Print pi to a number of decimal places

One of the challenges on w3resources is to print pi to 'n' decimal places. Here is my code:

from math import pi

fraser = str(pi)

length_of_pi = []

number_of_places = raw_input("Enter the number of decimal places you want to 
see: ")

for number_of_places in fraser:
    length_of_pi.append(str(number_of_places))

print "".join(length_of_pi)

For whatever reason, it automatically prints pi without taking into account of any inputs. Any help would be great :)

Why not just format using number_of_places:

''.format(pi)
>>> format(pi, '.4f')
'3.1416'
>>> format(pi, '.14f')
'3.14159265358979'

And more generally:

>>> number_of_places = 6
>>> '{:.{}f}'.format(pi, number_of_places)
'3.141593'

In your original approach, I guess you're trying to pick a number of digits using number_of_places as the control variable of the loop, which is quite hacky but does not work in your case because the initial number_of_digits entered by the user is never used. It is instead being replaced by the iteratee values from the pi string.

The proposed solutions using np.pi, math.pi, etc only only work to double precision (~14 digits), to get higher precision you need to use multi-precision, for example the mpmath package

>>> from mpmath import mp
>>> mp.dps = 20    # set number of digits
>>> print(mp.pi)
3.1415926535897932385

Using np.pi gives the wrong result

>>> format(np.pi, '.20f')
3.14159265358979311600

Compare to the true value:

3.14159265358979323846264338327...

Your solution appears to be looping over the wrong thing:

for number_of_places in fraser:

For 9 places, this turns out be something like:

for "9" in "3.141592653589793":

Which loops three times, one for each "9" found in the string. We can fix your code:

from math import pi

fraser = str(pi)

length_of_pi = []

number_of_places = int(raw_input("Enter the number of decimal places you want: "))

for places in range(number_of_places + 1):  # +1 for decimal point
    length_of_pi.append(str(fraser[places]))

print "".join(length_of_pi)

But this still limits n to be less than the len(str(math.pi)), less than 15 in Python 2. Given a serious n, it breaks:

> python test.py
Enter the number of decimal places you want to see: 100
Traceback (most recent call last):
  File "test.py", line 10, in <module>
    length_of_pi.append(str(fraser[places]))
IndexError: string index out of range
>

To do better, we have to calculate PI ourselves -- using a series evaluation is one approach:

# Rewrite of Henrik Johansson's (Henrik.Johansson@Nexus.Comm.SE)
# pi.c example from his bignum package for Python 3
#
# Terms based on Gauss' refinement of Machin's formula:
#
# arctan(x) = x - (x^3)/3 + (x^5)/5 - (x^7)/7 + ...

from decimal import Decimal, getcontext

TERMS = [(12, 18), (8, 57), (-5, 239)]  # ala Gauss

def arctan(talj, kvot):

    """Compute arctangent using a series approximation"""

    summation = 0

    talj *= product

    qfactor = 1

    while talj:
        talj //= kvot
        summation += (talj // qfactor)
        qfactor += 2

    return summation

number_of_places = int(input("Enter the number of decimal places you want: "))
getcontext().prec = number_of_places
product = 10 ** number_of_places

result = 0

for multiplier, denominator in TERMS:
    denominator = Decimal(denominator)
    result += arctan(- denominator * multiplier, - (denominator ** 2))

result *= 4  # pi == atan(1) * 4
string = str(result)

# 3.14159265358979E+15 => 3.14159265358979
print(string[0:string.index("E")])

Now we can take on a large value of n:

> python3 test2.py
Enter the number of decimal places you want: 100
3.141592653589793238462643383279502884197169399375105820974944592307816406286208998628034825342117067
>

As this question already has useful answers, I would just like to share how i created a program for the same purpose, which is very similar to the one in the question.

from math import pi
i = int(input("Enter the number of decimal places: "))
h = 0
b = list()
for x in str(pi):
    h += 1
    b.append(x)
    if h == i+2:
        break

h = ''.join(b)
print(h)

Thanks for Reading.

Abhishek Bahukhandi

For example the mpmath package

from mpmath import mp
def a(n):
   mp.dps=n+1
   return(mp.pi)

来源：https://stackoverflow.com/questions/45416626/print-pi-to-a-number-of-decimal-places

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