Counted loop: the number of iterations is known at start of loop
for syntax is often appropriate
called an indexed loop when the counting variable is used for indexing
Sentinel loop: the loop terminates when a special input value (the
sentinel) is encountered
Common loop patterns (continued)
Post-test loop: loop termination test is made somewhere in the body of the loop
common with sentinel loops
Accumulator loop: in the loop information is collected in one or more variables
(accumulators) for use after the loop
Nested loop: the loop is directly contained in the body of another loop
Example of while loop and equivalent for loop
def extend_list(lst1, lst2):
index = 0
while index < len(lst2):
lst1.append(lst2[index])
index = index + 1
def extend_list(lst1, lst2):
for index in range(len(lst2)):
lst1.append(lst2[index])
For loop syntax and semantics
Statement syntax:
forvariable_nameinrange(expression):
block
Semantics: expression is evaluated, and should yield a non-negative
integer, stop. The variable is assigned in tern the values 0, 1, 2, ..., stop-1,
with the block evaluated once after each assignment.
For loop translation to a while loop
A for loop of the form
forvariableinrange(expression):
block
is equivalent to the while loop (provided index is not used after the
loop and stop is not used anywhere else):
variable=0
stop=expression
whilevariable<stop:
block
index=index+1
Using simplified for loops
Such for loops can only be used in place of while loops that
exactly fits this pattern
You are responsible for knowing that such for loops behave this way
(reading knowledge), but in this course you can always use while loops
when writing programs
Python allows less restricted use of for loops, but such use is confusing
at first and not used in this course
Common mistake: forgetting to call len in the range expression
Exercise: for_sum
Replace the following two-line for loop with four lines that include a
while loop
def for_sum(lst):
answer = 0
for index in range(len(lst)):
answer = answer + lst[index]
return answer
Answer: while_sum
def while_sum(lst):
answer = 0
index = 0
while index < len(lst):
answer = answer + lst[index]
index = index + 1
return answer
This is the same function we saw in class last week
After the following definition, what does max_value1([1,20,8]) do?
def max_value1(list_of_numbers)
index = 0
while index < len(list_of_numbers)
value = list_of_numbers(index)
if value < answer
answer = value
return answer
Syntax error
Runtime error message
Returns 20
Returns 1
None of the above
Answer: A
After the following definition, what does max_value2([1,20,8]) do?
def max_value2(list_of_numbers): # colon added
index = 0
while index < len(list_of_numbers): # colon added
value = list_of_numbers(index)
if value < answer: # colon added
answer = value
return answer
Syntax error
Runtime error message
Returns 20
Returns 1
None of the above
Answer: B (fourth line)
After the following definition, what does max_value3([1,20,8]) do?
def max_value3(list_of_numbers):
index = 0
while index < len(list_of_numbers):
value = list_of_numbers[index] # [...] not (...)
if value < answer:
answer = value
return answer
Syntax error
Runtime error message
Returns 20
Returns 1
None of the above
Answer: B (fifth line: undefined variable)
After the following definition, what does max_value4([1,20,8]) do?
def max_value4(list_of_numbers):
answer = list_of_numbers[0] # new line
index = 0
while index < len(list_of_numbers):
value = list_of_numbers[index]
if value < answer:
answer = value
return answer
Syntax error
Runtime error message
Returns 20
Returns 1
None of the above
Answer: D
After the following definition, what does max_value5([1,20,8]) do?
def max_value5(list_of_numbers):
answer = list_of_numbers[0]
index = 0
while index < len(list_of_numbers):
value = list_of_numbers[index]
if value < answer:
answer = value
return answer # changed indentation
Syntax error
Runtime error
Returns 20
Returns 1
None of the above
Answer: E (infinite loop)
After the following definition, what does max_value6([1,20,8]) do?
def max_value6(list_of_numbers):
answer = list_of_numbers[0]
index = 0
while index < len(list_of_numbers):
value = list_of_numbers[index]
if value < answer:
answer = value
index = index + 1 # new line
return answer
Syntax error
Runtime error
Returns 20
Returns 1
None of the above
Answer: D
After the following definition, what does max_value7([1,20,8]) do?
def max_value7(list_of_numbers):
answer = list_of_numbers[0]
index = 0
while index < len(list_of_numbers):
value = list_of_numbers[index]
if value > answer: # changed < to >
answer = value
index = index + 1
return answer
Syntax error
Runtime error
Returns 20
Returns 1
None of the above
Answer: C (it works!)
Warning: it is very dangerous to add or delete elements from a list while you are iterating over it!
Consider this attempt to delete every other element from a list:
def bad_remove_blanks(los):
"""Remove from the given list of strings all elements containing
blank (whitespace-only) strings.
>>> los = ['spam', ' ', '\n', 'eggs']
>>> bad_remove_blanks(los)
>>> los # right answer is ['spam', 'eggs']
['spam', '\n', 'eggs']
>>>
"""
index = 0
while index < len(los):
if los[index].strip() == '':
del los[index]
index = index + 1
remove_blank done right
def remove_blanks(los):
"""Remove from the given list of strings all elements containing
blank (whitespace-only) strings.
>>> los = ['spam', ' ', '\n', 'eggs']
>>> remove_blanks(los)
>>> los
['spam', 'eggs']
>>>
"""
index = len(los) - 1
while index >= 0:
if los[index].strip() == '':
del los[index]
index = index - 1
Exercise: print_list
def print_list(lst):
"""Prints the elements of lst, one per line.
>>> print_list([1, 2])
1
2
>>>
"""
index = 0
#...
#...
index = index + 1
Solution: print_list
def print_list(lst):
"""Prints the elements of lst, one per line.
>>> print_list([1, 2])
1
2
>>>
"""
index = 0
while index < len(lst):
print lst[index]
index = index + 1
Exercise: multiply_list
def multiply_list(lst):
"""Takes a list of numbers and returns their product.
>>> multiply_list([1, 2, 3])
6
>>>
"""
# Hint; accumulating index loop pattern
#...
index = 0
while index < len(lst):
#...
index = index + 1
#...
Solution: multiply_list
def multiply_list(lst):
"""Takes a list of numbers and returns their product.
>>> multiply_list([1, 2, 3])
6
>>>
"""
# Hint; accumulating index loop pattern
answer = 1
index = 0
while index < len(lst):
answer = answer * lst[index]
index = index + 1
return answer
Exercise: concatenate_list
def concatenate_list(lst):
"""Takes a list of strings and returns their concatenation
>>> concatenate_list(['spam', ' ', 'eggs'])
'spam eggs'
>>>
"""
#...
index = 0
while index < len(lst):
#...
index = index + 1
#...
Solution: concatenate_list
def concatenate_list(lst):
"""Takes a list of strings and returns their concatenation
>>> concatenate_list(['spam', ' ', 'eggs'])
'spam eggs'
>>>
"""
answer = ''
index = 0
while index < len(lst):
answer = answer + lst[index]
index = index + 1
return answer
Exercise: concatenate_not_blank
def concatenate_not_blank(lst):
"""Takes a list of strings and returns the concatenation
of the non-whitespace elements.
>>> concatenate_not_blank(['spam', ' ', 'eggs'])
'spameggs'
>>>
"""
# Hint: a string is blank if the string strip() method
# returns the empty string.
answer = ''
index = 0
while index < len(lst):
#...
#...
index = index + 1
return answer
Solution: concatenate_not_blank
def concatenate_not_blank(lst):
"""Takes a list of strings and returns the concatenation
of the non-whitespace elements.
>>> concatenate_not_blank(['spam', ' ', 'eggs'])
'spameggs'
>>>
"""
# Hint: a string is blank if the string strip() method
# returns the empty string.
answer = ''
index = 0
while index < len(lst):
if lst[index].strip() != '':
answer = answer + lst[index]
index = index + 1
return answer
After the following definition, what does max_value1([1, 20, 8]) do?