In the following examples, input and output are distinguished by the presence orabsence of prompts (>>> and …): to repeat the example, you must typeeverything after the prompt, when the prompt appears; lines that do not beginwith a prompt are output from the interpreter. Note that a secondary prompt on aline by itself in an example means you must type a blank line; this is used toend a multi-line command.
In mathematics and digital electronics, a binary number is a number expressed in the base-2 numeral system or binary numeral system, which uses only two symbols: typically '0' and '1'. The base-2 numeral system is a positional notation with a radix of 2. Each digit is referred to as a bit. Jansson is a C library for encoding, decoding and manipulating JSON data. Its main features and design principles are: Simple and intuitive API and data model. Average follow-up in HERS II was an additional 2.7 years, for a total of 6.8 years overall. Rates of CHD events were comparable among women in the combined continuous CE plus MPA treatment group and the placebo group in HERS, HERS II, and overall.
Many of the examples in this manual, even those entered at the interactiveprompt, include comments. Comments in Python start with the hash character,
#
, and extend to the end of the physical line. A comment may appear at thestart of a line or following whitespace or code, but not within a stringliteral. A hash character within a string literal is just a hash character.Since comments are to clarify code and are not interpreted by Python, they maybe omitted when typing in examples.Some examples:
3.1. Using Python as a Calculator¶
Let’s try some simple Python commands. Start the interpreter and wait for theprimary prompt,
>>>
. (It shouldn’t take long.)3.1.1. Numbers¶
The interpreter acts as a simple calculator: you can type an expression at itand it will write the value. Expression syntax is straightforward: theoperators
+
, -
, *
and /
work just like in most other languages(for example, Pascal or C); parentheses (()
) can be used for grouping.For example:The integer numbers (e.g.
2
, 4
, 20
) have type int
,the ones with a fractional part (e.g. 5.0
, 1.6
) have typefloat
. We will see more about numeric types later in the tutorial.The return type of a division (
/
) operation depends on its operands. Ifboth operands are of type int
, floor division is performedand an int
is returned. If either operand is a float
,classic division is performed and a float
is returned. The //
operator is also provided for doing floor division no matter what theoperands are. The remainder can be calculated with the %
operator:With Python, it is possible to use the
**
operator to calculate powers 1:The equal sign (
=
Adobe dimension tutorial. ) is used to assign a value to a variable. Afterwards, noresult is displayed before the next interactive prompt:If a variable is not “defined” (assigned a value), trying to use it willgive you an error:
There is full support for floating point; operators with mixed type operandsconvert the integer operand to floating point:
In interactive mode, the last printed expression is assigned to the variable
_
. This means that when you are using Python as a desk calculator, it issomewhat easier to continue calculations, for example:This variable should be treated as read-only by the user. Don’t explicitlyassign a value to it — you would create an independent local variable with thesame name masking the built-in variable with its magic behavior.
In addition to
int
and float
, Python supports other types ofnumbers, such as Decimal
and Fraction
.Python also has built-in support for complex numbers,and uses the j
or J
suffix to indicate the imaginary part(e.g. 3+5j
).3.1.2. Strings¶
Besides numbers, Python can also manipulate strings, which can be expressedin several ways. They can be enclosed in single quotes (
'..'
) ordouble quotes ('..'
) with the same result 2.
can be usedto escape quotes:In the interactive interpreter, the output string is enclosed in quotes andspecial characters are escaped with backslashes. While this might sometimeslook different from the input (the enclosing quotes could change), the twostrings are equivalent. The string is enclosed in double quotes ifthe string contains a single quote and no double quotes, otherwise it isenclosed in single quotes. The
print
statement produces a morereadable output, by omitting the enclosing quotes and by printing escapedand special characters:If you don’t want characters prefaced by
to be interpreted asspecial characters, you can use raw strings by adding an r
beforethe first quote:String literals can span multiple lines. One way is using triple-quotes:
''..''
or ''..''
. End of lines are automaticallyincluded in the string, but it’s possible to prevent this by adding a
atthe end of the line. The following example:produces the following output (note that the initial newline is not included):
Strings can be concatenated (glued together) with the
+
operator, andrepeated with *
:Two or more string literals (i.e. the ones enclosed between quotes) nextto each other are automatically concatenated.
This feature is particularly useful when you want to break long strings:
This only works with two literals though, not with variables or expressions:
If you want to concatenate variables or a variable and a literal, use
+
:Strings can be indexed (subscripted), with the first character having index 0.There is no separate character type; a character is simply a string of sizeone:
Indices may also be negative numbers, to start counting from the right:
Note that since -0 is the same as 0, negative indices start from -1.
In addition to indexing, slicing is also supported. While indexing is usedto obtain individual characters, slicing allows you to obtain a substring:
Note how the start is always included, and the end always excluded. Thismakes sure that
s[:i]+s[i:]
is always equal to s
:Slice indices have useful defaults; an omitted first index defaults to zero, anomitted second index defaults to the size of the string being sliced.
One way to remember how slices work is to think of the indices as pointingbetween characters, with the left edge of the first character numbered 0.Then the right edge of the last character of a string of n characters hasindex n, for example:
The first row of numbers gives the position of the indices 0…6 in the string;the second row gives the corresponding negative indices. The slice from i toj consists of all characters between the edges labeled i and j,respectively.
For non-negative indices, the length of a slice is the difference of theindices, if both are within bounds. For example, the length of
word[1:3]
is2.Attempting to use an index that is too large will result in an error:
However, out of range slice indexes are handled gracefully when used forslicing:
Python strings cannot be changed — they are immutable.Therefore, assigning to an indexed position in the string results in an error:
If you need a different string, you should create a new one:
The built-in function
len()
returns the length of a string:See also
Strings, and the Unicode strings described in the next section, areexamples of sequence types, and support the common operations supportedby such types.
Both strings and Unicode strings support a large number of methods forbasic transformations and searching.
Information about string formatting with
str.format()
.The old formatting operations invoked when strings and Unicode strings arethe left operand of the
%
operator are described in more detail here.3.1.3. Unicode Strings¶
Starting with Python 2.0 a new data type for storing text data is available tothe programmer: the Unicode object. It can be used to store and manipulateUnicode data (see http://www.unicode.org/) and integrates well with the existingstring objects, providing auto-conversions where necessary.
Unicode has the advantage of providing one ordinal for every character in everyscript used in modern and ancient texts. Previously, there were only 256possible ordinals for script characters. Texts were typically bound to a codepage which mapped the ordinals to script characters. This lead to very muchconfusion especially with respect to internationalization (usually written as
i18n
— 'i'
+ 18 characters + 'n'
) of software. Unicode solvesthese problems by defining one code page for all scripts.Creating Unicode strings in Python is just as simple as creating normalstrings:
The small
'u'
in front of the quote indicates that a Unicode string issupposed to be created. If you want to include special characters in the string,you can do so by using the Python Unicode-Escape encoding. The followingexample shows how:The escape sequence
u0020
indicates to insert the Unicode character withthe ordinal value 0x0020 (the space character) at the given position.Stringed 2 2 7 Divided By 4 7
Other characters are interpreted by using their respective ordinal valuesdirectly as Unicode ordinals. If you have literal strings in the standardLatin-1 encoding that is used in many Western countries, you will find itconvenient that the lower 256 characters of Unicode are the same as the 256characters of Latin-1.
For experts, there is also a raw mode just like the one for normal strings. Youhave to prefix the opening quote with ‘ur’ to have Python use theRaw-Unicode-Escape encoding. It will only apply the above
uXXXX
conversion if there is an uneven number of backslashes in front of the small‘u’.2.2 Gd
The raw mode is most useful when you have to enter lots of backslashes, as canbe necessary in regular expressions.
Apart from these standard encodings, Python provides a whole set of other waysof creating Unicode strings on the basis of a known encoding.
The built-in function
unicode()
provides access to all registered Unicodecodecs (COders and DECoders). Some of the more well known encodings which thesecodecs can convert are Latin-1, ASCII, UTF-8, and UTF-16. The latter twoare variable-length encodings that store each Unicode character in one or morebytes. The default encoding is normally set to ASCII, which passes throughcharacters in the range 0 to 127 and rejects any other characters with an error.When a Unicode string is printed, written to a file, or converted withstr()
, conversion takes place using this default encoding.![Stringed 2 2 7 divided by 4 7 Stringed 2 2 7 divided by 4 7](https://upload.wikimedia.org/wikipedia/commons/8/8a/Go-in-China.jpg)
To convert a Unicode string into an 8-bit string using a specific encoding,Unicode objects provide an
encode()
method that takes one argument, thename of the encoding. Lowercase names for encodings are preferred.If you have data in a specific encoding and want to produce a correspondingUnicode string from it, you can use the
unicode()
function with theencoding name as the second argument.3.1.4. Lists¶
Python knows a number of compound data types, used to group together othervalues. The most versatile is the list, which can be written as a list ofcomma-separated values (items) between square brackets. Lists might containitems of different types, but usually the items all have the same type.
Like strings (and all other built-in sequence type), lists can beindexed and sliced:
All slice operations return a new list containing the requested elements. Thismeans that the following slice returns a new (shallow) copy of the list:
Lists also supports operations like concatenation:
Unlike strings, which are immutable, lists are a mutabletype, i.e. it is possible to change their content:
You can also add new items at the end of the list, by usingthe
append()
method (we will see more about methods later):Assignment to slices is also possible, and this can even change the size of thelist or clear it entirely:
The built-in function
len()
also applies to lists:Stringed 2 2 7 Cars
It is possible to nest lists (create lists containing other lists), forexample:
3.2. First Steps Towards Programming¶
Of course, we can use Python for more complicated tasks than adding two and twotogether. For instance, we can write an initial sub-sequence of the Fibonacciseries as follows:
This example introduces several new features.
Stringed 2 2 76
- The first line contains a multiple assignment: the variables
a
andb
simultaneously get the new values 0 and 1. On the last line this is used again,demonstrating that the expressions on the right-hand side are all evaluatedfirst before any of the assignments take place. The right-hand side expressionsare evaluated from the left to the right. - The
while
loop executes as long as the condition (here:b<10
)remains true. In Python, like in C, any non-zero integer value is true; zero isfalse. Klokki 1 1 3 download free. The condition may also be a string or list value, in fact any sequence;anything with a non-zero length is true, empty sequences are false. The testused in the example is a simple comparison. The standard comparison operatorsare written the same as in C:<
(less than),>
(greater than), (equal to),<=
(less than or equal to),>=
(greater than or equal to)and!=
(not equal to). - The body of the loop is indented: indentation is Python’s way of groupingstatements. At the interactive prompt, you have to type a tab or space(s) foreach indented line. In practice you will prepare more complicated inputfor Python with a text editor; all decent text editors have an auto-indentfacility. When a compound statement is entered interactively, it must befollowed by a blank line to indicate completion (since the parser cannotguess when you have typed the last line). Note that each line within a basicblock must be indented by the same amount.
- The
print
statement writes the value of the expression(s) it isgiven. It differs from just writing the expression you want to write (as we didearlier in the calculator examples) in the way it handles multiple expressionsand strings. Strings are printed without quotes, and a space is insertedbetween items, so you can format things nicely, like this:A trailing comma avoids the newline after the output:Note that the interpreter inserts a newline before it prints the next prompt ifthe last line was not completed.
Footnotes
Since
**
has higher precedence than -
, -3**2
will beinterpreted as -(3**2)
and thus result in -9
. To avoid thisand get 9
, you can use (-3)**2
.2+2 Club Piraeus
Unlike other languages, special characters such as
n
have thesame meaning with both single ('..'
) and double ('..'
) quotes.The only difference between the two is that within single quotes you don’tneed to escape '
(but you have to escape '
) and vice versa.