aboutsummaryrefslogtreecommitdiffstats
path: root/docs/source/lang/goto.rst
diff options
context:
space:
mode:
Diffstat (limited to 'docs/source/lang/goto.rst')
-rw-r--r--docs/source/lang/goto.rst130
1 files changed, 130 insertions, 0 deletions
diff --git a/docs/source/lang/goto.rst b/docs/source/lang/goto.rst
new file mode 100644
index 0000000..96a6262
--- /dev/null
+++ b/docs/source/lang/goto.rst
@@ -0,0 +1,130 @@
+.. 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