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Flow Control

Flow Control. Dr. Bernard Chen Ph.D. University of Central Arkansas July 9 th 2012 CS4HS@UCA. If Statement. The main statement used for selecting from alternative actions based on test results It’s the primary selection tool in Python and represents the Logic process.

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Flow Control

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  1. Flow Control Dr. Bernard Chen Ph.D. University of Central Arkansas July 9th 2012 CS4HS@UCA

  2. If Statement • The main statement used for selecting from alternative actions based on test results • It’s the primary selection tool in Python and represents the Logic process

  3. Some Logic Expression Equal: “==” NOT Equal: “!=” Greater: “>”, “>=” Less than: “<”, “<=”

  4. Outline • Simple If statement • If... else statement • If… else if … else statement

  5. Simple If statement • It takes the form of an if test • if <test>: <statement>

  6. Simple If statement >>>if 4>3: print ("it is true that 4>3") >>>if 1: print ("true") >>>a=10 >>>if a==10: print ("a=10")

  7. If... else statement • It takes the form of an if test, and ends with an optional else block • if <test>: <statement1> else: <statement2>

  8. If... else statement a=10 b=20 if a>b: print ("a is larger than b") else: print ("a is NOT larger than b")

  9. If... else statement a=10 b=20 if a>b: print ("a is larger than b") else: print (“b is larger than a")

  10. If… else if … else statement • It takes the form of an if test, followed by one or more optional elif tests, and ends with an optional else block if <test1>: <statement1> elif <test2>: <statement2> else: <statement3>

  11. If… else if … else statement a=15 if a>10: print ("a > 10") elif a==10: print ("a = 10") else: print ("a < 10")

  12. If… else if … else statement example number = 23 print("please guess a number (between 1~100)") guess = int(input()) if guess == number: print ('Congratulations, you guessed it.') # New block starts here elif guess < number: print ('No, it is a little higher than that') # Another block else: print ('No, it is a little lower than that') # you must have guess > number to reach here

  13. Some More Logic Expression • and >>> 5>4 and 8>7 True >>> 5>4 and 8>9 False

  14. Some More Logic Expression • or >>> 5>4 or 8>7 True >>> 5>4 or 8>9 True

  15. Lists • Lists are Python’s most flexibleordered collection object type • Lists can contain any sort of object: numbers, strings and even other lists

  16. Lists >>> aa=[] #An empty list >>> aa=[1,2,3,4] #4 items, index 0-3 >>> aa [1,2,3,4] >>> aa[0] 1 >>> aa[3] 4

  17. For loop • The for loop is a generic sequence iterator in Python • It can step through the items in ANYordered sequence object

  18. For loop • The Python for loop begins with a header line that specifies an assignment target along with an object that you want to step through for <target> in <object>: <statement> • When Python runs a for loop, it assigns item in the sequence object to the target “one by one”, and then executes the loop body

  19. For loop • The name used as the assignment target in a for header line is usually a variable in the scope where the for statement is coded • After the loop, this variable normally still refers to the last item visited

  20. For loop examples • The name x is assigned to each of the three items in the list in turn, from left to right >>> aa=[1,2,3,4] >>> for i in aa: print (i) 1 2 3 4

  21. For loop examples >>> aa=[1,2,3,4] >>> sum=0 >>> for i in aa: sum = sum + i >>> sum 10

  22. For loop examples >>> aa=[1,2,3,4] >>> product=1 >>> for i in aa: product = product * i >>> product 24

  23. For loop examples • The name x is assigned to each of the three items in the list in turn, from left to right >>> s=“string in python” >>> for i in s: print i

  24. Counter loops: range • The range function is really independent of for loops; although it’s used most often to generate indexes in a for loop >>> range(5) [0, 1, 2, 3, 4]

  25. Counter loops: range • With one argument, range generates a list with integers from zero up to but NOT including the argument’s value >>> range(10) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

  26. Counter loops: range • If you pass in two arguments, the first is taken as the lower bound >>> range(2,5) [2, 3, 4]

  27. range in for loop >>> sum=0 >>> for i in range(100): sum = sum + I >>> sum 4950 >>> product=1 >>> for i in range(5,10) product = product * 10 >>> product 15120

  28. Python Official Sites Python Tutorial • http://docs.python.org/tut/ Python Official Site • http://python.org/ Download Python Latest Version • http://python.org/download/

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