CS2304: Data Types - Virginia Techcourses.cs.vt.edu/~cs2304/spring2014/Notes/T02_DataTypes.pdfCS2304: Python for Java Programmers Monti 2014 Basic Output • Python provides the print()

CS2304: Python for Java Programmers

Monti 2014

CS2304: Data Types


Monti 2014

Side point: Comments

•  Like other languages Python lets you put free text inside of source code.

•  Comments are ignored by the Python interpreter and therefore do not execute.

•  There are two types of comments you can use: •  Whole line (or rest of line) comments start with a # and work like // in Java/C/C++.

•  Python also has multi-line comments: •  Three double quotes to start and stop the comment “”” “”” .

•  We are basically creating a big unused string. •  These strings/comments can also used as doc-strings.


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Side point: Using The Interpreter

•  You can follow along in the interactive interpreter (the part with >>>, just type ‘python3’.

•  Using the the interpreter you can treat Python kind of like a calculator.

•  It’s also convenient for quickly testing code. •  Everything we look at today will work in standard

Python code outside of the interpreter.


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Numeric Types and Operations

•  Python has integer and double types (as well as a few others) like Java/C/C++.

•  Variables can switch between integer and double types (or anything else) from line to line.

•  Python 3 reacts more gracefully (?) when dealing with integer division.

•  Python supports the standard mathematical operations you are familiar with: +, -, *, / and %.

•  You can also calculate powers with **.


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Identifiers and Variables

•  These have a similar naming scheme to Java/C++. •  Identifiers (variables, functions, etc.) must start

with a letter or an underscore. •  The underscore has a particular meaning we’ll see later on.

•  After the first character the rest can be: numbers, letters, and underscores.

•  Python suggests that variables and function names be all lower case with underscores.

•  There are different style rules for class names, modules: •  Style guide: http://www.python.org/dev/peps/pep-0008/


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Examples: Operators and Variables

•  Using variables and operators should work more or less as you expect.

•  However, variables don’t have to be declared in the same sense as Java/C++.

•  You will want to give variables a value though:

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


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An Important Difference: Division

•  By default (using /), division converts the operands and produces a floating point result.

•  Unlike Java/C++ where you need to carefully consider the types. •  Try 1 / 2 in Python vs. C++/Java, it should be 0.5 in Python.

•  Python has an additional operator “//”, which will discard the fractional part. •  So, 1 // 2 = 0 rather than 0.5.

•  Note: this is different from Python 2.X, which behaved like Java/C/C++.


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Strings in Python

•  A collection of characters ex. “Hello World”. •  Single and double quotes are pretty much

interchangeable: •  So ‘Hello World’ is the same as “Hello World”. •  While ‘’ or “” are acceptable, they do need to “match” so ‘ ”

won’t work. •  Remember “”” “”” can be used for multi-line comments.

•  Strings in Python are for the most part very flexible. •  “+” Will concatenate two or more strings. •  “*” Can be used as well, ex. “hello”*3.


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String Basics

•  Strings can use bracket notation like arrays/strings in other languages.

•  Oddly, at least coming from another language, you can also use negative indices.

•  You start at the end of the string and count backwards:

>>> s = “Hello World” >>> s[0] ‘H’ >>> s[3] ‘l’

>>> s[-1] ‘d’


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String Slicing

•  String “slicing” is supported, giving you substrings:

•  Strings are immutable, so trying to modify an individual cell will cause an error:

•  You can always create another string though.

>>> s = “Hello World” >>> s[0:3] ‘Hel’ >>> s[3:6] ‘lo ’

>>> s[0] = “L” # TypeError


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Iterating Through Strings

•  Recall: We can use a for loop to move through a string.

•  The loop will iterate through the characters, printing them one by one:

H e l …

for x in s: print(x)


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Basic Output

•  Python provides the print() function for basic I/O:

•  The print function is a little different in Python

2.X, it doesn’t need parentheses.

>>> print(s) Hello World >>> print(s[2:]) llo World >>> print(s[2:], 1, 0) llo World 1 0


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Basic Input

•  If you want to prompt for something, use the input function:

•  Be careful: ‘7’ is a string that need to be

converted to be used as a number:

>>> j = input("Enter a number: ") Enter a number: 7 >>> print(j) 7

>>> print(int(j)+1) 8

Documents

CS2304: Data Types - Virginia Techcourses.cs.vt.edu/~cs2304/spring2014/Notes/T02_DataTypes.pdfCS2304: Python for Java Programmers Monti 2014 Basic Output • Python provides the print()