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.. highlight:: cpp
.. _lang-comparison:
Comparison Operators
====================
The comparison operators ``==``, ``!=``, ``<``, ``>``, ``<=``, and
``>=`` are used to compare two numbers. They are :ref:`true
<lang-constants-true>` when the comparison is true, and :ref:`false
<lang-constants-false>` otherwise. They are based on the symbols
=, ≠, <, >, ≤, and ≥ from mathematics.
Here are some examples, with their meaning in comments::
// "eq" is true when x is equal to y
bool eq = (x == y);
// "neq" is true when x is different than y
bool neq = (x != y);
// "lt" is true when x is less than, but NOT equal to, y
bool lt = (x < y);
// "gt" is true when x is greater than, but NOT equal to, y
bool gt = (x > y);
// "lte" is true when x is less than or equal to y
bool lte = (x <= y);
// "gte" is true when x is greater than or equal to y
bool gte = (x >= y);
The parentheses are optional; they are present only for clarity. For
example, the following two lines are the same::
bool eq = x == y;
bool eq = (x == y);
Uses
----
Comparison operators, along with :ref:`boolean operators
<lang-boolean>`, are useful inside the conditionals of :ref:`if
<lang-if>` statements. Here's one example::
if (x < 50) {
// only execute these lines if x is less than 50
SerialUSB.println("delaying:");
SerialUSB.println(x);
delay(x);
}
.. warning::
Beware of accidentally using the single equal sign (``=``) when you
meant to test if two numbers are equal (``==``). This is a common
mistake inside of ``if`` statement conditionals, e.g.::
// DON'T MAKE THIS MISTAKE
if (x = 10) {
// body
}
The single equal sign is the assignment operator, and sets x to 10
(puts the value 10 into the variable x). Instead use the double equal
sign (e.g. ``if (x == 10)``), which is the comparison operator, and
tests *whether* x is equal to 10 or not. The latter statement is only
true if x equals 10, but the former statement will always be true.
This is because C evaluates the statement ``if (x=10)`` as follows: 10
is assigned to x (remember that the single equal sign is the
:ref:`assignment operator <lang-assignment>`), so x now
contains 10. Then the 'if' conditional evaluates 10, which evaluates
to :ref:`true <lang-constants-true>`, since any non-zero number
evaluates to ``true``.
Consequently, the conditional of an ``if`` statement like ``if (x =
10) {...}`` will always evaluate to ``true``, and the variable x
will be set to 10, which is probably not what you meant.
(This sometimes has uses, though, so just because an assignment
appears within a conditional doesn't mean it's automatically wrong.
Be careful to know what you mean.)
.. include:: cc-attribution.txt
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