Writing Simple Programs

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Writing Simple Programs

• Chapter Two

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(No Transcript)
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convert.py A program to convert Celsius
temps to Fahrenheit by Susan Computewell def
main() celsius input("What is the Celsius
temperature? ") fahrenheit 9.0 / 5.0
celsius 32 print "The temperature is",
fahrenheit, "degrees Fahrenheit." main()
Note that indentation defines statements in main()
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Workshop

• Change the convert.py program so it converts
  dollars to Euros.
• Find the current rate online
• Work in pairs
• To launch the program, type gtgtgt import filename
• Make changes in your text editor, and save
• Then in the terminal window, type
  gtgtgtreload(filename)

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• Change the convert.py program so it converts
  dollars to Euros.
• Find the current rate online
• Work in pairs

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Names modules, functions, variables

• All names are called identifiers.
• Python rules all identifier smust begin with a
  letter or underscore (_)
• Identifiers are case sensitive
• Choose identifiers that describe what is being
  named
• Reserved words cannot be used as identifiers

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Reserved Words

• and assert break
• class continue def
• del elif else
• except exec finally
• for from global
• if import in
• is lambda not
• or pass print
• raise return try
• while yield

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Expressions

• Expressions are code fragments which manipulate
  data
• When you hard code data or characters into your
  code, you are creating literals
• Name Errors are produced when Python cant find a
  value associated with an identifer
• Spaces dont matter inside expressions.
• Arithmetic operators - / (the last
  one is exponentiation)

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Workshop

• Using the Python interpreter, create a variable
  called subtotal equal to 118.45
• Print subtotal
• Write an expression which adds a tax of 8 to
  subtotal
• Ask the interpreter to print a variable called
  item_cost

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Printing output

• The print command displays output on the screen
• Try these examples
• print by itself produces a blank line
• Separating values with a comma means you can
  print multiple values print 8, 9, 10
• You can print expressions print (8 8) / 2
• You can also print string literals (characters
  between quotations which the programmer hard
  codes) print The area of a circle is , 4 4
  3.14

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Simple assignment statements

• ltvariablegt ltexprgt
• Example euro dollar 1.39
• Assignment is ALWAYS right to left. In other
  words, the value on the right is loaded into
  the variable on the left.
• You can assign a new value to an existing
  variable. newVar 21 newVar 55

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Garbage Collection

• When you assign a new value to a variable, the
  old value is still in memory, just without
  reference.

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newVar
21
newVar
55
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Getting user input

• Python makes getting user input really easy. The
  reserved word input gets data from the user and
  loads it into a variable.
• dollars input(Please enter the dollar amount
  you want converted to euros )
• Python evaluates the prompt and displays it on
  screen. Python pauses and waits for user input.

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Simultaneous assignment

• Python allows you to simultaneously assign
  several variablesits a shortcut
• tax, subtotal, total .08, 0, 0
• Simultaneous assignment can use expressions as
  well as literalssale_price, total unit_price
  .75, subtotal .08

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avg2.py A simple program to average two
exam scores. Illustrates use of multiple
input. def main() print "This program
computes the average of two exam scores."
print score1, score2 input("Enter two
scores separated by a comma ") average
(score1 score2) / 2.0 print "The average
of the scores is", average main()
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Definite loops

• A definite loop just repeats a number of actions
  (or statements) a definite number of times.
  Python knows when to stop the loop (because you
  tell it so)
• This is also called iteration, and a definite
  loop is an iterative loop or iterative control
  structure
• For ltvargt in ltsequencegt ltexprgt ltexprgt

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Example of definite loop
Loop index changes value each time the body
statements are executed

• for i in range(10) x 3.9 x (1-x) print x
• range(10) is a built-in Python command which
  controls the number of times the loop is
  executed. It will start at 0 and quit after 9
  (always by integers)

Body
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Lists of values as sequencers

• The command range(10) is really a convenient
  shorthand for a list of values 0, 1, 2, 3,
  4, 5, 6, 7, 8, 9
• The definite loop changes the value of the loop
  index to match the progression of values in the
  sequence.
• For x in 7, 14, 21, 28 print x
• And the screen result will be7142128

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Workshop

• Have the Python Interpreter print the squares of
  the odd numbers between 0 and 20, using a
  definite loop

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futval.py A program to compute the value
of an investment carried 10 years into the
future def main() print "This program
calculates the future value of a 10-year
investment." principal input("Enter the
initial principal ") apr input("Enter the
annual interest rate ") for i in
range(10) principal principal (1
apr) print "The value in 10 years is",
principal main()
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Workshop

• Alter the futval.py program so that it displays
  the value of the investment each year
• Alter the futval.py program so that the user
  controls the number of years the investment will
  accrue