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.. highlight:: cpp
.. _lang-goto:
Labels and goto
===============
A *label* gives a name to a line of code within a function. You can
label a line by writing a name for it, then a colon (``:``), before
the line starts. The ``goto`` keyword allows program flow to transfer
to a labeled line from anywhere within the same function.
.. warning:: The use of ``goto`` is discouraged in C and C++
programming. It is *never necessary* to use ``goto`` to write a
program.
Unless you know what you're doing, using ``goto`` tends to
encourage code which is harder to debug and understand than
programs without ``goto`` that do the same thing. That said,
however, it's sometimes useful; :ref:`see below <goto-when-to-use>`
for a concrete example.
Using Labels and goto
---------------------
Labels and ``goto`` are probably best explained through example.
Let's start with an example of how to label lines. The first line
(``int x = analogRead(some_pin);``) in the :ref:`loop <lang-loop>`
function below has label ``readpin``. The third line (``delay(x);``)
has label ``startdelay``. The second line (``SerialUSB.println(x);``)
does not have a label::
void loop() {
readpin:
int x = analogRead(some_pin);
SerialUSB.println(x); // for debugging
startdelay:
delay(x);
// ... more code ...
}
Anything which can be a :ref:`variable <lang-variables>` name can
be a label.
Let's say that we wanted to print ``x`` only if it was very large, say
at least 2000. We might want to do this just so anybody watching on a
:ref:`serial monitor <ide-serial-monitor>` would know they were in for
a longer wait than usual. We can accomplish this through the use of a
``goto`` statement that skips the printing if ``x`` is less than
2000::
void loop() {
readpin:
int x = analogRead(some_pin);
if (x < 2000) {
goto startdelay;
}
SerialUSB.println(x); // for debugging
startdelay:
delay(x);
// ... more code ...
}
In this modified program, whenever ``x`` is less than 2000, the body
of the :ref:`if <lang-if>` statement in the second line is
executed. The ``goto`` statement inside the ``if`` body skips
straight to the line labeled ``startdelay``, passing over the line
doing the printing.
A ``goto`` does not have to "move forwards"; it can go "backwards",
too. For example, the following program prints "5" forever (why?)::
void loop() {
printfive:
SerialUSB.println(5);
goto printfive;
SerialUSB.println(6);
}
.. _goto-when-to-use:
When to Use goto
----------------
As mentioned above, use of ``goto`` is `generally discouraged
<http://en.wikipedia.org/wiki/Goto#Criticism_and_decline>`_. However,
when used with care, ``goto`` can simplify certain programs. One
important use case for ``goto`` is breaking out of deeply nested
:ref:`for <lang-for>` loops or :ref:`if <lang-if>` logic blocks.
Here's an example::
for(int r = 0; r < 255; r++) {
for(int g = 255; g > -1; g--) {
for(int b = 0; b < 255; b++) {
if (analogRead(0) > 250) {
goto bailout;
}
// more statements ...
}
// innermost loop ends here
}
}
bailout:
// more code here
In the above example, whenever the :ref:`analog reading
<lang-analogread>` on pin 0 was greater than 250, the program would
jump to the line labeled ``bailout``, exiting all three loops at once.
While there is already a :ref:`break <lang-break>` keyword for
breaking out of a loop, it will only break out of the *innermost*
loop. So, if instead of saying "``goto bailout;``", there was a
"``break;``" instead, the program would only exit from the loop with
header "``for(int b = 0; b < 255; b++)``". The program would continue
at the line which reads "``// innermost loop ends here``", which is
clearly undesirable if you wanted to leave all three loops at once.
More examples of when ``goto`` is a good choice are given in Donald
Knuth's paper, "Structured Programming with go to Statements"; see
below for a link.
See Also
--------
- Dijkstra, Edsger W. `Go To Statement Considered Harmful <http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.92.4846&rep=rep1&type=pdf>`_ (PDF)
- Knuth, Donald. `Structured Programming with go to Statements <http://pplab.snu.ac.kr/courses/adv_pl05/papers/p261-knuth.pdf>`_ (PDF)
.. include:: cc-attribution.txt
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