F

that will accept an input argument of a single temperature in degrees Celsius

function tempC2f(single_temperature_in_degrees_Celcius)

and return the equivalent temperature in degrees Fahrenheit.

function equivelent_temperature_in_degrees_Fahrenheit = tempC2f(single_temperature_in_degrees_Celcius)

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Steven Lord 2024-2-27

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You've combined two unrelated sections of code in the same file. This code that calls your function would be typed in the Command Window or written in some other file, not in the file that contains AreaSphere as its main function. [I'm intentionally omitting discussion of local functions or functions in script for simplicity right now.]

r = randi(10, 3, 1)
Area = AreaSphere(r) % Call the function

The code that defines your function would be in a function file named AreaSphere.m since the main (first) function in the function file should have the same name as the file itself.

function A = AreaSphere(radius) % Define the function
A = 4*pi.*radius.^2;
end

It can be a little difficult to make the distinction here on MATLAB Answers, since usually we type all the code directly in the Answers editor.

Since it seems like you're fairly new to MATLAB, I suggest you start with the free MATLAB Onramp tutorial to quickly learn the essentials of MATLAB. The Onramp is more interactive than simply reading through documentation pages, so it might better match your learning style. [Some people learn best by reading, some by hearing, some by watching, and some by doing.]

Steven Lord 2024-2-29

I didn't know it was so obvious I am so new

\begin{Yoda mode}

When for more than 20 years you have been answering questions about MATLAB, novice users will you be able to spot :)

\end{Yoda mode}

Spaceman 2024-3-2

Hilarious as you are wise XD

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Answer 3

Aquatris 2024-2-27

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在 MATLAB Online 中打开

So if you want to define them in a single script, the function needs to be at the bottom.

And as others mentioned you might want to check out the documentation on how to define functions. But here is your code arrange to work.

tC = 100;
tF = tempC2F(tC)
tF = 212
function tFahr = tempC2F(tCelcius)
tFahr=(1.8*tCelcius)+32;
end

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Aquatris 2024-2-28

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Just to make sure there are no confusion, here are a couple things that might be helpful to your understanding.

The function name is 'tempC2F'
The function output is 'tFahr'
'tFahr' variable only lives within the function. Outside of the function, nobody knows what 'tFahr' is (there might be a completely different variable with the same name and that's why you might want to checkout the topic of 'workspaces' here).
The syntax matlab uses to define a function is literally how you would use that function from another piece of code with a 'function' attached to the front to define it as a function.

% function definition
function functionOutput = myFunctionName(functionInput)
% whatever the function needs to do, 
% in this case it just multiplies the input by 5 and returns that value
    functionOutput = 5*functionInput; 
end

And to call the function from outside as:

x = myFunctionName(10); % this will make x = 50

To call the function, the script that holds the function needs to be in your matlab path. Because it was not in your matlab path you got the error 'Unrecognized function or variable 'tempC2F' 'in previous comments.

Spaceman 2024-3-2

This. I actually finally learned about functions and this explains it perfectly. Thank you. I assume when it gets more complicated with loops and such I will be right back here asking questions, lol.

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Defining and calling functions in MATLAB

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