Question

below is my program for finding approximate root for a some 5000th-degree polynomial that is given as a series:

def s(r, z):
    sm = 0
    for k in range(1, z+1):
        sm += (900-3*k) * r ** (k-1)
    return sm

target = -600000000000

n = 1
dr = .125
curr = 0
while abs(curr - target) > 1:
    curr = s(n, 5000)
    if curr > target : n+= dr
    else : n-=dr
    dr /= 2

I wanted it rounded to 12 digits behind decimal point, hence

print round(n, 12)

which gave me :

1.00232210863

number is approximate enough, but it is now rounded to 12 digits. I fired up python console and found it myself :

>>> n
1.0023221086328755
>>> round(n, 12)
1.002322108633

Why is my round() function working only inside python console prompt?

Answer 1

By deafault python’s print rounds numbers to 11 digits when printing.
Try instead

print '%0.12f'%n

To be a little more clear, when you are in the interactive console and do:

>>> x = 1.123456789012345
>>> x
1.123456789012345
>>> print x
1.12345678901

Remember that the the first call is actually a call to

x.__repr__() 

which will print as much information about x as it can, while the print statement translates to

print x.__str__()

and the builting str() method on ints rounds to 11 digits if necessary. You can force as many digits as you like using python’s % formatting as shown above.