【Udemy Python3入門+応用】　10.数値

**※この記事はUdemyの
「[現役シリコンバレーエンジニアが教えるPython3入門+応用+アメリカのシリコンバレー流コードスタイル](https://www.udemy.com/course/python-beginner/ "現役シリコンバレーエンジニアが教えるPython3入門+応用+アメリカのシリコンバレー流コードスタイル")」
の講座を受講した上での、自分用の授業ノートです。
講師の[酒井潤](https://twitter.com/sakaijun "酒井潤")さんから許可をいただいた上で公開しています。**

##■特殊な四則演算
#####◆商の整数部分を求める
```:division_integer
>>> 17 // 3
```
```:result
5
```
`//`により、
17÷3 = 5.6666666
の整数部分が返る。

#####◆商のあまりを求める
```:division_remainder
>>> 17 % 3
```
```:result
2
```
`%`により、
17÷3 = 5 あまり2
のあまりの部分が返る。

#####◆べき乗(指数)
```:exponentiation
>>> 2 ** 10
```
```:result
1024
```
`**`が「^」に相当する。

<br>

##■文字を用いる
```:use_alphabet
>>> x = 5
>>> y = 2
>>> x * y
```
```:result
10
```
文字に数値を代入しておき、その文字を演算に用いることができる。

```:use_alphabet_error
>>> a
```
```:result
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'a' is not defined
```
何も代入されていない文字を用いるとエラーになる。

<br>

##■小数点以下を丸める
```:round
>>> pie = 3.14159265358979
>>> round(pie, 2)
```
```:result
3.14
```
`round(pie, 2)`により、pieに代入された値の小数点以下第2位まで丸めることができる。

<br>

##■mathライブラリを使ってみる
```python:math_library
import math

x = math.sqrt(25)
print(x)

y = math.log(2)
print(y)
```
```:result
5.0
0.6931471805599453
```
`math.sqrt()`は√、
`math.log()`は自然対数(ln)になる。

```python:math_library_help
import math

help(math)
```
```:result
Help on module math:

NAME
    math

MODULE REFERENCE
    https://docs.python.org/3.7/library/math
    
    The following documentation is automatically generated from the Python
    source files.  It may be incomplete, incorrect or include features that
    are considered implementation detail and may vary between Python
    implementations.  When in doubt, consult the module reference at the
    location listed above.

DESCRIPTION
    This module provides access to the mathematical functions
    defined by the C standard.

FUNCTIONS
    acos(x, /)
        Return the arc cosine (measured in radians) of x.
    
    acosh(x, /)
        Return the inverse hyperbolic cosine of x.
    
    asin(x, /)
        Return the arc sine (measured in radians) of x.
    
    asinh(x, /)
        Return the inverse hyperbolic sine of x.
    
    atan(x, /)
        Return the arc tangent (measured in radians) of x.
    
    atan2(y, x, /)
        Return the arc tangent (measured in radians) of y/x.
        
        Unlike atan(y/x), the signs of both x and y are considered.
    
    atanh(x, /)
        Return the inverse hyperbolic tangent of x.
    
    ceil(x, /)
        Return the ceiling of x as an Integral.
        
        This is the smallest integer >= x.
    
    copysign(x, y, /)
        Return a float with the magnitude (absolute value) of x but the sign of y.
        
        On platforms that support signed zeros, copysign(1.0, -0.0)
        returns -1.0.
    
    cos(x, /)
        Return the cosine of x (measured in radians).
    
    cosh(x, /)
        Return the hyperbolic cosine of x.
    
    degrees(x, /)
        Convert angle x from radians to degrees.
    
    erf(x, /)
        Error function at x.
    
    erfc(x, /)
        Complementary error function at x.
    
    exp(x, /)
        Return e raised to the power of x.
    
    expm1(x, /)
        Return exp(x)-1.
        
        This function avoids the loss of precision involved in the direct evaluation of exp(x)-1 for small x.
    
    fabs(x, /)
        Return the absolute value of the float x.
    
    factorial(x, /)
        Find x!.
        
        Raise a ValueError if x is negative or non-integral.
    
    floor(x, /)
        Return the floor of x as an Integral.
        
        This is the largest integer <= x.
    
    fmod(x, y, /)
        Return fmod(x, y), according to platform C.
        
        x % y may differ.
    
    frexp(x, /)
        Return the mantissa and exponent of x, as pair (m, e).
        
        m is a float and e is an int, such that x = m * 2.**e.
        If x is 0, m and e are both 0.  Else 0.5 <= abs(m) < 1.0.
    
    fsum(seq, /)
        Return an accurate floating point sum of values in the iterable seq.
        
        Assumes IEEE-754 floating point arithmetic.
    
    gamma(x, /)
        Gamma function at x.
    
    gcd(x, y, /)
        greatest common divisor of x and y
    
    hypot(x, y, /)
        Return the Euclidean distance, sqrt(x*x + y*y).
    
    isclose(a, b, *, rel_tol=1e-09, abs_tol=0.0)
        Determine whether two floating point numbers are close in value.
        
          rel_tol
            maximum difference for being considered "close", relative to the
            magnitude of the input values
          abs_tol
            maximum difference for being considered "close", regardless of the
            magnitude of the input values
        
        Return True if a is close in value to b, and False otherwise.
        
        For the values to be considered close, the difference between them
        must be smaller than at least one of the tolerances.
        
        -inf, inf and NaN behave similarly to the IEEE 754 Standard.  That
        is, NaN is not close to anything, even itself.  inf and -inf are
        only close to themselves.
    
    isfinite(x, /)
        Return True if x is neither an infinity nor a NaN, and False otherwise.
    
    isinf(x, /)
        Return True if x is a positive or negative infinity, and False otherwise.
    
    isnan(x, /)
        Return True if x is a NaN (not a number), and False otherwise.
    
    ldexp(x, i, /)
        Return x * (2**i).
        
        This is essentially the inverse of frexp().
    
    lgamma(x, /)
        Natural logarithm of absolute value of Gamma function at x.
    
    log(...)
        log(x, [base=math.e])
        Return the logarithm of x to the given base.
        
        If the base not specified, returns the natural logarithm (base e) of x.
    
    log10(x, /)
        Return the base 10 logarithm of x.
    
    log1p(x, /)
        Return the natural logarithm of 1+x (base e).
        
        The result is computed in a way which is accurate for x near zero.
    
    log2(x, /)
        Return the base 2 logarithm of x.
    
    modf(x, /)
        Return the fractional and integer parts of x.
        
        Both results carry the sign of x and are floats.
    
    pow(x, y, /)
        Return x**y (x to the power of y).
    
    radians(x, /)
        Convert angle x from degrees to radians.
    
    remainder(x, y, /)
        Difference between x and the closest integer multiple of y.
        
        Return x - n*y where n*y is the closest integer multiple of y.
        In the case where x is exactly halfway between two multiples of
        y, the nearest even value of n is used. The result is always exact.
    
    sin(x, /)
        Return the sine of x (measured in radians).
    
    sinh(x, /)
        Return the hyperbolic sine of x.
    
    sqrt(x, /)
        Return the square root of x.
    
    tan(x, /)
        Return the tangent of x (measured in radians).
    
    tanh(x, /)
        Return the hyperbolic tangent of x.
    
    trunc(x, /)
        Truncates the Real x to the nearest Integral toward 0.
        
        Uses the __trunc__ magic method.

DATA
    e = 2.718281828459045
    inf = inf
    nan = nan
    pi = 3.141592653589793
    tau = 6.283185307179586
```
ライブラリのhelpを表示させることもできる。
ライブラリについて詳しくは後の授業で。