Drawing Sinus curve in Python

[Steven D'Aprano]

On Sun, 01 Jun 2014 05:17:07 -0700, Farzad Torabi wrote:

> Hi Experts
> 
>  I am trying to draw a sine curve in Python , well first I had a script
>  that i could draw a function curve in this way :
> 
> xMax = 25.0
> points = []
> for i in range(100):
>   x = (float(i)/99)*xMax
>   y = math.sqrt(x)
>   points.append([x,y])
> 
> s.Spline(points=points)

What is s? Where does it come from?



> first i have questions that : what does the line x = (float(i)/99)*xMax
> do ? why do we multiply it by

In older versions of Python, division / with integer arguments does 
integer division, like C, instead of calculator division. For example:

1/2 => returns 0

instead of 1/2 returning 0.5 like a calculator does. In these older 
versions of Python, you can fix that by converting one of the arguments 
to a float first:

1/2.0 => 0.5

So "float(i)/99" converts the loop index i to a float, then divides by 
99. An easier way to get the same result is "i/99.0".

Then, multiplying by xMax simply scales the result to be between 0 and 
xMax, in this case 25.0. Look at the results:

when i = 0, x = 0/99.0*25 = 0.0
when i = 99, x = 99/99.0*25.9 = 25.0

every other value of i gives a corresponding value between 0 and 25.


> and then when I wanted to draw a sine curve I found this one :
> 
> import math
> 
> for angle in range(????):
>     y = math.sin(math.radians(angle))
>     print(y)
> 
> first , here instead of ???? can we put 2*pi ?


No. The Python built-in range() function only accepts integer values. It 
is quite tricky to *accurately* produce floating point ranges. While it 
is easy to make a floating point range, it is much harder to make it 
accurate. You can see some discussion about the problem, and some sample 
code, here:

http://code.activestate.com/recipes/577068
http://code.activestate.com/recipes/577878
http://code.activestate.com/recipes/577881




> second i wanted to try this method instead:
> 
> xMax = pi
> Lamda = 200
> points = []
> for i in range(Lamda):
>   x = (float(i)/99)*xMax
>   y = math.sin(x)
>   points.append([x,y])
> 
> it actually works much better and creates an actual sine curve but the
> lengths are not really what i want , also if i want to draw a straight
> line I use this command :
> 
> xMax = 1
> Lamda = 200
> points = []
> for i in range(Lamda):
>   x = (float(i)/99)
>   y = xMax
>   points.append([x,y])

In this example, you are calculating points from a straight line. x 
varies from 0.0 to 2.0202 in steps of 1/99, and y is always the same 
value, 1.


> but then the problem will be that I can not control the length of this
> line and the sine curve , that should be equal

You have to get the maths right, otherwise the graph will be wrong.



-- 
Steven D'Aprano
http://import-that.dreamwidth.org/