How to round up a complex number?

Question

How can I round up a complex number (e.g. 1.9999999999999998-2j) as 2-2j?

When I tried using

print(round(x,2))

it showed

Traceback (most recent call last):
  File "C:\Python34\FFT.py", line 22, in <module>
    print(round(x,2))
TypeError: type complex doesn't define __round__ method

Solution 1

Round real part and imaginary part separately and combine them:

>>> num = 1.9999999999999998-2j
>>> round(num.real, 2) + round(num.imag, 2) * 1j
(2-2j)

Solution 2

If all you want to do is represent the value rounded as shown, rather than modify the value itself, the following works:

>>> x=1.9999999999999998-2j
>>> print("{:g}".format(x))
2-2j

See: Format Specification Mini-Language.

Solution 3

Id say the best way to do it is as such

x = (1.542334+32.5322j)
x = complex(round(x.real),round(x.imag))

if you don't want to repeat that every time you want to do it, you could put it in a function.

def round_complex(x):
    return complex(round(x.real),round(x.imag))

Additional optional arguments can then be added to this, so if you only want to round one part for example, or if you only want to round to a certain number of decimal places on either the real or complex part

def round_complex(x, PlacesReal = 0, PlacesImag = 0, RoundImag = True, RoundReal = True):
     if RoundImag and not RoundReal:
         return complex(x.real,round(x.imag,PlacesImag))

     elif RoundReal and not RoundImag:
         return complex(round(x.real,PlacesReal),x.imag)

     else: #it would be a waste of space to make it do nothing if you set both to false, so it instead does what it would if both were true
         return complex(round(x.real,PlacesReal),round(x.imag,PlacesImag))

as the variables are auto set to true or 0, you don't need to input them unless you specifically want too. But they are handy to have

Comments

[Zero Piraeus]

Do you want to actually change the value, or just print a rounded representation of it?

[prav]

i want to change the value

[Carl Walsh]

To round up numbers that aren't cause by floating point precision i.e. 2.9+1j you can use a small precision format like f"{2.9+1j:.0g}". Beware this hack does not work for small numbers: f"{.9+1j:.0g}" = '0.9+1j'.

[Carl Walsh]

You can force rounding small numbers with format f instead of g, like f"{.9+1j:.0f}" gives '1+1j' (I don't know why I didn't find that a year ago)