April 8th, 2013, 12:57 AM

New to python, need help
Hi Friends
I'm a DBA and new to python. I failed to solve a question in a interview exam. But interested to know the process. Here is the question, please help me with answer:
Here is an infinite series of alternating positive and negative terms: 1  1/3 + 1/5  1/7 + ... This series converges to PI/4. Write a Python generator for it and return the first 20 values from the generator
April 8th, 2013, 06:47 AM

hey every one i am a novice to python programming.I learned from my friend that it is quite helpful in understanding java. Please do comment on this, anyone..
April 8th, 2013, 09:20 AM

Without testing, I wrote following code.
Code:
import fractions
def pi_over_4_terms():
'''
T = 1  1/3 + 1/5  1/7
>>> L = []
>>> for (i,term,) in enumerate(pi_over_4_terms()):
... L.append(term)
... if i == 3:
... break
...
>>> [term*denominator for (term,denominator,) in zip(L, (1, 3, 5, 7))]
[1, 1, 1, 1]
>>> (1.01.0/3+1.0/51.0/7) == float(sum(L))
True
'''
n = 0
while True:
yield fractions.Fraction((1)**n,n*2+1)
n += 1
Turns out that when I do test my code both doctests fail. As you see, I didn't realize that integral Fraction remain as Fraction. I wasn't too sure that my sum test would pass because of intolerant comparisons. But I know better than this, and in retrospect should have written that test as
Code:
abs(sum(reversed((1.0,1.0/3,1.0/5,1.0/7)))float(sum(L))) < 1e6
I'm quite sure I would have passed the interview question with my code.
Code:
$ ( cd /tmp && python3 m doctest s.py )
**********************************************************************
File "s.py", line 13, in s.pi_over_4_terms
Failed example:
[term*denominator for (term,denominator,) in zip(L, (1, 3, 5, 7))]
Expected:
[1, 1, 1, 1]
Got:
[Fraction(1, 1), Fraction(1, 1), Fraction(1, 1), Fraction(1, 1)]
**********************************************************************
File "s.py", line 15, in s.pi_over_4_terms
Failed example:
(1.01.0/3+1.0/51.0/7) == float(sum(L))
Expected:
True
Got:
False
**********************************************************************
1 items had failures:
2 of 4 in s.pi_over_4_terms
***Test Failed*** 2 failures.
Comments on this post
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April 10th, 2013, 12:30 PM

April 17th, 2013, 02:30 AM

Not getting required results
Hi
What it the exact code should I use to get a Python generator for it and return the first 20 values from the generator.
April 17th, 2013, 08:49 AM

pi_over_4_terms_iterator = iter(pi_over_4_terms())
print([next(pi_over_4_terms_iterator) for i in range(20)])
In future, you spend time with the python interpreter, tutorials, and documents.
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April 18th, 2013, 02:25 AM

Got error while printing
Hi I used the following code, every thing worked fine but got a error while printing:
Code:
>>> def sqr(x):
return x*x
>>> def firstn(g, n):
for i in range(n):
yield g.next()
>>> def pi_series():
sum = 0
i = 1.0; j = 1
while(1):
sum = sum + j/i
yield 4*sum
i = i + 2; j = j * 1
>>> def euler_accelerator(g):
s0 = g.next() # Sn1
s1 = g.next() # Sn
s2 = g.next() # Sn+1
while 1:
yield s2  (sqr(s2  s1))/(s0  2*s1 + s2)
s0, s1, s2 = s1, s2, g.next()
>>> if __name__ == '__main__': print (list(firstn(euler_accelerator(pi_series()), 8)))
Traceback (most recent call last):
File "<pyshell#9>", line 1, in <module>
if __name__ == '__main__': print (list(firstn(euler_accelerator(pi_series()), 8)))
File "<pyshell#3>", line 3, in firstn
yield g.next()
AttributeError: 'generator' object has no attribute 'next'
April 18th, 2013, 09:30 AM

Maybe the troubles in "everything worked except for my program" are python 2.7 versus python 3 issues. Use the next function, not a .next method, and make iterators.
In python3:
Code:
def sqr(x):
return x*x
def firstn(it, n):
for i in range(n):
yield next(it)
def pi_series():
sum = 0
i, j = 1.0, 1
while True:
sum += j/i
yield 4*sum
i += 2
j *= 1
def euler_accelerator(g):
it = g()
s0 = next(it) # Sn1
s1 = next(it) # Sn
s2 = next(it) # Sn+1
while 1:
yield s2  (sqr(s2  s1))/(s0  2*s1 + s2)
s0, s1, s2 = s1, s2, next(it)
if __name__ == '__main__':
print(list(firstn(euler_accelerator(pi_series), 8)))
Last edited by b49P23TIvg; April 18th, 2013 at 09:32 AM.
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Code tags[/code] are essential for python code and Makefiles!
April 18th, 2013, 09:23 PM

Worked great thanks
Worked great thanks
Originally Posted by b49P23TIvg
Maybe the troubles in "everything worked except for my program" are python 2.7 versus python 3 issues. Use the next function, not a .next method, and make iterators.
In python3:
Code:
def sqr(x):
return x*x
def firstn(it, n):
for i in range(n):
yield next(it)
def pi_series():
sum = 0
i, j = 1.0, 1
while True:
sum += j/i
yield 4*sum
i += 2
j *= 1
def euler_accelerator(g):
it = g()
s0 = next(it) # Sn1
s1 = next(it) # Sn
s2 = next(it) # Sn+1
while 1:
yield s2  (sqr(s2  s1))/(s0  2*s1 + s2)
s0, s1, s2 = s1, s2, next(it)
if __name__ == '__main__':
print(list(firstn(euler_accelerator(pi_series), 8)))