Можно ли в Python во float отбросить k знаков после запятой?
Не совсем, тем не менее, спасибо за ответ, это я некорректно вопрос сформулировал, видимо.
Нужно не выводить на печать последние две цифры, а хранить в памяти число с двумя последними цифрами
Михаил: round — стандартная bult-in функция, существующая для выполнения конкретной задачи, именно она и является стандартным способом, впрочем, таки да, вы не обязаны со мной соглашаться, но указанный вами способ вызывает у меня стойкое чувство жжения в районе поясницы.
Limiting floats to two decimal points
I want a to be rounded to 13.95. I tried using round , but I get:
For the analogous issue with the standard library Decimal class, see How can I format a decimal to always show 2 decimal places?.
36 Answers 36
You are running into the old problem with floating point numbers that not all numbers can be represented exactly. The command line is just showing you the full floating point form from memory.
With floating point representation, your rounded version is the same number. Since computers are binary, they store floating point numbers as an integer and then divide it by a power of two so 13.95 will be represented in a similar fashion to 125650429603636838/(2**53).
Double precision numbers have 53 bits (16 digits) of precision and regular floats have 24 bits (8 digits) of precision. The floating point type in Python uses double precision to store the values.
If you are after only two decimal places (to display a currency value, for example), then you have a couple of better choices:
- Use integers and store values in cents, not dollars and then divide by 100 to convert to dollars.
- Or use a fixed point number like decimal.
You can do the same as:
Note 1: the above returns a string. In order to get as float, simply wrap with float(. ) :
Note 2: wrapping with float() doesn’t change anything:
The built-in round() works just fine in Python 2.7 or later.
Let me give an example in Python 3.6’s f-string/template-string format, which I think is beautifully neat:
It works well with longer examples too, with operators and not needing parentheses:
I feel that the simplest approach is to use the format() function.
This produces a float number as a string rounded to two decimal points.
Most numbers cannot be exactly represented in floats. If you want to round the number because that’s what your mathematical formula or algorithm requires, then you want to use round. If you just want to restrict the display to a certain precision, then don’t even use round and just format it as that string. (If you want to display it with some alternate rounding method, and there are tons, then you need to mix the two approaches.)
And lastly, though perhaps most importantly, if you want exact math then you don’t want floats at all. The usual example is dealing with money and to store ‘cents’ as an integer.
Because the latter may lead to output errors when trying to output multiple variables (see comments).
Try the code below:
The rounding problem of input and output has been solved definitively by Python 3.1 and the fix is backported also to Python 2.7.0.
Rounded numbers can be reversibly converted between float and string back and forth:
str -> float() -> repr() -> float() . or Decimal -> float -> str -> Decimal
A Decimal type is not necessary for storage anymore.
Results of arithmetic operations must be rounded again because rounding errors could accumulate more inaccuracy than that is possible after parsing one number. That is not fixed by the improved repr() algorithm (Python >= 3.1, >= 2.7.0):
The output string function str(float(. )) was rounded to 12 valid digits in Python < 2.7x and < 3.1, to prevent excessive invalid digits similar to unfixed repr() output. That was still insufficientl after subtraction of very similar numbers and it was too much rounded after other operations. Python 2.7 and 3.1 use the same length of str() although the repr() is fixed. Some old versions of Numpy had also excessive invalid digits, even with fixed Python. The current Numpy is fixed. Python versions >= 3.2 have the same results of str() and repr() function and also output of similar functions in Numpy.
Documentation
Conversions between floating-point numbers and strings are now correctly rounded on most platforms. These conversions occur in many different places: str() on floats and complex numbers; the float and complex constructors; numeric formatting; serializing and de-serializing floats and complex numbers using the marshal , pickle and json modules; parsing of float and imaginary literals in Python code; and Decimal-to-float conversion.
Related to this, the repr() of a floating-point number x now returns a result based on the shortest decimal string that’s guaranteed to round back to x under correct rounding (with round-half-to-even rounding mode). Previously it gave a string based on rounding x to 17 decimal digits.
Округление чисел в Python
Статьи
Введение
В данной статье разберём различные способы округления чисел в языке программирования Python.
Округление чисел встроенными функциями
Округление чисел функцией round() в Python
Функция round() округляет число до указанного количества знаков после запятой.
Округление чисел функцией int() в Python
Функция int() преобразовывает данные в целочисленный тип округляя их в сторону нуля.
Если нужно, чтобы число преобразовалось по математическим правилам, то требуется выполнить некоторые действия:
- Если число является положительным, то прибавить к нему 0,5;
- если же число является отрицательным, то прибавить к нему -0,5.
Округление чисел функциями из модуля math
Для использования функция из модуля math мы его импортируем:
Округление чисел функцией ceil() в Python
Функция ceil() служит для округления чисел до наименьшего целого числа.
Округление чисел функцией floor()
Функция floor() служит для округления чисел до наибольшего целого числа.
Заключение
В ходе статьи мы с Вами разобрали различные способы округления чисел в языке программирования Python. Надеюсь Вам понравилась статья, желаю удачи и успехов! ��
10 ways to round to 2 decimal places in Python
In this post, we are going to learn different 10 ways to round to 2 decimal places in Python by using the python built-in round() function, str. format(),f-string, and all possible ways to round Float to 2 decimal places in python
What is Python round() function
The Python’s round() function rounds off a given number based on the number of digits passed for round off and returns a floating-point number. If the number of digits for round-off is not specified, the default value will be 0 and it returns the nearest integer.
Syntax
Parameters
- number: The number that we have to round off.
- digits: The number of digits up to the number to be rounded off.
1. How to Round to 2 decimal Places python 3
In this example, we are using Python’s inbuilt round() function to round off a float number to 2 decimal places and using the print() function to print the result.
Program Exmaple
3. Python str.format() to round to 2 Decimal Places
To round off up to 2 decimal places we can use the inbuilt python string function str.format() with “<:2f>” a string and pass a float number as a parameter to the format() method. The round-off can be changed to different places <:3f>for 3 decimal places and so on.
Program Example
4. Python f-string to Round off to 2 Decimal Places
In this example, we are using the f-string to round off a number to 2 decimal palaces.
Program Example to round off to 2 decimal places
5. Using decimal Module
To get the correct precision, we can use the decimal module decimal() method of Python. Let us understand with an example, how to achieve this
Program Example
Output
6. Regex to Truncate float Number to 2 Decimal Places
In this example, we are using the regular expression to Truncate floating Numbers to 2 Decimal Places, by defining a custom method. Here we are passing the number which we want to truncate to 2 decimal places and print the result after truncation.
Program Example
7. Custom function to Truncate float to 2 decimal places
In this example, we are using a custom truncate function that takes two arguments as float numbers & the number of digits up to where we have to truncate the number. Here we are truncating float numbers to 2 decimal places.
Program Example
8. Round a float number keeping ending zero
In this example, we are rounding off a float number to 2 decimal palaces keeping the ending zero and print the result using the print() function.
Python Program to round a float number keeping ending zero
9. Add extra zero after decimal in Python
In this example, we are adding extra zero after decimal places by using the format() method.
Program to Add extra zero after decimal in Python
10.Math module for Python Handling precision
The python math module has some built-in functionality to handle the precision.