An Introduction To Python - WHILE Loop
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An Introduction To Software Development Using Python Spring Semester, 2015 Class #7: WHILE Loop
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Transcript of An Introduction To Python - WHILE Loop
An Introduction To Software
Development Using Python
Spring Semester, 2015
Class #7:
WHILE Loop
How Do You Do Things Over And Over Again?
• In Python, loop statements repeatedly execute instructions until a goal has been reached.
• In Python, the while statement implements such a repetition. It has the form:
while condition : statement1statement 2
• As long as the condition remains true, the statements inside the while statement are executed.
Image Credit: etc.usf.edu
Body
Condition
Remember Mr. Bank Account Interest Rate Problem?
Problem:
You put $10,000 into a bank account that earns 5 percent interest per year.
How many years does it take for the account balance to be double the original?
(over and over & over again)
Some people call this loop an event-controlled loop.
How Can I Loop For A Given Number Of Times?
• You can use a while loop that is controlled by a counter:
counter = 1 # Initialize the counter.while counter <= 10 : # Check the counter.
print(counter)counter = counter + 1 # Update the loop variable
Note: Some people call this loop count-controlled.
Image Credit: www.dreamstime.com
Loop Challenge!
• Create a program to print the sum of all odd numbers between a and b (inclusive).
Image Credit: www.dreamstime.com
Infinite Loops
• Def: a loop that runs forever and can be stopped only by killing the program or restarting the compute
year = 1while year <= 20 :
interest = balance * RATE / 100balance = balance + interest
Image Credit: www.canstockphoto.com
The Loop Problem:“Off By-One Errors”
year = 0while balance < TARGET :
year = year + 1interest = balance * RATE / 100balance = balance + interest
print("The investment doubled after", year, "years.")
• Should year start at 0 or at 1?
• Should you test for balance < TARGET or for balance <= TARGET?
• It is easy to be off by one in these expressions.
• Think through simple test cases to avoid this type of error.
Image Credit: www.wisegeek.org
Sentinel Values
• One common programming task isto read and process a sequence of input values.
• You need to have some method of indicating the end of the sequence.
– 0?
– -1?
• Such a value, which is not an actual input,but serves as a signal for termination, is called a sentinel
Sentinel Value Example:Average Salary
• Create a program that computes the average of a set of salary values.
• In our sample program, we will use any negative value as the sentinel.
• Any negative number can end the loop, but we prompt for a sentinel of –1 so that theuser need not ponder which negative number to enter.
Image Credit: www.clipartpanda.com
What Do We Use Loops For?
• Sum and Average Value
• Counting Matches
– How many values fulfill a particular condition.
• Prompting Until a Match is Found
• Maximum and Minimum
• Comparing Adjacent Values
Image Credit: www.clipartpanda.com
Remember The Python ATM Machine?
Image Credit: www.canstockphoto.com
Create software that will provide an ATM user
with the proper change for any dollar amount
up to $200.
Example: Run the code for $200 and for $19
What’s In Your Python Toolbox?
print() math strings I/O IF/Else elif While
What We Covered Today
1. While Loop
2. Infinite Loops
3. Sentinel Values
Image Credit: http://www.tswdj.com/blog/2011/05/17/the-grooms-checklist/
What We’ll Be Covering Next Time
1. For Loop
Image Credit: http://merchantblog.thefind.com/2011/01/merchant-newsletter/resolve-to-take-advantage-of-these-5-e-commerce-trends/attachment/crystal-ball-fullsize/
