Print pi to a number of decimal places

Question

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 :)

Answer 1

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...

Answer 2

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.

Answer 3

For example the mpmath package

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

Answer 4

Great answers! there are so many ways to achieve this. Check out this method I used below, it works any number of decimal places till infinity:

#import multp-precision module
from mpmath import mp
#define PI function
def pi_func():
    while True:
        #request input from user
        try:
             entry = input("Please enter an number of decimal places to which the value of PI should be calculated\nEnter 'quit' to cancel: ")
             #condition for quit
             if entry == 'quit':
                 break
             #modify input for computation
             mp.dps = int(entry) +1
         #condition for input error
         except:
                print("Looks like you did not enter an integer!")
                continue
         #execute and print result
         else:
              print(mp.pi)
              continue

Good luck Pal!