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CS A131: Lecture 5

Nadia Ahmed

Orange Coast College, Computer Science

CS1A

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 1/29 1/29

Overview

• Counters ◦ Augmented Assignment Operators ◦ Increment and Decrement Operators

• Repetition Statements ◦ while loop ◦ for loop

• Counter-controlled repetition ◦ Example average.py

• Sentinel-controlled repetition ◦ Example average2.py

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 2/29 2/29

Overview

• Think! • Structured Programming

◦ Control flow charts ◦ Sequential statements ◦ Conditional statements

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

◦ Repetition statements • while loop • for loop

◦ Example interest.py

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 3/29 3/29

Augmented Assignment Operators

• Assignment operator: = ◦ evaluates right-hand side ◦ assigns result to left-hand side

• Augmented assignment operators: +=, *=, ... ◦ evaluates right-hand side as temporary result ◦ applies operation to left-hand side and temporary result ◦ assigns result of operation to left-hand side

• Example: Counter ◦ c = 0 # counter starting from 0 ◦ c = c + 1 # counting by regular assignment ◦ c += 1 # counting by augmented assignment

• Augmented assignment operators: ◦ +=, -=, *=, /=, %=, <<=, >>=

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 4/29 4/29

Repetition Statement

• Repetition ◦ repeated execution of a block of statements ◦ counter-controlled

• counter determines number of repetitions (often predefined at compile time)

◦ sentinel-controlled • sentinel condition determines number of repetitions (usually

determined at run-time)

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 5/29 5/29

Control flow chart

while cond:

code block

False

True

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 6/29 6/29

Repetition Statement while loop • Control flow statement for repetition (iteration)

◦ Repeats execution depending on specified condition ◦ All statements in the while block must be consistently indented

for the interpreter to tell where the block begins and ends. • Example

product = 2 while product < 1000:

product *=2 # elihw print( "Product␣is␣%d" % product)

• Syntax while construct consists of ◦ keyword while ◦ condition: expression evaluated to True or False ◦ body statement block

• Semantics: the body is repeatedly executed as long as the condition evaluates to true ◦ the condition is evaluated at the beginning of each loop

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 7/29 7/29

Counter Controlled Example: average.py # ######################################################## # average . py : compute the average of a set of numbers # author : Nadia Ahmed # modifications : # 07/12/2016 NA initial version # ######################################################## def main():

# input and computation section counter = 1; total = 0.0; # while loop while counter <= 10:

value = int(input("Please␣enter␣a␣value:␣")) total += value counter += 1

# elihw # computation section average = total/ 10.0 # output section print("The␣average␣is:␣%f" % average)

# EOF # function call main()

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 8/29 8/29

Sentinel Controlled Example: average2.py

/# ############################################################ # average2 . py : compute the average of a set of numbers # author : Nadia Ahmed # modifications : # 07/12/2016 NA sentinel controlled loop # 07/12/2016 NA initial version # #############################################################/ def main():

# variable initialization counter = 0 total = 0.0

# initialize the loop control variable value value = float(input("Please␣enter␣a␣value␣(or␣-1␣to␣quit):␣")) ...

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 9/29 9/29

Sentinel Controlled Example: average2.py

... while value != -1.0: # check the loop control variable value

total += value counter += 1 # update the loop control variable value value = float(input("Please␣enter␣a␣value␣(or␣-1␣to␣quit):␣"))

# elihw print("%d␣value␣entered." % counter) if counter >= 1:

average = total/counter print("The␣average␣is␣%f" % average)

# fi # EOF # function call main()

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 10/29 10/29

Programming == Thinking

• Programming ◦ not a mechanical procedure ◦ requires thinking

• Program ◦ writing requires human intelligence ◦ execution can be done by a dumb machine.

• General programming steps 1. Understand the problem 2. Define the input and output data 3. Develop the algorithm (e.g. use pseudo code) 4. Define the control flow (e.g. use control flow charts) 5. Write the program in programming language 6. Test and debug the program

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 11/29 11/29

Control Flow Charts Graphical representation of program control flow

• Sequential Execution

• Selection

• Termination

Start

Input

Compute

while cond:

Compute

Output

Finish

False

True

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 12/29 12/29

Structured Programming

Empty statement blocks • empty compound statement • does nothing (no operation, no-op) • Example

# nothing do nothing

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 13/29 13/29

Structured Programming

Sequential Execution in Python • Statement blocks: Compound statements

# statement 1

# statement 2

# statement 3

...

# statement n

Statement 1

Statement 2

Statement 3

Statement n

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 14/29 14/29

Selection: if statement

• Flow chart:

if expression:

process true

True

False

• Example: if grade >= 60:

print("You␣passed.") # fi

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 15/29 15/29

Selection: if-else statement

• Flow chart:

if expression:

process False

false

process True

true

• Example: if grade >= 60:

print("You␣passed.") # fi else:

print("You␣failed.") # esle

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 16/29 16/29

Multiple Selections: if else-if statement • Flow chart:

if expr_1: process block_1

elif expr_2: process block_2

elif expr_n: process block_n

else: default_block

True

False

True

False

• Example: if grade >= 90:

print("Excellent!") # fi elif grade >= 80:

print("Satisfactory.") . . .

# file elif grade >=60:

print("You␣passed.") # file else:

print("Failed.") # esle

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 17/29 17/29

Python Repetition: while loop

• Flow Chart:

while cond:

code block

False

True

• Example: product = 2; while product < 1000:

product *= 2; # elihw

• Note: The condition is evaluated at the beginning of each loop! while loop is a pretest loop!

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 18/29 18/29

Python Repetition: for loop

• Flow Chart:

initialization

condition:

code block

increment

False

True

• Example: for i in [0,1,2,3,4,5,6,7,8,9]:

print("i␣=␣%d␣" % i) # rof

• Syntax: for variable in [val1, val2,...]:

# statement # statement # etc.

• Note: The condition is evaluated at the beginning of each loop! for is a pretest loop!

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 19/29 19/29

Python Repetition: for loop using range()

• Flow Chart:

initialization

condition:

code block

increment

False

True

• Example: # range counts from 0 to one # less than the input argument for i in range(10):

print("i␣=␣%d␣" % i) # rof

• Syntax: for variable in range(bound):

# statement # statement # etc.

• Note: The condition is evaluated at the beginning of each loop! for is a pretest loop!

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 20/29 20/29

Choosing the Best Loop based on Application

• If you can determine in advance the number of repetitions use the for loop or the counter controlled while loop

• If you do not know and cannot determine the number of repetitions needed, and it could be zero use a while loop

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 21/29 21/29

Example Program Compound interest: interest.py

• Write a program that calculates the interest accumulated in a savings account. Given an initial deposit amount an an annual percentage rate (APR), compute the yearly interest earned and the resulting balance, for a period of ten years.

• the output should be listed in the table as follows: Interest for year 1 is $ 45.00, total balance is $ 1045.00. Interest for year 2 is $ 47.02, total balance is $ 1092.03. Interest for year 3 is $ 49.14, total balance is $ 1141.17. ...

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 22/29 22/29

Example Program • Compound interest: interest.py • Assignment

◦ Write a program that calculates the interest accumulated in a savings account. Given an initial deposit amount and an annual percentage rate (APR), compute the yearly interest earned and the resulting balance, for a period of ten years.

• Step 1: Understand the problem ◦ What is given?

• deposit amount • annual percentage rate

◦ What is asked for? • yearly interest • resulting balance

◦ How do we compute what is asked for? • interest = amount * APR/100 • balance = amount + interest

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 23/29 23/29

Example Program

• Compound interest: interest.py • Assigment

• Step 2: Define the input and output data ◦ Input

• Initial deposit amount: floating point value, amount • Annual percentage rate: floating point value, rate

◦ Output • Current year: integral value, year • Interest earned: floating point value, interest • Resulting balance: floating point value, balance

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 24/29 24/29

Example Program

• Compound interest: interest.py • Assignment

• Step 3: Develop the algorithm ◦ First, input amount and rate ◦ Next, compute interest on the amount for the year ◦ Next, compute new balance at the end of the year ◦ Then, print year, interest and balance in tabular format ◦ Finally, set the amount to the new balance ◦ Repeat the previous 4 steps for 10 years ◦ Done!

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 25/29 25/29

Example Program

• Compound interest: interest.py • Assignment

• Step 4: Write the program in programming language ... amount = float(input("Please␣enter␣the␣initial␣amount␣in␣$:␣")) rate = float(input("Please␣enter␣the␣interest␣rate␣in␣%␣:␣")) ...

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 26/29 26/29

Example Program: interest.py part (1/2)

/# ############################################################ # interest . py : compound interest on savings account # author : Nadia Ahmed # modifications : # 07/19/2016 NA distinguish amount and balance # 07/20/2016 NA initial version # #############################################################/

def main(): # input section amount = float(input("Please␣enter␣the␣initial␣amount␣in␣$:␣")) rate = float(input("Please␣enter␣the␣interest␣rate␣in␣%:␣")) ...

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 27/29 27/29

Example Program: interest.py part (2/2)

... # computation and output section for year in range(1,11):

interest = amount * (rate/100.0); balance = amount + interest; print("Interest␣for␣year␣%d␣is␣$␣%.2f,␣total␣balance␣is␣$␣%.2f

." % (year, interest, balance)) amount = balance;

# rof main()

Nadia Ahmed nahmed@occ.cccd.edu Python Lecture 5 28/29 28/29

Example Program

• Compound interest: interest.py • Assignment

• Step 6: Test (and debug) the program

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