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diff --git a/source/lang/goto.rst b/source/lang/goto.rst deleted file mode 100644 index ff2f248..0000000 --- a/source/lang/goto.rst +++ /dev/null @@ -1,130 +0,0 @@ -.. 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 |