How to display two decimal points in python, when a number is perfectly divisible?

Question

Currently I am trying to solve a problem, where I am supposed to print the answer upto two decimal points without rounding off. I have used the below code for this purpose

import math
a=1.175                            #value of a after some division
print(math.floor(a*100)/100)

The output we get is:

1.17                              #Notice value which has two decimal points & not rounded

But the real problem starts when I try to print a number which is evenly divisible, after the decimal point only one zero is displayed. I have used the same code as above, but now

a=25/5                                   #Now a is perfectly divisible
print(math.floor(a*100)/100)

The output displayed now is

5.0                                      #Notice only one decimal place is printed

what must be done rectify this bug?

Answer 1

The division works and returns adequate precision in result.

So your problem is just about visualization or exactly:

• string-representation of floating-point numbers

Formatting a decimal

You can use string-formatting for that. For example in Python 3, use f-strings:

twoFractionDigits = f"{result:.2f}"

or print(f"{result:.2f}")

The trick does .2f, a string formatting literal or format specifier that represents a floating-point number (f) with two fractional digits after decimal-point (.2).

See also:

• Fixed digits after decimal with f-strings
• How to format a floating number to fixed width in Python

Try on the Python-shell:

Python 3.6.9 (default, Dec  8 2021, 21:08:43) 
[GCC 8.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import math
>>> a=1.175                            #value of a after some division
>>> result = math.floor(a*100)/100
>>> result
1.17
>>> print(result)
1.17
>>> a=25/5                                   #Now a is perfectly divisible
>>> result = math.floor(a*100)/100
>>> result
5.0
>>> print(result)
5.0
>>> print(f"{result:.2f}")
5.00

Formatting a decimal as percentage

Similar you can represent the ratio as percentage: print(f"{result:.2f} %")

prints:

5.00 %

A formatting shortcut for percentage can be: print(f"{25/100:.2%}") Which converts the result of 25/100 == 0.25 to:

25.00%

Note: The formatting-literal .2% automatically converts from ratio to percentage with 2 digits after the decimal-point and adds the percent-symbol.

Formatting a decimal with specific scale (rounded or truncated ?)

Now the part without rounding-off, just truncation. As example we can use the repeating decimal, e.g. 1/6 which needs to be either rounded or truncated (cut-off) after a fixed number of fractional digits - the scale (in contrast to precision).

>>> print(f"{1/6:.2}")
0.17
>>> print(f"{1/6:.2%}")
16.67%

Note how the formatted string is not truncated (to 0.16) but rounded (to 0.17). Here the scale was specified inside formatting-literal as 2 (after the dot).

See also:

• Truncate to three decimals in Python
• How do I interpret precision and scale of a number in a database?
• What is the difference between precision and scale?

Formatting many decimals in fixed width (leading spaces)

Another example is to print multiple decimals, like in a column as right-aligned, so you can easily compare them.

Then use string-formatting literal 6.2f to add leading spaces (here a fixed-width of 6):

>>> print(f"{result:6.2f}")
  5.00
>>> print(f"{100/25*100:6.2f}")
400.00
>>> print(f"{25/100*100:6.2f}")
 25.00

See also

All the formatting-literals demonstrated here can also be applied using

• old-style %-formatting (also known as "Modulo string formatting") which was inherited from printf method of C language. Benefit: This way is also compatible with Python before 3.6).
• new-style .format method on strings (introduced with Python 3)

See theherk's answer which demonstrates those alternatives.

Learn more about string-formatting in Python:

• Real Python: Python 3's f-Strings: An Improved String Formatting Syntax (Guide)
• Real Python: Python String Formatting Best Practices

Answer 2

You can find this recommendation in the official Python tutorial: 15. Floating Point Arithmetic: Issues and Limitations.

For more pleasant output, you may wish to use string formatting to produce a limited number of significant digits

print("%.2f" % 3.0)
3.00

or

format(3.0, ".2f")
'3.00'