string
is an alias in C# for System.String
.
So technically, there is no difference. It's like int
vs. System.Int32
.
As far as guidelines, it's generally recommended to use string
any time you're referring to an object.
e.g.
string place = "world";
Likewise, I think it's generally recommended to use String
if you need to refer specifically to the class.
e.g.
string greet = String.Format("Hello {0}!", place);
This is the style that Microsoft tends to use in their examples.
It appears that the guidance in this area may have changed, as StyleCop now enforces the use of the C# specific aliases.
Maybe a bit of example code will help: Notice the difference in the call signatures of foo
, class_foo
and static_foo
:
class A(object):
def foo(self, x):
print(f"executing foo({self}, {x})")
@classmethod
def class_foo(cls, x):
print(f"executing class_foo({cls}, {x})")
@staticmethod
def static_foo(x):
print(f"executing static_foo({x})")
a = A()
Below is the usual way an object instance calls a method. The object instance, a
, is implicitly passed as the first argument.
a.foo(1)
# executing foo(<__main__.A object at 0xb7dbef0c>, 1)
With classmethods, the class of the object instance is implicitly passed as the first argument instead of self
.
a.class_foo(1)
# executing class_foo(<class '__main__.A'>, 1)
You can also call class_foo
using the class. In fact, if you define something to be
a classmethod, it is probably because you intend to call it from the class rather than from a class instance. A.foo(1)
would have raised a TypeError, but A.class_foo(1)
works just fine:
A.class_foo(1)
# executing class_foo(<class '__main__.A'>, 1)
One use people have found for class methods is to create inheritable alternative constructors.
With staticmethods, neither self
(the object instance) nor cls
(the class) is implicitly passed as the first argument. They behave like plain functions except that you can call them from an instance or the class:
a.static_foo(1)
# executing static_foo(1)
A.static_foo('hi')
# executing static_foo(hi)
Staticmethods are used to group functions which have some logical connection with a class to the class.
foo
is just a function, but when you call a.foo
you don't just get the function,
you get a "partially applied" version of the function with the object instance a
bound as the first argument to the function. foo
expects 2 arguments, while a.foo
only expects 1 argument.
a
is bound to foo
. That is what is meant by the term "bound" below:
print(a.foo)
# <bound method A.foo of <__main__.A object at 0xb7d52f0c>>
With a.class_foo
, a
is not bound to class_foo
, rather the class A
is bound to class_foo
.
print(a.class_foo)
# <bound method type.class_foo of <class '__main__.A'>>
Here, with a staticmethod, even though it is a method, a.static_foo
just returns
a good 'ole function with no arguments bound. static_foo
expects 1 argument, and
a.static_foo
expects 1 argument too.
print(a.static_foo)
# <function static_foo at 0xb7d479cc>
And of course the same thing happens when you call static_foo
with the class A
instead.
print(A.static_foo)
# <function static_foo at 0xb7d479cc>
Best Answer
First find the difference between the start point and the end point (here, this is more of a directed line segment, not a "line", since lines extend infinitely and don't start at a particular point).
Then calculate the angle (which runs from the positive X axis at
P1
to the positive Y axis atP1
).But
arctan
may not be ideal, because dividing the differences this way will erase the distinction needed to distinguish which quadrant the angle is in (see below). Use the following instead if your language includes anatan2
function:EDIT (Feb. 22, 2017): In general, however, calling
atan2(deltaY,deltaX)
just to get the proper angle forcos
andsin
may be inelegant. In those cases, you can often do the following instead:(deltaX, deltaY)
as a vector.deltaX
anddeltaY
by the vector's length (sqrt(deltaX*deltaX+deltaY*deltaY)
), unless the length is 0.deltaX
will now be the cosine of the angle between the vector and the horizontal axis (in the direction from the positive X to the positive Y axis atP1
).deltaY
will now be the sine of that angle.EDIT (Feb. 28, 2017): Even without normalizing
(deltaX, deltaY)
:deltaX
will tell you whether the cosine described in step 3 is positive or negative.deltaY
will tell you whether the sine described in step 4 is positive or negative.deltaX
anddeltaY
will tell you which quadrant the angle is in, in relation to the positive X axis atP1
:+deltaX
,+deltaY
: 0 to 90 degrees.-deltaX
,+deltaY
: 90 to 180 degrees.-deltaX
,-deltaY
: 180 to 270 degrees (-180 to -90 degrees).+deltaX
,-deltaY
: 270 to 360 degrees (-90 to 0 degrees).An implementation in Python using radians (provided on July 19, 2015 by Eric Leschinski, who edited my answer):
All tests pass. See https://en.wikipedia.org/wiki/Unit_circle