python - Formatting Complex Numbers

Question

For a project in one of my classes we have to output numbers up to five decimal places.It is possible that the output will be a complex number and I am unable to figure out how to output a complex number with five decimal places. For floats I know it is just:

print "%0.5f"%variable_name

Is there something similar for complex numbers?

Answer 1

You could do it as is shown below using the str.format() method:

>>> n = 3.4+2.3j
>>> n
(3.4+2.3j)
>>> '({0.real:.2f} + {0.imag:.2f}i)'.format(n)
'(3.40 + 2.30i)'
>>> '({c.real:.2f} + {c.imag:.2f}i)'.format(c=n)
'(3.40 + 2.30i)'

To make it handle both positive and negative imaginary portions properly, you would need a (even more) complicated formatting operation:

>>> n = 3.4-2.3j
>>> n
(3.4-2.3j)
>>> '({0:.2f} {1} {2:.2f}i)'.format(n.real, '+-'[n.imag < 0], abs(n.imag))
'(3.40 - 2.30i)'

Update - Easier Way

Although you cannot use f as a presentation type for complex numbers using the string formatting operator %:

n1 = 3.4+2.3j
n2 = 3.4-2.3j

try:
    print('test: %.2f' % n1)
except Exception as exc:
    print('{}: {}'.format(type(exc).__name__, exc))

Output:

TypeError: float argument required, not complex

You can however use it with complex numbers via the str.format() method. This isn't explicitly documented, but is implied by the Format Specification Mini-Language documentation which just says:

'f'??Fixed point. Displays the number as a fixed-point number. The default precision is 6.

. . .so it's easy to overlook. In concrete terms, the following works in both Python 2.7.14 and 3.4.6:

print('n1: {:.2f}'.format(n1))
print('n2: {:.2f}'.format(n2))

Output:

n1: 3.10+4.20j
n2: 3.10-4.20j

This doesn't give you quite the control the code in my original answer does, but it's certainly much more concise (and handles both positive and negative imaginary parts automatically).

Update 2 - f-strings

Formatted string literals (aka f-strings) were added in Python 3.6, which means it could also be done like this in that version or later:

print(f'n1: {n1:.2f}')  # -> n1: 3.40+2.30j
print(f'n2: {n2:.3f}')  # -> n2: 3.400-2.300j

In Python 3.8.0, support for an = specifier was added to f-strings, allowing you to write:

print(f'{n1=:.2f}')  # -> n1=3.40+2.30j
print(f'{n2=:.3f}')  # -> n2=3.400-2.300j