Programming in Python Part I

1
Programming in Python Part I

• Dr. Fatma Cemile Serçe
• Atilim University
• 2009-2010

2
The Python Programming Language

• High-level language (like C, C, Perl, Java,
  etc.)
• Interpreted language
• Python programs executed by an interpreter
• Two ways to use the interpreter
• command-line mode
• script mode

3
The Python Programming Language

• In Command-line Mode
• type Python programs and the interpreter prints
  the result
• The first line of this example is the command
  that starts the Python interpreter.
• The next two lines are messages from the
  interpreter
• The third line starts with gtgtgt, prompt the
  interpreter uses to indicate it is ready
• Type print (11) program, and interpreter replied
  2

4
The Python Programming Language

• In Script Mode
• write a program in a file and use the interpreter
  to execute the contents of the file. The file is
  called a script
• file name ends with .py
• Ex use text editor to create a file
• named hello.py.
• Then tell interpreter the name of the script
• python hello.py

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First Program Hello World

• in the Python
• print ("Hello, World! ")
• in the C
• include ltstdio.hgt
• int main(void)
• printf("hello, world\n")
• return 0
• in the C
• include ltiostream.hgt
• void main()
• cout ltlt "Hello, world." ltlt endl
• in the Java
• public class HelloWorld
• public static void main(String args)
• System.out.println(Hello, World!)

6
Values and Types

• A value is one of the fundamental thingslike a
  letter or a numberthat a program manipulates
• Ex
• 2
• Hello, World!
• These values are belongs to different types
• 2 integer
• Hello, World! string

7
Values and Types

• If you are not sure what type a value has, the
  interpreter can tell you
• gtgtgt type(Hello, World!)
• ltclass strgt
• gtgtgt type(17)
• ltclass intgt
• gtgtgt type(3.2)
• ltclass floatgt
• str-gtString, int-gtInteger, float-gtfloating-point

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Exercise 1

• What about values like 17 and 3.2?
• gtgtgt type(17)
• lttype strgt
• gtgtgt type(3.2)
• lttype strgt
• Theyre strings.

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Exercise 2

• What is the output?
• gtgtgt print (1,000,000)
• 1.000.000 ? NO
• 1 0 0 ? YES
• a semantic error the code runs without producing
  an error message, but it doesnt do the right
  thing.

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Variables

• A variable is a name that refers to a value
• The assignment statement creates new variables
  and gives them values
• gtgtgt message Hello, World!
• gtgtgt n 17
• gtgtgt pi 3.14159

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Variables (cont.)

• The print statement also works with variables
• gtgtgt print (message)
• Hello, World!
• gtgtgt print (n)
• 17
• gtgtgt print (pi)
• 3.14159

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Variables (cont.)

• Variables also have types again, we can ask the
  interpreter what they are
• gtgtgt type(message)
• lttype strgt
• gtgtgt type(n)
• lttype intgt
• gtgtgt type(pi)
• lttype floatgt
• The type of a variable is the type of the value
  it refers to.

13
Variable names and keywords

• choose meaningful names
• both letters and numbers, but begin with a letter
• Message and message are different (use lowercase
  by convention)
• use underscore character (_) in names with
  multiple words
• person_name

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Variable names and keywords

• If you give a variable an illegal name, you get a
  syntax error
• gtgtgt 76tables seventy six tables
• SyntaxError invalid syntax
• gtgtgt more 1000000
• SyntaxError invalid syntax
• gtgtgt class COMPE 111
• SyntaxError invalid syntax
• 76trombones is illegal because it does not begin
  with a letter.
• more is illegal because it contains an illegal
  character, the dollar sign
• But whats wrong with class? It turns out that
  class is one of the Python keywords.

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Variable names and keywords

• Keywords define the languages rules and
  structure
• Keywords cannot be used as variable names
• Python has twenty-nine keywords

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Statements

• A statement is an instruction that the Python
  interpreter can execute
• print and assignment
• The result of a print statement is a value.
• Assignment statements dont produce a result.
• A script usually contains a sequence of
  statements.
• Ex the script
• print (1)
• x 2
• print (x)
• produces the output
• 1
• 2

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Operators and Operands

• Operators are special symbols that represent
  computations like addition and multiplication
• The values the operator uses are called operands
• 2032
• hour-1
• hour60minute
• minute/60
• 52
• (59)(15-7)

The symbols , -, and /, and the use of
parenthesis for grouping, mean in Python what
they mean in mathematics The asterisk () is the
symbol for multiplication is the symbol for
exponentiation modulo
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Operators and Operands(cont.)

• When a variable name appears in the place of an
  operand, it is replaced with its value before the
  operation is performed
• gtgtgt minute 59
• gtgtgt minute/60
• 0.983888

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Order of Operations

• When more than one operator appears in an
  expression, the order of evaluation depends on
  the rules of precedence.
• Python follows the same precedence rules for its
  mathematical operators that mathematics does.
• The acronym PEMDAS is a useful way to remember
  the order of operations

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Order of Operations

• PEMDAS
• Parentheses have the highest precedence
• 2 (3-1) is 4, and (11)(5-2) is 8
• Exponentiation has the next highest precedence,
• 211 is 3 and not 4, and 313 is 3 and not 27
• Multiplication and Division have the same
  precedence, which is higher than Addition and
  Subtraction
• 23-1yields 5 rather than 4, and 2/3-1 is -1, not
  1
• Operators with the same precedence are evaluated
  from left to right.
• 6100/60 yields 10

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Operations on Strings

• In general, you cannot perform mathematical
  operations on strings, even if the strings look
  like numbers
• The following are illegal
• message-1
• Hello/123
• messageHello
• 152
• operator work with strings. It does
  concatenation, means joining the two operands by
  linking them end-to-end
• fruit banana
• bakedGood nut bread
• print fruit bakedGood
• Output banana nut bread
• operator also works on strings it performs
  repetition.
• Fun3 is FunFunFun

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Warning!

• There are limits on where you can use certain
  expressions.
• For example, the left-hand side of an assignment
  statement has to be a variable name, not an
  expression.
• The following is illegal
• minute1 hour

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Comments

• Notes to your programs to explain in natural
  language what the program is doing, called
  comments, and they are marked with the symbol
• Everything from the to the end of the line is
  ignoredit has no effect on the program
• compute the percentage of the hour that has
  elapsed
• percentage (minute100)/60 cautioninteger
  division

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The Keyboard Input

• Python provides built-in functions that get input
  from the keyboard.
• The simplest is called input.
• the program stops and waits for the user to type
  something
• when the user presses Return or the Enter key,
  the program resumes and raw input returns what
  the user typed as a string
• gtgtgt name input ("What...is your name? ")
• What...is your name? F. Cemile Serce
• gtgtgt print (name)
• F. Cemile Serce
• gtgtgt age input (how old are you?)
• print (10 int(age))
• gtgtgt how old are you?30
• 40