1. Loops Upsorn Praphamontripong CS 1110 Introduction to Programming
Spring 2017

2. Walking and Shaking – if
Given a list of Python experts in this room Upsorn-bot wants to shake hands the Python experts who wear glasses
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3. “Repeated” Process – if
Walking and shaking
Given a list of Python experts in this room
Write code that
print “shake” if an expert wears glasses and
print “no” if an expert does not wear glasses
if expert_1 == "wear":
print("shake")
else:
print("no")
if expert_2 == "wear":
...
if expert_n == "wear":
100 experts = 100 if-else blocks ??
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4. Looping (in action)
Given a list of Python experts in this room Upsorn-bot wants to shake hands the Python experts who wear glasses 1 2 3 4
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5. “Repeated” Process – while loop
Walking and shaking
Given a list of Python experts in this room
Write code that
print “shake” if an expert wears glasses and
print “no” if an expert does not wear glasses
while
# there are more experts
# if an expert wears glasses
# print "shake"
# otherwise
# print "no"
ctr = 0
while ctr < len(experts):
expert = experts[ctr]
if expert == "wear":
print("shake")
else:
print("no")
ctr += 1
experts = ["wear", "no", "no", "wear", "wear", "no"]
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6. Common mistake: infinite loop
experts = ["wear", "no", "no", "wear", "wear", "no"]
ctr = 0
while ctr < len(experts):
if experts[ctr] == "wear":
print("shake")
else:
print("no")
# ctr = ctr + 1
shake
shake
shake
shake
shake
. . .
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7. Common mistake: index out of range
experts = ["wear", "no", "no", "wear", "wear", "no"]
ctr = 0
while ctr >= 0:
if experts[ctr] == "wear":
print("shake")
else:
print("no")
ctr = ctr + 1
shake no no
shake
shake no
Index out of range
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8. Looping What is looping? Why do we do looping?
The process of doing something in the same order again and again
Why do we do looping?
To automated some repeated processes
To shorten the code, less time/effort, less chance of errors
To make code more readable and maintainable
To save memory as few instances are created
When should we do looping?
How do we write loops?
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9. Loops in Python while loop – a condition-controlled loop
for loop using the range operator – a count-controlled loop
for loop using the in operator with a list
Nested loops
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10. While Loop
while condition statements handler to break the condition such that the loop terminates
Condition (Boolean expression)
index = 1 while index <= 10: print(index) index += 1
index = 1 while True:
if index > 10:
break
print(index) index += 1
Bad condition
indented
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11. While Loop (2)
while input("Do you want to continue (Y/N) ? ") == "Y": print("let's continue")
done = "" while done != "quit": print("let's continue -- not done yet") done = input("Continue ? (type quit to quit) ")
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12. While Loop (3) Variable not found !! Infinite loop !!1 2 3 4 5 6 7 8 …
cnt = 1 while infinite > -1: print(cnt) cnt += 1
Variable not found !!
Infinite loop !!
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13. For Loop (using in operator)
for iterate_var in list statements
animals = ['dog', 'cat', 'fish'] for animal in animals: print('Current animal :', animal)
for letter in 'Python': print('Current Letter :', letter)
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14. Example: for Loop (using in operator)
Walking and shaking
Given a list of Python experts in this room
Write code that
print “shake” if an expert wears glasses and
print “no” if an expert does not wear glasses
for
# expert in list of experts
# if an expert wears glasses
# print "shake"
# otherwise
# print "no"
for expert in experts: if expert == "wear": print("shake") else: print("no")
experts = ["wear", "no", "no", "wear", "wear", "no"]
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15. For Loop (using range operator)
for iterate_var in range(start, stop, step) statements
default = 1
Iterate upto this but not include
for num in range(1, 100, 5): print(num)
for num in range(1, 100): print(num)
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16. Example: for Loop (using range operator)
Imagine the user enters a string and a substring. Remove the substring from the string without using any built-in functions/methods besides len() (i.e., you may not use .index()). The substring will be at most one character long
string1 = "for loop, using range" string2 = "o"
result = "" size1 = len(string1) position = 0 for position in range(0, size1): if string1[position] != string2: result = result + string1[position] position += 1 print("result = " + result)
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17. While Loop vs. For Loop While loop For loop
Use when you don’t know the number of iterations
Need to explicitly handle the termination
Use when you know the number of iterations
Loop stops as it reaches the end of range, or the end of list
i = 5 while i > 0: print(i * i * i) i -= 1
i = 5 for i in range(5, 0, -1): print(i * i * i)
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18. Exercise (while loop) Write a program using a while loop
Your program will takes the number of courses a user enrolled
Your program will take a list of 3 exam grades from a user (for each course), separated by spaces (i.e., each grade is separated by a single space)
Calculate the average of the grades for each course, and the letter grade (according to our letter grade system found in the syllabus). The scores are not weighted in any way - just do a straight up average.
Print to the screen the average along with the letter grade of each course
A run of the program might look like this:
Input the number of courses: 2
Input your scores, separated by spaces: Average: , B-
Input your scores, separated by spaces: Average: , A-
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19. Exercise (for loop) Write a program using a for loop
Your program will takes the number of courses a user enrolled
Your program will take a list of 3 exam grades from a user (for each course), separated by spaces (i.e., each grade is separated by a single space)
Calculate the average of the grades for each course, and the letter grade (according to our letter grade system found in the syllabus). The scores are not weighted in any way - just do a straight up average.
Print to the screen the average along with the letter grade of each course
A run of the program might look like this:
Input the number of courses: 2
Input your scores, separated by spaces: Average: , B-
Input your scores, separated by spaces: Average: , A-
CS 1110