An Introduction To Python - FOR Loop

An Introduction To Software
Development Using Python
Spring Semester, 2015
Class #8:
FOR Loop

Programming Challenge:
Print Each Character Of A String
• The user will enter their name
• Your program will then step through the string
that they entered and print out each character
by itself on a line listing their name vertically.
Image Credit: megmedina.com

userName = input(“Enter your name: ”)
i = 0
while i < len(userName) :
letter = userName[i]
print(letter)
i = i + 1
Image Credit: thegraphicsfairy.com

The For Statement
The for loop can be used to iterate over the contents of any container, which is an
object that contains or stores a collection of elements. Thus, a string is a container
that stores the collection of characters in the string.

userName = input(“Enter your name: ”)
for letter in userName :
print(letter)
Image Credit: thegraphicsfairy.com

What’s The Difference Between
While Loops and For Loops?
• In the for loop, the element variable is
assigned stateName[0] , stateName[1] , and so
on.
• In the while loop, the index variable i is
assigned 0, 1, and so on.
Image Credit: www.clipartpanda.com

The Range Function
• Count-controlled loops that iterate over a range of integer values are very
common.
• To simplify the creation of such loops, Python provides the range function
for generating a sequence of integers that can be used with the for loop.
• The loop code:
for i in range(1, 10) : # i = 1, 2, 3, ..., 9
print(i)
prints the sequential values from 1 to 9. The range function generates a
sequence of values based on its arguments.
• The first argument of the range function is the first value in the sequence.
• Values are included in the sequence while they are less than the second
argument
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You Can Do The Same Thing With
While And For Loops
for i in range(1, 10) : # i = 1, 2, 3, ..., 9
print(i)
i = 1
while i < 10 :
print(i)
i = i + 1
Image Credit: www.rgbstock.com
Note that the ending value (the second argument to the range function) is not included
in the sequence, so the equivalent while loop stops before reaching that value, too.

Stepping Out…
• By default, the range function creates the
sequence in steps of 1. This can be changed
by including a step value as the third
argument to the function:
for i in range(1, 10, 2) : # i = 1, 3, 5, ..., 9
print(i)
Image Credit: megmedina.com

Loop Challenge #2!
• Create a program to print the sum of all odd
numbers between a and b (inclusive).
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For Loop With One Argument
• You can use the range function with a single
argument.
• When you do, the range of values starts at
zero.
for i in range(10) : # i = 0, 1, 2, ..., 9
print("Hello") # Prints Hello ten times
Image Credit: www.canstockphoto.com

Review: The Many Different Forms
Of A Range

Describe The Steps Necessary For
Finding A Solution To A 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?

Count Iterations
• Finding the correct lower and upper bounds for a loop can be
confusing.
– Should you start at 0 or at 1?
– Should you use <= b or < b as a termination condition?
• The following loops are executed b - a times:
• These asymmetric bounds are particularly
useful for traversing the characters in a string
int i = a
while i < b :
. . .
i = i + 1
for i in range(a, b) :
for i in range(0, len(str)) :
do something with i and str[i]
Image Credit: imgarcade.com

The +1 Problem
• How many times is the loop with symmetric bounds
executed?
int i = a
while i <= b :
. . .
i = i + 1
• b - a + 1 times. That “+1” is the source of many programming
errors.
• When a is 10 and b is 20, how many times does the loop
execute?
• In a Python for loop, the “+1” can be quite noticeable:
for year in range(1, numYears + 1) :
Image Credit: www.clipartbest.com

Loop Challenge #3!
• Read twelve temperature values (one for
each month), and display the number of the
month with the highest temperature. For
example the average maximum temperatures
for Death Valley are (in order by month, in
degrees Celsius):
18.2, 22.6, 26.4, 31.1, 36.6, 42.2, 45.7, 44.5, 40.2, 33.1, 24.2, 17.6

Secret Form Of print Statement
• Python provides a special form of the print function that
prevents it from starting a new line after its arguments are
displayed.
print(value1, value2, end="")
• By including end="" as the last argument to the first print
function, we indicate that an empty string is to be printed
after the last argument is printed instead of starting a new
line.
• The output of the next print function starts on the same line
where the previous one left off.

Loop Challenge #4!
• Create a program to print the following table
that contains the first four power values for
the numbers 1-10.
Image Credit: www.rafainspirationhomedecor.com

Counting Matches In Strings
• Count the number of uppercase letters in a
string:
uppercase = 0
for char in string :
if char.isupper() :
uppercase = uppercase + 1
• Count the number of occurrences of multiple
characters within a string.
vowels = 0
for char in word :
if char.lower() in "aeiou" :
vowels = vowels + 1

Finding All Matches In A String
• When you need to examine every character within a string,
independent of its position, you can use the for statement to
iterate over the individual characters.
• For example, suppose you are asked to print the position of
each uppercase letter in a sentence.
sentence = input("Enter a sentence: ")
for i in range(len(sentence)) :
if sentence[i].isupper() :
print(i)

Finding the First or Last Match
In A String
• If your task is to find a match, then you can
stop as soon as the condition is fulfilled.
found = False
position = 0
while not found and position < len(string) :
if string[position].isdigit() :
found = True
else :
position = position + 1
if found :
print("First digit occurs at position", position)
else :
print("The string does not contain a digit.")

What’s In Your Python Toolbox?
print() math strings I/O IF/Else elif While For

What We Covered Today
1. For Loop
2. Range
3. Using loops to process
strings
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What We’ll Be Covering Next Time
1. Lists, Part 1
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An Introduction To Python - FOR Loop

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