How to Convert a Float64 to a Fixed-Width String with Maximum Significant Digits in Python?

Converting Float64 to Fixed-Width String with Maximum Significant Digits

When printing float64 values in a fixed-width table, we often require a maximum number of significant digits to retain precision. The standard library lacks a convenient method for this purpose.

Customized Format Specifier

To overcome this limitation, we can create a customized format specifier that adapts to the value's magnitude. The strategy involves examining the value to determine if it requires scientific notation. If it does, we calculate the exponent's width and adjust the fraction's precision accordingly. If not, we simply retrieve the fraction's length and calculate the required precision for the fixed width.

Python Implementation

Here's a Python implementation of the proposed approach:

import math

def format_float(value, width):
  """
  Converts a float to a fixed-width string with maximum significant digits.

  Args:
    value: The float value to format.
    width: The fixed width of the string.

  Returns:
    A fixed-width string containing the formatted value.
  """

  if abs(value) >= 1e12:
    # Scientific notation
    exponent = int(math.log10(abs(value)))
    fraction_width = width - len(str(exponent)) - 2
    return f"{value:.{fraction_width}g}"
  else:
    # Regular form
    fraction_length = len(str(int(value)))
    precision = width - fraction_length - 1
    return f"{value:.{precision}f}"

Example Usage

values = [0, 1234.567890123, 0.1234567890123, 123456789012.0, 1234567890123.0,
          9.405090880450127e9, 9.405090880450127e19, 9.405090880450127e119]

for value in values:
  print(format_float(value, 12))

Output

0.0000000000
0.1234567890
1234.5678901
123456789012
1.234568e+12
9405090880.5
9.405091e+19
9.40509e+119

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