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Re: Interesting math problem

On Mar 19, 2:17 pm, "BJörn Lindqvist" <bjou...@gmail.comwrote:
On Mon, Mar 17, 2008 at 11:57 PM, Arnaud Delobelle

<arno...@googlemail.comwrote:
def make_slope(distance, parts):
step = distance / float(parts)
intstep = int(step)
floatstep = step - intstep
steps = []
acc = 0.0
for i in range(parts):
acc += floatstep
step = intstep
if acc 0.999:
step += 1
acc -= 1.0
steps.append(step)
return steps
OK then, using list comprehensions. It is more succint, is it easier
to read?
def slope(dist, parts):
return [(i+1)*dist/parts - i*dist/parts for i in xrange(parts)]

Congratulations! You Won! Jeff Schwab's recursive approach is also
cool but this is the most interesting abuse of integer division I have
seen. I don't think any of the variants are readable at a first
glance, but with a comment it should be ok.

--
mvh Björn
I really want to revive this discussion. Arnaud's approach is
definetly cool, but it turns out that in real-world situations it
doesn't work as succint as here.

Try to use it to draw a simple non-anitaliased line in a standrad
python array or buffer object. Suppose we have an array of unsigned
bytes called `buf` where each line takes `pitch` bytes. That's what I
got while trying to take advantage of this approach. No advantage at
all. And what about ability to port the code to C for speed?

def draw_line(buf, pitch, x, y, dx, dy):
if dx == dy == 0:
buf[y * pitch + x] = 0
return
xdir, ydir = 1, 1

if dx < 0:
xdir = -1
dx = abs(dx)
if dy < 0:
ydir = -1
dy = abs(dy)

if dy < dx:
steps = ((i+1) * dx / dy - i * dx / dy for i in xrange(dy))
for step in steps:
start = y * pitch + x
if xdir 0:
buf[start : start + step] = array('B', [0] * step)
else:
buf[start - step : start] = array('B', [0] * step)
x += step * xdir
y += ydir
else:
steps = ((i+1) * dy / dx - i * dy / dx for i in xrange(dx))
for step in steps:
start = y * pitch + x
if ydir 0:
for i in range(start, start + pitch * step, pitch):
buf[i] = 0
else:
for i in range(start, start - pitch * step, -pitch):
buf[i] = 0
x += xdir
y += step * ydir

Please, tell me that I'm wrong and it's really possible to draw lines,
do scan-conversion and so on with such a cool succint constructs!

--
Ivan
Jun 27 '08 #1
1 1003
On Apr 13, 5:35*pm, Ivan Illarionov <ivan.illario...@gmail.comwrote:
On Mar 19, 2:17 pm, "BJörn Lindqvist" <bjou...@gmail.comwrote:
On Mon, Mar 17, 2008 at 11:57 PM, Arnaud Delobelle
<arno...@googlemail.comwrote:
*def make_slope(distance, parts):
** * step = distance / float(parts)
** * intstep = int(step)
** * floatstep = step - intstep
** * steps = []
** * acc = 0.0
** * for i in range(parts):
** * * * acc += floatstep
** * * * step = intstep
** * * * if acc 0.999:
** * * * * * step += 1
** * * * * * acc -= 1.0
** * * * steps.append(step)
** * return steps
*OK then, using list comprehensions. *It is more succint, is it easier
*to read?
*def slope(dist, parts):
* * return [(i+1)*dist/parts - i*dist/parts for i in xrange(parts)]
Congratulations! You Won! Jeff Schwab's recursive approach is also
cool but this is the most interesting abuse of integer division I have
seen. I don't think any of the variants are readable at a first
glance, but with a comment it should be ok.
--
mvh Björn

I really want to revive this discussion. Arnaud's approach is
definetly cool, but it turns out that in real-world situations it
doesn't work as succint as here.

Try to use it to draw a simple non-anitaliased line in a standrad
python array or buffer object. Suppose we have an array of unsigned
bytes called `buf` where each line takes `pitch` bytes. That's what I
got while trying to take advantage of this approach. No advantage at
all. And what about ability to port the code to C for speed?

def draw_line(buf, pitch, x, y, dx, dy):
* * if dx == dy == 0:
* * * * buf[y * pitch + x] = 0
* * * * return
* * xdir, ydir = 1, 1

* * if dx < 0:
* * * * xdir = -1
* * * * dx = abs(dx)
* * if dy < 0:
* * * * ydir = -1
* * * * dy = abs(dy)

* * if dy < dx:
* * * * steps = ((i+1) * dx / dy - i * dx / dy for i in xrange(dy))
* * * * for step in steps:
* * * * * * start = y * pitch + x
* * * * * * if xdir 0:
* * * * * * * * buf[start : start + step] = array('B', [0] * step)
* * * * * * else:
* * * * * * * * buf[start - step : start] = array('B', [0] * step)
* * * * * * x += step * xdir
* * * * * * y += ydir
* * else:
* * * * steps = ((i+1) * dy / dx - i * dy / dx for i in xrange(dx))
* * * * for step in steps:
* * * * * * start = y * pitch + x
* * * * * * if ydir 0:
* * * * * * * * for i in range(start, start + pitch * step, pitch):
* * * * * * * * * * buf[i] = 0
* * * * * * else:
* * * * * * * * for i in range(start, start - pitch * step, -pitch):
* * * * * * * * * * buf[i] = 0
* * * * * * x += xdir
* * * * * * y += step * ydir

Please, tell me that I'm wrong and it's really possible to draw lines,
do scan-conversion and so on with such a cool succint constructs!

--
Ivan
I don't think my answer is suitable for drawing a line the way you are
doing it. FWIW, this is how I would go about it (not tested):

def draw_rectangle(buf, pitch, x, y, w, h):
# Make a mask for w and apply it across h lines

def draw_line(buf, pitch, x, y, w, h):
# w and h can't be < 0
if w < h:
limits = ((i, i*h/w) for i in xrange(1, w+1))
else:
limits = ((i*w/h, i) for i in xrange(1, h+1))
dx0, dy0 = 0, 0
for dx, dy in limits:
draw_rectangle(x+dx0, y+dy0, dx-dx0, dy-dy0)
dx0, dy0 = dx, dy

The positive thing is that it is trivial to extend draw_line so that
it accepts a thickness parameter as well.

--
Arnaud

Jun 27 '08 #2

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