Python Calculator

Python Calculator

Let’s try some simple Python commands. Start the interpreter and wait for the primary prompt, >>>. (It shouldn’t take long.)

Python Calculator

Numbers

The interpreter works as a simple calculator (Python Calculator): you can type an expression at it and it will print the value. Expression syntax is straightforward: the operators +, -, * and / work similar to in most alternative languages (for example, Pascal or C); parentheses (()) may be used for grouping. For example:

>>> 2 + 2
4
>>> 50 - 5*6
20
>>> (50 - 5*6) / 4
5.0
>>> 8 / 5  # division always returns a floating point number
1.6

Integer numbers (such as 2, 4, 20) have type int, float in a numerator (eg 5.0, 1.6). 

We will see more about numerical types later in the tutorial.


Division (/) always returns a float. To split the floor and get integer results 

(Discarding any partial result) You can use the // operator;
You can use% to calculate the remainder:

>>> 17 / 3  # classic division returns a float
5.666666666666667
>>>
>>> 17 // 3  # floor division discards the fractional part
5
>>> 17 % 3  # the % operator returns the remainder of the division
2
>>> 5 * 3 + 2  # result * divisor + remainder
17





It is possible to use the ** operator to calculate powers 1 with Python :


>>> 5 ** 2  # 5 squared
25
>>> 2 ** 7  # 2 to the power of 7
128



An equal sign (=) is used to assign a value to a variable. After this, no result is 

displayed before the next interactive prompt:





>>> width = 20
>>> height = 5 * 9
>>> width * height
900





If a variable is "not defined" (a value specified), then trying to use it will give you 

an error:




>>> n  # try to access an undefined variable
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'n' is not defined




There is full support for floating-point; Operators with mixed type operands convert 

integer operators to floating points:




>>> 4 * 3.75 - 1
14.0




In interactive mode, the last printed expression variable is assigned to _. 
This means that when you are using Python calculator, it is slightly easier to 

continue calculations, for example:

>>> tax = 12.5 / 100
>>> price = 100.50
>>> price * tax
12.5625
>>> price + _
113.0625
>>> round(_, 2)
113.06




This variable should be treated as read-only. Do not explicitly specify a value for this - 

you will create an independent local variable that will have the same name masking the 

built-in variable with your magical behavior.



Strings
In addition to numbers, Python can also manipulate strings, which can be expressed in 

many ways. They can be enclosed in a single quote ('...') or
Double Result ("...") with the same result. 2. \ _ can be used to avoid citations:

>>> 'spam eggs'  # single quotes
'spam eggs'
>>> 'doesn\'t'  # use \' to escape the single quote...
"doesn't"
>>> "doesn't"  # ...or use double quotes instead
"doesn't"
>>> '"Yes," they said.'
'"Yes," they said.'
>>> "\"Yes,\" they said."
'"Yes," they said.'
>>> '"Isn\'t," they said.'
'"Isn\'t," they said.'




In the interactive interpreter, the output string is enclosed in quotes and special 

characters are escaped with a backslash. Although it can sometimes look different from 

the input (the enclosed quotation may change), the two strings are equivalent. If the 

string contains a single quote and there are no double quotes, the string is enclosed 

in double quotes, otherwise, it is enclosed in single quotes.
The print () function produces a more readable output, except for the enclosed citations 

and saved and special print Characters:



>>> '"Isn\'t," they said.'
'"Isn\'t," they said.'
>>> print('"Isn\'t," they said.')
"Isn't," they said.
>>> s = 'First line.\nSecond line.'  # \n means newline
>>> s  # without print(), \n is included in the output
'First line.\nSecond line.'
>>> print(s)  # with print(), \n produces a new line
First line.
Second line.





If you do not want characters preceded to be \ interpreted as special characters, you can 

use the raw string by adding r before the first quotation:




>>> print('C:\some\name')  # here \n means newline!
C:\some
ame
>>> print(r'C:\some\name')  # note the r before the quote
C:\some\name



String literals can span multiple lines. One method is using triple-quotes: "" "..." "" 

or '"' ... '".
The end of the rows is automatically included in the string, but it is possible to stop 

it by adding a \ "at the end of the row. The following example:

print("""\
Usage: thingy [OPTIONS]
     -h                        Display this usage message
     -H hostname               Hostname to connect to
""")




Strings can be combined with the + operator (glued together), and repeated with *:



>>> # 3 times 'un', followed by 'ium'
>>> 3 * 'un' + 'ium'
'unununium'




Two or more string literals (ie enclosed between blocks) are automatically leveled next to 
each other.



>>> 'Py' 'thon'
'Python'




This feature is particularly useful when you want to break a long strings:


>>> text = ('Put several strings within parentheses '
...         'to have them joined together.')
>>> text
'Put several strings within parentheses to have them joined together.'




This only works with two literals though, not with variables or expressions:



>>> prefix = 'Py'
>>> prefix 'thon'  # can't concatenate a variable and a string literal
  File "<stdin>", line 1
    prefix 'thon'
                ^
SyntaxError: invalid syntax
>>> ('un' * 3) 'ium'
  File "<stdin>", line 1
    ('un' * 3) 'ium'
                   ^
SyntaxError: invalid syntax





If you want to concatenate variables or a variable and a literal, use +:




>>> prefix + 'thon'
'Python'





Strings can be indexed, with the first character index being 0. There is no separate 
character type; A character is simply a string of one size:




>>> word = 'Python'
>>> word[0]  # character in position 0
'P'
>>> word[5]  # character in position 5
'n'





Indices may also be negative numbers, to start counting from the right:




>>> word[-1]  # last character
'n'
>>> word[-2]  # second-last character
'o'
>>> word[-6]
'P'





Note that since -0 is the same as 0, negative indices start from -1.



In addition to sequencing, slicing is also supported. Indexing is used to obtain individual 
characters, slicing allows you to get substrings:



>>> word[0:2]  # characters from position 0 (included) to 2 (excluded)
'Py'
>>> word[2:5]  # characters from position 2 (included) to 5 (excluded)
'tho'






Note how the beginning is always included, and the end is always excluded. This ensures
 that s [: i] + s [i:] is always equal to s:



>>> word[:2] + word[2:]
'Python'
>>> word[:4] + word[4:]
'Python'






Slice indices have useful defaults; A omitted first index defaults to zero, an augmented 
second index default is truncated to the size of the string.



>>> word[:2]   # character from the beginning to position 2 (excluded)
'Py'
>>> word[4:]   # characters from position 4 (included) to the end
'on'
>>> word[-2:]  # characters from the second-last (included) to the end
'on'









One way to remember how the slice works is to think about pointing the indices between 
characters, with the first character's left edge being 0.. Then the right edge of the last 
character of a string of n characters is the index n, for example:




 +---+---+---+---+---+---+
 | P | y | t | h | o | n |
 +---+---+---+---+---+---+
 0   1   2   3   4   5   6
-6  -5  -4  -3  -2  -1





The first line of numbers gives the position of the indices 0… 6 in the string; The second 
line indicates the same negative. Slices from i to j contain all the characters between 
labeled edges i and j, respectively.