CSCI 1101B Introduction to Python Mohammad T . Irfan New office - PDF document

2/12/2014 CSCI 1101B Introduction to Python Mohammad T . Irfan New office hours: M 10-12, Th 3-5 2/4/14 Basic Elements of Python 1

2/12/2014 Statement  print ‘Welcome to Python’  print “What’s your name?”  total = 0  x = [10, 20, 5]  for number in x: total = total + number  print total Function (example from lab) More on Parameters Function name functions later Header on ... def oddEven(): xStr = raw_input('Enter a number: ') x = int(xStr) if x%2 == 0: print x, 'is even' else: print x, 'is odd' Body – must use indentation 2

2/12/2014 Objects  Scalar – 4 types  int (example: -10000, 200, 53)  float (example: -37.59, 28.0)  bool (example: True, False)  None  Non-scalar  String (example: “hello”, “57”)  And many other ... (You will define and create your own non-scalar objects later on) Expressions  Arithmetic  x + y  x – y  x * y  x/y  Caution: when x and y are both int, then the result is also int (Python 2.7 truncates all decimal digits after the decimal point) – i t’s called “integer division”  Better practice: Use x//y when x and y are both int // operator specifically means integer division  x % y: remainder of the division x/y for int x & y  x ** y: x raised to the power y  comparison: == (equal), != (not eq), >, >=, <, <=  Precedence – usual 3

2/12/2014 Expressions  Logical operators (on bool )  a and b  a or b  not a Variables and assignment  Variable: named storage space def area(): pi = 3.14159 Note the indentation r = 10 area = pi * r**2 print area  You don’t need to specify type of a variable – it’s automatically determined  Naming a variable  Must start with a letter (upper or lower case) or _, may contain digits  Use sensible names. Cannot use reserved words. 4

2/12/2014 Conditional Statements if-else  An if block can be optionally associated with an else block def oddEven(): x = int(raw_input('Enter a number: ')) if x%2 == 0: Note the indentation print x, 'is even' else: print x, 'is odd' 5

2/12/2014 if-elif-else  if block , followed by any number of elif blocks , followed by an optional else block def showMinimum(x, y, z): if x < y and x < z: print x, 'is min' elif y < z: print y, 'is min' else: print z, 'is min‘  Note: parameter passing to a function  To “call” this function from JES, load it first and then enter the following command into the black window  showMinimum (20, 30, 5)  if statements can be nested Recap 6

2/12/2014 Scalar vs. non-scalar types 34,654.01 12.998 31,364 Floats 12 1.01 Integers 0.01 -12 Bowdoin College Mark 85 5 th Street NW Inside the computer, these are all just bits Strings Quiz  What would be the output of the following?  print 1/2 7

2/12/2014 Values & names with the same value are interchangeable! >>> print 12 * 3 >>> name = " Mark " 36 >>> print name >>> value = 12 Mark >>> print value >>> print name * 3 12 MarkMarkMark >>> print value * 3 >>> print " Mark " * 3 36 MarkMarkMark Non-scalar Type: String 8

2/12/2014 String  Examples  “Hello and welcome”  “Bowdoin College”  Assignment statement  college = “Bowdoin College” What is the difference between “57” and 57?  Main difference is representation  You can apply all arithmetic operators on 57  57 + 2  57 * 2  Arithmetic operations are meaningless for “57”  “57” + 2 # ERROR  “57” * 2 # ‘5757’ 9

2/12/2014 Operations on String  college = “Bowdoin College”  Length of a string  len(college) # 15  Indexing  college[2] # ‘w’  college[-1] # ‘e’  college[-2] # ‘g’  Slicing  college[0:2] # ‘Bo’  college[0:len(college)] # ‘Bowdoin College’  Splitting  college.split() # ['Bowdoin', 'College'] Taking a string as input  x = raw_input (‘Enter your name’)  y = raw input (‘How old are you?’)  Suppose user enters 20 as his/her age  y’s value is “20”, not 20  if you want to convert string “20” to number 20,  y = int(y) # Can use a different variable name on LHS 10

2/12/2014 Iterative Statements/ Loops for loop  General form (not actual code)  for loop_variable in sequence : body of for loop  Visualization of for loop  Use boxes to replace the for loop 11

2/12/2014 range function  sequence is commonly specified using the built-in range function with 3 parameters:  start (optional, default is 0)  end (must, actually ends before this value)  increment (optional, default is 1)  Examples  range(10, 70, 20) # [10, 30, 50]  range (0, 5, 1) # [0, 1, 2, 3, 4]  range (0, 5) # [0, 1, 2, 3, 4]  range (5) # [0, 1, 2, 3, 4] for loop example: Finding the average – 2 ways average.py Loop variable: Choose any name you want Caution: integer division! Better to write: avg = float(total)/len(x) 12