overwriting built in function

if i make a conditional statement where i compare 2 stings, i need to do this:

 if strcmpi(string_1,string_2), 
 ...
 end

but the following looks much more elegant, and readable:

 if string_1 == string_2
 ...
 end

that is why i wrote the function

 function y = eq(a,b)
 y=strcmpi(a,b);
 end

but this only works when i put the string in a cell like this:

 if {string_1} == {string_2}
 ...
 end

does anyone know why/ how to overrule the built-inn? also, why does an anonymous function ( eq=@(a,b)strcmpi(a,b); ) not work?

[Wesley Ooms]

i could create my very own precompiler where i fopen the current mfilename and replace every == for the strcmpi, but that sounds like a lot of work for a small problem.

Answer 1

[Matt J]

You can put your eq.m in a directory @char/. Its parent directory (not its contents) must be on the MATLAB path and higher up on the path than the builtin char methods.

However, I think it is a bad idea to overwrite builtin methods. You don't know what MathWorks-provided mfunctions might rely on the original str1==str2 definition, which you would be breaking.

[Matt J]

also, why does an anonymous function ( eq=@(a,b)strcmpi(a,b); ) not work?

Probably because anonymous functions are not on the MATLAB search path. I guess they are treated like variables.

Answer 2

[Sean de Wolski]

I would strongly recommend against this for a few additional reasons to what have already been provided.

1. Other MATLAB functions require the current syntax so basically anything you do with the product is untested by MathWorks.
2. If you give your code to someone else, it won't work for them, and your motivation about "looks elegant" is not enough for most users to want to break their software.
3. I imagine the performance will be significantly slower.
4. Debugging the errors that you get from this can be nearly impossible since a p-file might be expecting the above syntax and getting incorrect results.
5. People with MATLAB experience can look at the code and have a good idea of what is going on. If the definitions of builtin functions change out from under them, this no longer holds true.

This is the kind of thing I see done when you want to prank someone temporarily on a short business week. "Hey you called strcmpi!" not something to ever be done for something serious.

My $0.02, not those of my employer.

Answer 3

[Guillaume]

You can't do what you want. The function you want to overload is @char\eq which is built-in (hence unmodifiable) according to

which eq -all

And it is a good thing as well. What you want to do is very wrong on two levels:

1. It corrupts the meaning of == which means compare the two matrices elementwise and returns which elements differ and which are the same. This applies to any matrix, including matrices of characters
2. strcmpi is a case insensitive comparison. Many people would not associate that with equality. At the very least, strcmp, a case sensitive comparison, would make more sense.

The reason your eq works when you wrap the strings into a cell, is because eq is not defined for cell arrays. However, your function will now be called for cell arrays of anything including numbers which makes no sense:

{45} == {45}  %call your function, return 0 for some reason
{'abcd', 'efgh'} == {'ijkl'} %call your function, return [0 0]
{'aa', 'bb', 'cc'} == {'dd', 'ee'} %call you function, error in strcmpi

So don't do this, == has a different meaning from strcmpi, strcmp, strncmp and strncmpi. They're not interchangeable, so use the right one in the right situation.

Also remember that somebody else may have to read / debug your code. If you start changing the rules of the language, they stand no chance of understanding it.