Defining a function in one file and using it in a second file

I have one code file that defines two functions. The second function operates on the output of the first function:

function SoilResp_gC = SoilResp(Temp,Precip)
SoilResp_gC = F.*(exp(Q*Temp)).*(Precip./(K+Precip));
end

function SoilResp_gO2 = UnitConvert(SoilResp_gC)
    SoilResp_molesC = SoilResp_gC./12.01;
    SoilResp_molesO2 = SoilResp_molesC;
    SoilResp_gO2 = SoilResp_molesO2.*32;
end

I have a second file which defines some constants (including those required by these functions, Temp and Precip) and then runs for loop to iteratively add data to a matrix. The first steps in this loop are the functions I defined in the first file:

for i = 2:NumTimesteps % for each timestep
% 	1. Soil Respiration % as per Reich, 2002
    SoilResp_gC(Temp,Precip)
    SoilResp_gO2(SoilResp_gC)
    ...

I first run the file with the functions in. I get no errors. I then run the file with the loop, and get the following error message:

Error in Filename (line 38) SoilResp_gC(Temp,Precip)

Undefined function 'SoilResp_gC' for input arguments of type 'double'.

What do I need to do to make sure my function is recognised and properly defined, so that the second file will run correctly?

Answer 1

[Star Strider]

I would save each of your functions to a separate .m file. That way, they should always have access to each other.

NOTE: The functions are actually named 'SoilResp' and ''UnitConvert'.You have to call them by their assigned names.

[Vicki]

Thanks! I have now separated them out into two separate files, and changed the second code file to call the assigned names of 'SoilResp' and 'UnitConvert'. However, when I run the code that calls the function, I still get the same error. When I look in the directory sidebar, I notice that matlab displays different icons for my two functions, calling one a function and one a script:

The function that is first called (where the error appears) is the one with the normal script icon. How do I make this have the function icon (and hopefully prevent the error occuring?

[Star Strider]

I'm not certain what you're doing.

If you want 'SoilResp_gC' you have to call 'SoilResp' with the 'Temp' and 'Precip' arguments and return the output to a variable.

Perhaps:

for i = 2:NumTimesteps % for each timestep

SoilResp_gC(i) = SoilResp(Temp,Precip);

SoilResp_gO2(i) = UnitConvert(SoilResp_gC(i));

...

end

or something similar.

.

EDIT -- (2 Dec 2025 at 12:36)

The function name and the .m file name should be the same. (Missed that befOre. Early here.)

.

EDIT -- (2 Dec 2025 at 15:20)

The script .m file name cannot be the same as any local function defined within it. That confuses MATLAB.

The function .m file name is usually the name of the function defined within it. There can be exceptions to that, however making the function .m file name the same as the function name is preferred.

.

EDIT -- (2 Dec 2025 at 15:49)

'How do I convert the file to a function file from a script file?'

The easiest way is probably to rename the script file and save it with the new name, then copy the function to a new Editor tab, then save it as the function name.

.

Answer 2

[Torsten]

The calls must be

for i = 2:NumTimesteps % for each timestep
% 	1. Soil Respiration % as per Reich, 2002
    SoilResp_gC = SoilResp(Temp,Precip)
    SoilResp_gO2 = UnitConvert(SoilResp_gC)
    ...

not

for i = 2:NumTimesteps % for each timestep
% 	1. Soil Respiration % as per Reich, 2002
    SoilResp_gC(Temp,Precip)
    SoilResp_gO2(SoilResp_gC)
    ...
        

And the functions should be named

    SoilResp.m
    UnitConvert.m
    

not

    SoilResp_Func.m
    UnitConvert_Func.m

[Vicki]

Hi Torsten,

I now get this error:

File: SoilResp.m Line: 14 Column: 24

Local function name must be different from the script name.

If I give the file the same name, it says the local function name must be different, but if I give it a different name, it doesn't recognise the function at all :/

[Torsten]

This is the script - name it "script.m" and run it from the editor:

for i = 2:NumTimesteps % for each timestep
% 	1. Soil Respiration % as per Reich, 2002
    SoilResp_gC = SoilResp(Temp,Precip)
    SoilResp_gO2 = UnitConvert(SoilResp_gC)
    ...
        

This is function SoilResp - name it "SoilResp.m" and save it in your working directory:

    function SoilResp_gC = SoilResp(Temp,Precip)
SoilResp_gC = F.*(exp(Q*Temp)).*(Precip./(K+Precip));
    end
    

This is function UnitConvert - name it "UnitConvert.m" and save it in your working directory:

    function SoilResp_gO2 = UnitConvert(SoilResp_gC)
    SoilResp_molesC = SoilResp_gC./12.01;
    SoilResp_molesO2 = SoilResp_molesC;
    SoilResp_gO2 = SoilResp_molesO2.*32;
    end
    

Maybe you want to learn the MATLAB basics free of costs to get used to the new software:

https://matlabacademy.mathworks.com/details/matlab-onramp/gettingstarted

[Vicki]

The only other thing in the file SoilResp.m is the definition of F, Q & K (and some comments). The final lines of the file are the function as outlined above, which I understand is needed for a function file. From Steve's comment, it appears the issue is that this is a script file defining a local function. How do I convert this to a function file?

I'm new to functions rather than writing 1000+ lines of code in one file, I'm not new to matlab. I'm just trying to make this project more human readable than my previous code.

[Torsten]

Post the complete files and we will show you how to do it.

Answer 3

[Steven Lord]

Script files

Inside of a script file, all functions defined inside it are local functions. That means they can only be called directly from within that script file. [There is a way to indirectly call them, if the script file creates a function handle variable that other code has access to.]

Local functions inside a script file cannot have the same name as the script file itself. [If I had a script foo.m with a local function foo inside it, and I called foo inside that script file, would that be an attempt to call the local function with 0 input arguments or an attempt to rerun the script file? It's ambiguous, so we disallow it.]

Function files

Inside of a function file, only the first function in the file (the main function) is directly callable from outside the function file. All others are local functions and can only be called directly from within that function file. [The same comment about function handles applies here, though in this case the main function would need to return that variable or find some other way to make it available to other code in order for that other code to use it.] From the "Contents of Functions and Files" section on that documentation page to which I linked:

"Program files can contain multiple functions. If the file contains only function definitions, the first function is the main function, and is the function that MATLAB associates with the file name. Functions that follow the main function or are included in script code are called local functions. Local functions are only available within the file."

A note on names: if the name of the main function in the function file and the name of the function file itself differ, you will need to use the file name to call the main function. So if I had a file timestwo.m and its main function was named timesthree, I would need to call the function as timestwo (and should change one of the names to match the desired functionality.)

Class files

This gets a little tricky. Class files can have methods that may or may not be accessible to outside files (depending on how they are defined, in particular the Access attribute in their methods block) or local functions (which are not accessible outside the class.) I'm not going to go into detail here to try to avoid confusing you. [Probably too late.]

[Vicki]

Okay, thanks. It looks to me like I've made one of these files (UnitConvert.m) a function file, and the other (SoilResp.m) a local function inside of a script. How do I convert the file to a function file from a script file?

[Stephen23]

The difference between a script and a function is simple: anything that is outside of FUNCTION ... END makes it a script. So to make an Mfile a function you must make sure that it has absolutely no code outside of FUNCTION ... END, for example by deleting all code outside of FUNCTION ... END (or moving it to another script or function) or by placing FUNCTION ... END around all code in an Mfile.

[Vicki]

That solves it, thank you!