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matlab_return.html
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<html>
<head>
<title>
MATLAB_RETURN - Using RETURN May Be Expensive
</title>
</head>
<body bgcolor="#EEEEEE" link="#CC0000" alink="#FF3300" vlink="#000055">
<h1 align = "center">
MATLAB_RETURN <br> Using RETURN May Be Expensive
</h1>
<hr>
<p>
<b>MATLAB_RETURN</b>
is a directory of MATLAB programs which
illustrate the puzzling cost of the MATLAB RETURN statement.
</p>
<p>
A MATLAB M-file may contain several functions. It is enough to simply
use the "function" statement itself to indicate the beginning of each
function in such a file, as follows:
<pre>
function a ( )
...body of function
function b ( )
...body of function
function c ( )
...body of function
</pre>
</p>
<p>
The MATLAB "end" statement may be used to indicate the termination of
a function, as well as that of "if" and "for" statements:
<pre>
function a ( )
...body of function
end
function b ( )
...body of function
end
function c ( )
...body of function
end
</pre>
Using the "end" statement in this way would seem to be merely a fussy
formality, or, depending on how you think, a means of improving program
readability.
</p>
<p>
The MATLAB "return" statement may be used at any point in a function to
indicate that execution should return to the calling function. A function
may have several return statements. Reaching the end of the function definition
(end of file, or the beginning of a new function) will also result in
an implicit return to the calling function. We can use an explicit return
as an additional marker in our file:
<pre>
function a ( )
...body of function
return
end
function b ( )
...body of function
return
end
function c ( )
...body of function
return
end
</pre>
Using the "return" statement is again unnecessary, but would seem to me
to me a more careful way of indicating that the function's calculations
have been completed.
</p>
<p>
Call the three skeleton files above "version1", "version2" and "version3".
What will you think when I tell you that versions 1 and 2 run in 2 seconds,
and version three takes 22 seconds? (The real problem I wanted to run
is 100 times bigger, and there the discrepancy is far worse.)
</p>
<p>
The online documentation for the RETURN statement simply states that it
causes return to the calling function, and nowhere suggests that there is
a significant overhead incurred when calling RETURN explicitly. Nor is
there any explanation for a difference between an implicit and explicit RETURN.
</p>
<p>
Since absolutely anything can be "explained" after the fact, it's easy to
suppose that the explicit RETURN somehow "exits the entire file", causing all
the file memory to be wiped clean, and requiring a reinitialization on reentry,
whereas the implicit RETURN from, say, function b, would smoothly jump back
to function a (assuming a called b) without exiting the file and its memory.
But that's just my vague attempt at a plausible explanation, and there's no
reason to believe it.
</p>
<p>
What bothers me is that I can't see WHY the return has to cost so much;
and I've already written a heap of MATLAB M-files with multiple functions
that politely return/end all over the place. So now I'm stuck with the
worry of going back and cleaning them up.
</p>
<p>
By the way, it is a peculiarity of MATLAB that:
<ul>
<li>
conventionally, the name of the file and the name of the first function
ought to match, but in any case, invoking the file name causes the
execution of the first function in the file, no matter what its name;
</li>
<li>
except for the first function, which is invoked by the file name,
no other function in the file can be directly invoked by an external command.
</li>
</ul>
</p>
<h3 align = "center">
Licensing:
</h3>
<p>
The computer code and data files described and made available on this web page
are distributed under
<a href = "../../txt/gnu_lgpl.txt">the GNU LGPL license.</a>
</p>
<h3 align = "center">
Related Data and Programs:
</h3>
<p>
<a href = "../../m_src/feynman_kac_2d/feynman_kac_2d.html">
FEYNMAN_KAC_2D</a>,
a MATLAB program which
demonstrates the use of the Feynman-Kac algorithm
to solve Poisson's equation in a 2D ellipse by averaging
stochastic paths to the boundary.
</p>
<p>
<a href = "../../m_src/matlab/matlab.html">
MATLAB</a>,
MATLAB programs which
illustrate the use of the MATLAB programming language;
</p>
<h3 align = "center">
Source Code:
</h3>
<p>
<b>EXAMPLE</b> is an extremely simplified program which simply calls two
versions of the same function many times. One version has a return statement
and one doesn't.
<ul>
<li>
<a href = "example.m">example.m</a>, the program.
</li>
<li>
<a href = "example_output.txt">example_output.txt</a>, the output file.
</li>
</ul>
</p>
<p>
<b>VERSION1</b> uses no RETURN and no END statements and runs in 1.9 seconds.
<ul>
<li>
<a href = "version1.m">version1.m</a>, the program.
</li>
<li>
<a href = "version1_output.txt">version1_output.txt</a>, the output file.
</li>
</ul>
</p>
<p>
<b>VERSION2</b> uses END, but no RETURN statements and runs in 1.9 seconds.
<ul>
<li>
<a href = "version2.m">version2.m</a>, the program.
</li>
<li>
<a href = "version2_output.txt">version2_output.txt</a>, the output file.
</li>
</ul>
</p>
<p>
<b>VERSION3</b> uses RETURN and END statements and runs in 22 seconds.
<ul>
<li>
<a href = "version3.m">version3.m</a>, the program.
</li>
<li>
<a href = "version3_output.txt">version3_output.txt</a>, the output file.
</li>
</ul>
</p>
<p>
You can go up one level to <a href = "../m_src.html">
the MATLAB source codes</a>.
</p>
<hr>
<i>
Last revised on 27 August 2011.
</i>
<!-- John Burkardt -->
</body>
</html>