[✓] Save a long function on disk for further use

Questions
,
1

I have long functions that I use often in code. So instead to put them every time in a cell and execute it, I would like to save them on the hard drive and then reuse it inside other code in order to shorten it. I tried Put, Get, DumpSave and so on but either I do not use these correctly or they are not intended for this.

What I would like to to is the following:

Define the function f[x_]:=…

Save it giving it a name

Get it back in code:

code1

code2

f[x]

code3

…

I have the impression that this is something very easy to do but I am unable to get the info out of the Mathematica documentation. So can anyone help me? Thanks in advance.

Edit: to be more clear, here is what I made:

Put[x^2, "test.m"];
f = Get["test.m"]
g[x_] := f
g[2]

The wished output is 4 of course but what I get is x^2. I tried out a handful of other combinations but never obtained what I am looking for…

2

You may try a combo of Hold and ReleaseHold

In[1]:= Hold[g[x_]:=f]/.f->Get["test.m"]
Out[1]= Hold[g[x_]:=x^2]
In[2]:= ReleaseHold[%]
In[3]:= g[2]
Out[3]= 4

code.png
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3

Thanks Shenghui,
Can you tell me why this does not work?

Hold[g[x_] := Get["test.m"]]
ReleaseHold[%]
g[2]

Would be simpler.

Edit: I found another solution:

g[x_] := Evaluate[Get["test.m"]]
4

So if you break the code, the rhs for SetDelay is just load the package

1211code.png
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every time you call the function g, it just load the file and return x^2. Your second method works because Evaluate around the second argument overwrites the HoldAll attribute of SetDelayed.

In[9]:= Attributes[SetDelayed]
Out[9]= {HoldAll, Protected, SequenceHold}
5

Ok thanks for your info. Let’s go a step further now. How would you store and get back this function:

f[z_] := Block[{n}, n = 1; Do[n = n + 1, {z}]; Return[n]];

If I use Put, it will be evaluated and I get the error Do::iterb: Iterator {z} does not have appropriate bounds. When I use Unevaluated before Block, I can Put it but then I get the same message with

f[z_] := Evaluate[Get["test.m"]]
6

Ulrich,

From what I understand, in your first example, you are saving an expression, your second example, you are saving a procedure.

I recommend using Set and not SetDelayed for the first case. Using your example:

Put[x^2, "test.m"];
g[x_] = Get["test.m"];
g[2]

4

This is not unique to Put/Get. Anytime you need to evaluate an expression and then convert it to a “function”, I find using Set (=), to be more clear than trying to deal with Holding, etc.

As to your second example, I would consider using Save[] or DumpSave[] to save your procedure. For example:

In[1]:= ff[z_] := Block[{n}, n = 1; Do[n = n + 1, {z}]; Return[n]]
Save["test.m", ff]
Clear[ff]
Get["test.m"]
ff[3]

Out[5]= 4

The difference is that the Block expression must be delayed so I would use SetDelay here and save it as a function. I hope this helps.

Regards,

Neil

7

Great, this works. Meanwhile I also found a solution with packages but this is even simpler. Now, my codes will get much shorter.
Thank you very much, Neil.