Member Avatar for dustbunny000 
import math
import random

choices=[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36,\
37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73,\
 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 270, 271, 272, 273, 274, 275, 276, 277, 278, 279, 280, 281, 282, 283, 284, 285,\
 286, 287, 288, 289, 290, 291, 292, 293, 294, 295, 296, 297, 298, 299, 300, 301, 302, 303, 304, 305, 306, 307, 308, 309, 310, 311, 312, 313, 314, 315,\
 316, 317, 318, 319, 320, 321, 322, 323, 324, 325, 326, 327, 328, 329, 330, 331, 332, 333, 334, 335, 336, 337, 338, 339, 340, 341, 342, 343, 344, 345,\
 346, 347, 348, 349, 350, 351, 352, 353, 354, 355, 356, 357, 358, 359, 360]
z=math.radians(random.choice(choices))
newx=0
newy=0

for n in range(1000):
	newx=newx+math.sin(z)
	newy=newy+math.cos(z)
print newx
print newy

How do I make it so that z is the same in both the newx and newy calculations?

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  • 11 Hours Discussion Span
  • Latest Post Latest Post by TrustyTony

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Generated

The original behavior comes from assigning z once before the loop, so every iteration uses the same angle. That is why 's prints show a fixed z while newx and newy change: the accumulators add the same sine and cosine repeatedly. 's follow-up asking for z to change each time is exactly the situation addressed by selecting a new angle (or its precomputed sine/cosine) inside the loop.

A clear trade-off exists between correctness and performance. Selecting a new angle every iteration (calling the random selection inside the loop) makes z change each time. Computing sin and cos once for the chosen z and reusing those two values for that iteration avoids repeated trig work. Precomputing a table of sine/cosine pairs for the allowed angles and sampling from that table reduces CPU work further at the cost of a small memory footprint. For continuous sampling, random.uniform gives a uniformly chosen angle; random.choice or random.randrange works for discrete sets. See the Python docs for details on the math and random functions used: Python math docs and Python random docs.

A few practical notes: the chosen angle ranges (e.g., 0..90 plus 270..360) bias the direction of the accumulated steps, so the long-term sum will reflect that bias. For many iterations, vectorized libraries such as NumPy can compute large batches of sines/cosines much faster — see NumPy docs. Also remember the radians/ degrees distinction: Python trig functions expect radians.

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Member Avatar for woooee

"z" is the same as a print statement will verify, although newx and newy do change.

for n in range(10):     ## only print 10 times
        print "adding math.sin(%f) to %f" % (z, newx)
	newx=newx+math.sin(z)
        print "adding math.cos(%f) to %f" % (z, newy)
	newy=newy+math.cos(z)
print newx
print newy
##
## note that it is inefficient to calaculate sin(z) and cos(z) on every loop
##
sin_z = math.sin(z)
cos_z = math.cos(z)
for n in range(10):
	newx=newx+sin_z
	newy=newy+cos_z
print newx
print newy
##
##   an easier way to generate choices
##
choices = [x for x in range(0, 91)]
choices += [x for x in range(270, 361)]
Edited by woooee because: n/a
Member Avatar for dustbunny000

"z" is the same as a print statement will verify, although newx and newy do change.

for n in range(10):     ## only print 10 times
        print "adding math.sin(%f) to %f" % (z, newx)
	newx=newx+math.sin(z)
        print "adding math.cos(%f) to %f" % (z, newy)
	newy=newy+math.cos(z)
print newx
print newy
##
## note that it is inefficient to calaculate sin(z) and cos(z) on every loop
##
sin_z = math.sin(z)
cos_z = math.cos(z)
for n in range(10):
	newx=newx+sin_z
	newy=newy+cos_z
print newx
print newy
##
##   an easier way to generate choices
##
choices = [x for x in range(0, 91)]
choices += [x for x in range(270, 361)]

What if I want to change z every time?

Member Avatar for TrustyTony

You want to do like this?

import math
import random
import itertools
choices= [(math.sin(z), math.cos(z)) for z in itertools.chain(range(91), range(270,361))]
newx, newy = 0, 0

for n in range(1000):
    sinz, cosz = random.choice(choices)
    newx, newy = newx + sinz, newy+cosz
    if n < 100: print newx, newy # print hundred first as test
Edited by TrustyTony because: n/a
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