xiaojuan cai computational thinking 1 lecture 7 decision structure xiaojuan cai （蔡小娟）...

Xiaojuan Cai Computational Thinking 1

Lecture 7Decision Structure

Xiaojuan Cai（蔡小娟）[email protected]

Fall, 2015

mailto:[email protected]


Objective• To understand the decision structures:

• simple decision: if statement• two-way decision: if-else statement• multi-way decision: if-elif-else statement

• To understand the idea of exceptions• To know the bool data type• Algorithm design: decision structure


Roadmap• Decision structures

• if statement• if-else statement• if-elif-else statement

• Exception handling• Algorithm design: Max of three


The control flow graph• So far, programs are executed as sequences. • Control structures allow us to alter this

sequential program flow.• Decision structures are statements that allow

a program to execute different sequences of instructions for different cases.


Example: temperature warning• Susan在温度转换程序中希望加入一个警告，当温度超过90华氏度或低于30华氏度的时候发出警告，避免外出。

• If fahrenheit > 90 print a heat warning If fahrenheit < 30 print a cold warning

• Convert2.py


The flow chart


if-statement• Syntax

• if <condition>:

<body>

• Semantics• First, the condition in the heading is evaluated.• If the condition is true, the body is executed, and then the

next statement.• If the condition is false, the body are skipped.


Conditions• <expr> <relop> <expr>

• <relop> is short for relational operatorPython Mathemati

csMeaning

< < Less than<= ≤ Less than or equal to== = Equal to>= ≥ Greater than or equal

to> > Greater than!= ≠ Not equal to


Boolean expressions• Notice the use of == for equality.• A common mistake is using = in conditions!• The ordering of string is lexigraphic: (“Bbbb” < “aaaa”)

• Conditions are of type bool with values True and False, named for the English mathematician George Boole.


Example: Conditional program execution• Some modules are designed to be run directly

(programs or scripts).• Others are made to be imported (libraries).• We want to create a hybrid: a stand-alone program

and as a library.• When the program is imported: __name__ = <modulename>, otherwise __name__ = ‘__main__’


Example: Conditional program execution• If a module is imported, __name__ is the name

of the module.• When a file is run directly, __name__ is the

value ‘__main__’.• We can change the final lines of our programs

to:if __name__ == '__main__': main()

• Virtually every Python module ends this way!


Two-way decisions• Review the quadratic program: the case of b2-4ac<0 .

• Previously, just crash – ValueError• Use simple decision, nothing happened• Another if? Could be, but there is

better one: if-else statement.


• if <condition>:

<statements>

else:

<statements>

• Semantics• First, the condition in the heading is evaluated.• If the condition is true, the body under if is executed, and then

pass the control to the next statement.• If the condition is false, the body under else is executed, and

then pass the control to next statement.

if-else statement


Multi-way decision• Review the quadratic program: the case of b2-4ac<0 .

• if b2-4ac=0, we want to print only one root.• Three cases:

• b2-4ac<0

• b2-4ac=0

• b2-4ac>0


if-elif-else statement• Multi-way decision can be implemented by

using nested if-else. However, if-elif-else is a better choice.

• if <condition1>: <case1 statements>elif <condition2>: <case2 statements>elif <condition3>: <case3 statements>…[else: <default statements>]

else part is optional.


Exception handling• Decision structures are used to protect against rare

but possible errors.• Sometimes the algorithm becomes hard to follow

due to so many checks.• Exception: a mechanism to solve this design

problem.• Informally, “Do these steps, and if any problem crops

up, handle it this way.”


Exception handling• The try-except statement try:

<body>except <ErrorType1>[,<id1>]:

<handler1>except <ErrorType2>[,<id2>]:

<handler2>…[except:

<handler3>]

• The semantics: If an error occurs in the body, looks for a matching error type. If one is found, then [assign the exception to <id>, and] the handler code is executed.

except part is optional.Without this statement, the program will crash if none is match.


Exceptions• ValueError: e.g., performing sqrt to a negative

number• NameError: the number of params are wrong• TypeError: operator and operands not match• SyntaxError: • ZeroDivisionError: divided by zero• …


Study in design: Max3• Problem: max of three numbers

• Input: three numbers• Output: the maximum one

• def main():

x1, x2, x3 = input("Please enter three values: ")

# sets max to the value of the largest

print "The largest value is", max


Algo 1: three-way decision•if x1 >= x2 >= x3:

max = x1

• Python does allow this statement. (most languges do not.)

• Bad design, you need 3! cases.


Algo 1: three-way decision• if x1 >= x2 and x1 >= x3:

max = x1

elif x2 >= x1 and x2 >= x3:

max = x2

elif x3 >= x1 and x3 >= x2:

max = x3

• Better, perform 3*(3-1) compares in worst case.• Can we do better?


Algo 2: decision tree

• Worst case: 3-1 comparisons• Optimal solution, but difficult to implement


Algo 2: decision tree • if x1 >= x2:

if x1 >= x3:

max = x1

else:

max = x3

else:

if x2 >= x3:

max = x2

else

max = x3


Algo 3: sequencial• How would you solve this problem?• Sequential processing

• Initially max = x1• Always compare max with xi, and update max• Return max

• Worst case: 3-1 comparisons, optimal


Algo 3: sequencial• Easy to implement and easy to expand to n-

numbers:max = x1

if x2 > max:

max = x2

if x3 > max:

max = x3


Alternative: built-in function• Python has a built-in function called max that returns the largest of its parameters.

def main():

x1, x2, x3 = input("Please enter three

values: ")

print "The largest value is", max(x1, x2, x3)


Lessons• There’s usually more than one way.

• Find a correct algorithm. Then strive for better one: clarity, simplicity, efficiency, scalability, and elegance.

• Generality is good.• If the max of n is as easy to write as the max of three,

write the more general program.

• Don’t reinvent the wheel.• find out if there’s already a solution!


Conclusion• Decision structures allow a program to execute

different sequences.

• if, if-else, if-elif-else statements

• Boolean expressions: <, <=, >, >=, ==, !=

• Bool type: True, False

• Exception handling makes programs more “bulletproof”.

• Algorithm design: correct, efficient, and understandable


Homework• Textbook: Chapter 3

• [Zelle]: • True/False • Multiple choice

xiaojuan cai computational thinking 1 lecture 7 decision structure xiaojuan cai （蔡小娟）...

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