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diff --git a/source/lang/constants.rst b/source/lang/constants.rst deleted file mode 100644 index bc5c894..0000000 --- a/source/lang/constants.rst +++ /dev/null @@ -1,304 +0,0 @@ -.. _lang-constants: - -Constants -========= - -Constants are like predefined variables, whose values can't -change. They are used to make the programs easier to read and modify. -This page describes the most commonly used constants. - -.. contents:: Contents - :local: - -.. _lang-constants-bool: - -Boolean Constants ------------------ - -There are two constants used to represent truth and falsity: ``true``, -and ``false``. - -.. _lang-constants-false: - -false -^^^^^ - -``false`` is the false ``bool`` value. An integer which is 0 evaluates -to ``false`` as a boolean. - -.. _lang-constants-true: - -true -^^^^ - -``true`` is the true ``bool`` value. As an integer, ``true`` is often -said to be 1. This is correct in the sense that ``true`` evaluates to -1 as an integer. However, any integer which is *non-zero* is ``true`` -as a :ref:`bool <lang-booleanvariables>`. So -1, 2 and -200 are all -"true", in the sense that these numbers are treated the same as -``true`` in a boolean context. - -Note that the ``true`` and ``false`` constants are typed in lowercase; -unlike e.g. ``HIGH``, ``LOW``, ``INPUT``, and ``OUTPUT`` (which are -described below). - - -Pin Levels: HIGH and LOW ------------------------- - -When reading or writing to a digital pin there are only two possible -values a pin can be set to: ``HIGH`` and ``LOW``. - -.. _lang-constants-high: - -HIGH -^^^^ - -The meaning of ``HIGH`` (in reference to a pin) is somewhat different -depending on whether the pin is set to ``INPUT`` or ``OUTPUT``. When a -pin is configured as an ``INPUT`` (using :ref:`pinMode() -<lang-pinmode>`), and read with :ref:`digitalRead() -<lang-digitalread>`, the microcontroller will report ``HIGH`` if a -voltage of 3 volts or more is present at the pin. - -.. TODO? Following seems false; check it out sometime, leave out for now: - -.. A pin may also be configured as an ``INPUT`` with ``pinMode()``, and -.. subsequently made ``HIGH`` with :ref:`digitalWrite() -.. <lang-digitalwrite>`, this will set the internal pullup resistors, -.. which will *steer* the input pin to a HIGH reading unless it is pulled -.. LOW by external circuitry. - -When a pin is configured to ``OUTPUT`` with pinMode, and set to -``HIGH`` with :ref:`digitalWrite() <lang-digitalwrite>`, the pin is at -3.3 volts. In this state it can *source* current, e.g. light an LED -that is connected through a series resistor to ground, or to another -pin configured as an output and set to ``LOW``. - -.. _lang-constants-low: - -LOW -^^^ - -The meaning of ``LOW`` also has a different meaning depending on -whether a pin is set to ``INPUT`` or ``OUTPUT``. When a pin is -configured as an ``INPUT`` with :ref:`pinMode() <lang-pinmode>`, and -read with :ref:`digitalRead() <lang-digitalread>`, the microcontroller -will report ``LOW`` if a voltage of 2 volts or less is present at the -pin. - -When a pin is configured to ``OUTPUT`` with ``pinMode()``, and set to -``LOW`` with :ref:`digitalWrite() <lang-digitalwrite>`, the -microcontroller will attempt to keep that pin's voltage at 0V. In this -state it can *sink* current, e.g. light an LED that is connected -through a series resistor to +3.3V, or to another pin configured as an -output, and set to ``HIGH``. - -Pin Modes ---------- - -Digital pins can be used in a variety of modes. The basic modes, -``INPUT`` and ``OUTPUT``, have been introduced above. Changing a pin -from ``INPUT`` TO ``OUTPUT`` with :ref:`pinMode() <lang-pinmode>` -drastically changes the electrical behavior of the pin. - -This section describes the basic digital pin modes (``INPUT`` and -``OUTPUT``) only. For a detailed description of all possible pin -modes, see the :ref:`pinMode() <lang-pinmode>` reference page. - -.. _lang-constants-input: - -INPUT -^^^^^ - -Maple (STM32) pins configured as ``INPUT`` are said to be in a -high-impedance state. One way of explaining this is that pins -configured as ``INPUT`` make extremely small demands on the circuit -that they are sampling. This makes them useful for reading a sensor, -but not powering an LED. - -.. _lang-constants-output: - -OUTPUT -^^^^^^ - -Pins configured as ``OUTPUT`` with :ref:`pinMode() <lang-pinmode>` are -said to be in a low-impedance state. This means that they can provide -a substantial amount of current to other circuits. STM32 pins can -source (provide positive current) or sink (provide negative current) -up to 50 mA (milliamps) of current to other devices/circuits. This -makes them useful for powering LEDs, but useless for reading sensors. - -Pins configured as outputs can also be damaged or destroyed if short -circuited to either ground or 3.3V power rails. The amount of current -provided by an STM32 pin is also not enough to power most relays or -motors, and some interface circuitry will be required. - -.. _lang-constants-integers: - -Integer Constants ------------------ - -Integer constants (or more properly speaking, integer *literals*) are -numbers used directly in a sketch, like ``123``. By default, an -integer literal is treated as a (signed) :ref:`int <lang-int>`, but -you can change this with the U and L modifiers (see :ref:`below -<lang-constants-integers-u-l>`). You can specify negative numbers by -putting a minus sign in front, like ``-123``. - -Normally, integer literals are treated as base 10 (decimal) integers, -but special notation (formatters) may be used to enter numbers in -other bases. These are summarized in the following table: - -.. list-table:: - :header-rows: 1 - - * - Base - - Example - - Formatter - - Comment - - * - 10 (decimal) - - ``123`` - - None - - - - * - 2 (binary) - - ``0b1111011`` - - Leading "0b" - - GCC extension; not standard C++ - - * - 8 (octal) - - ``0173`` - - Leading "0" - - Characters 0-7 valid - - * - 16 (hexadecimal) - - ``0x7B`` - - Leading "0x" - - Characters 0-9, A-F (or a-f) valid - -Binary constants (like ``B1111011``) for values between 0 and 255 are -supported for compatibility with Arduino only. Their use in new -programs is discouraged. - -.. _lang-constants-integers-dec: - -**Decimal** is base 10. This is the common number system we learn in -school. Integer literals without other prefixes are assumed to be in -decimal format. - -For example, the decimal literal ``101`` is one hundred and one: 1×10\ -:sup:`2` + 0×10\ :sup:`1` + 1×10\ :sup:`0` = 101. - -.. _lang-constants-integers-bin: - -**Binary** is base two. Only characters 0 and 1 are valid. Binary -literals are indicated by the prefix ``0b`` (this is a :ref:`GCC -<arm-gcc>` extension; it's not standard C++). - -For example, the binary literal ``0b101`` is five: 1×2\ :sup:`2` + -0×2\ :sup:`1` + 1×2\ :sup:`0` = 5. - -.. _lang-constants-integers-oct: - -**Octal** is base eight. Only characters 0 through 7 are valid. Octal -literals are indicated by the prefix ``0``. - -For example, the octal literal ``0101`` is sixty five: 1×8\ :sup:`2` + -0×8\ :sup:`1` + 1×8\ :sup:`0` = 65. - -.. warning:: Bugs sometimes result by (unintentionally) including a - leading "0" before an integer literal, which makes the compiler - interpret it in octal. - -.. _lang-constants-integers-hex: - -**Hexadecimal** (or "hex") is base sixteen. Valid characters are 0 -through 9 and letters A through F; A has the value 10, B is 11, up to -F, which is 15. Hex values are indicated by the prefix ``0x``. A-F -may be typed in upper or lower case (a-f). - -For example, the hexadecimal literal ``0x101`` is two hundred fifty -seven: 1×16\ :sup:`2` + 0×16\ :sup:`1` + 1×16\ :sup:`0` = 257. - -The hexadecimal literal ``0xCF2`` is three thousand, three hundred -fourteen: 12×16\ :sup:`2` + 15×16\ :sup:`1` + 2×16\ :sup:`0` = 3314. - -(Remember that in hex, ``A`` means 10, and counting up, ``B``\ =11, so -``C``\ =12 and ``F``\ =15). - -.. _lang-constants-integers-u-l: - -U and L Suffixes -^^^^^^^^^^^^^^^^ - -By default, an integer constant is treated as an :ref:`int -<lang-int>`, with the attendant :ref:`limitations in values -<lang-int-overflow>`. To specify an integer constant with another data -type, follow it with: - -- a ``u`` or ``U`` to interpret the constant as an unsigned value. - For example, ``33U`` is an :ref:`unsigned int <lang-unsignedint>`. - -- an ``l`` or ``L`` to interpret the constant as a long value. For - example, ``100000L`` is a :ref:`long <lang-long>`. - -- a ``ul`` or ``UL`` to do both. For example, ``32767UL`` is an - :ref:`unsigned long <lang-unsignedlong>`. - -.. _lang-constants-fp: - -Floating-Point Constants ------------------------- - -Similar to integer literals, floating point constants (properly: -floating-point *literals*) are used to make code more readable. -Floating point literals are swapped at compile time for the value to -which the expression evaluates. - -A floating point literal is any number which includes a decimal point. -For instance, ``3.0`` is a floating-point literal for the number 3. -By default, a floating-point literal is a :ref:`double <lang-double>`. -In order for the literal to be interpreted as a :ref:`float -<lang-float>`, you can write ``f`` directly after it. For example, -``3.0f`` is a floating-point literal with type ``float``. - -Floating point constants can also be expressed in a variety of -scientific notation. ``E`` and ``e`` are both accepted as valid -exponent indicators. Some examples are given in the following table: - - -.. list-table:: - :header-rows: 1 - - * - Floating-point literal - - Evaluates to - - Alternate expression - - * - ``10.0`` - - 10 - - - - * - ``2.34E5`` - - 2.34×10\ :sup:`5` - - ``234000.0`` - - * - ``67e-12`` - - 67.0×10\ :sup:`-12` - - ``0.000000000067`` - -See Also --------- - -- :ref:`pinMode() <lang-pinmode>` -- :ref:`Boolean Variables <lang-booleanvariables>` -- :ref:`#define <lang-define>` -- :ref:`int <lang-int>` -- :ref:`unsigned int <lang-unsignedint>` -- :ref:`long <lang-long>` -- :ref:`unsigned long <lang-unsignedlong>` -- :ref:`float <lang-float>` -- :ref:`double <lang-double>` - -.. include:: cc-attribution.txt |