Additional Loop Considerations

Additional Loop Considerations#

A few other pieces of Python syntax we want to be aware of as we work with loops:

range()#

Python’s range() function allows us to generate a list of integer values. The general syntax:

range(START VALUE, END VALUE, STEP INTERVAL)

The default start value for range() is 0, and the default step interval is 1.

So for example, range(6) would include the values [0, 1, 2, 3, 4, 5]. While range(1, 6, 2) would include the values [1, 3, 5].

We can use range() in combination with list() to generate a list of numbers.

  • list(range(6)) would generate the list [0, 1, 2, 3, 4, 5]

We can also use range() as part of a for loop to iterate over a list of numeric values (without having to create that list manually).

# for loop that iterates over values in range
for i in range(0, 3):
 print(i)

Additional Information#

For more information on Python’s range() function:

enumerate()#

In a previous lab, we talked about how each item in a list has an index, or a number that indicates its position in the list. We can use the enumerate() function to generate a list of pairs containing each item in the list and its index.

We can use the enumerate() function as part of a for loop.

# for loop that iterates over list index and values
for index, letter in enumerate('abc'):
 print(index, letter)

In this last example, for index, letter instructed Python to iterate over both components in the enumerate() output. print(index, letter) instructed Python to print both components for each element.

Additional Resources#

For more information on Python’s enumerate() function:

Infinite Loop#

Loops that have no endpoint are called infinite loops. For example, given the following program:

# assign count variable
count = 1

# while loop
while count <= 5: # initial condition
   print ("Python") # print statement
   count = count + 1 # reassign count

# final print statement
print ("Done")

What would happen if we removed count = count + 1 from the loop? The value of count would never change, the initial condition’s truth value (count <= 5) would never change (because count would always equal 1), and we would have an infinite loop.

Break & Continue#

We can exit a loop immediately by using the break statement. break will stop or exit the while loop even if the condition is true.

# assign i variable
i = 1

# while loop
while i < 6: # initial condition

	print(i) # print statement

	if i == 3: # if statement
		break # break statement

	i += 1 # reassign i

1
2
3

In this example, the loop breaks as soon as the i == 3 condition is True.

We can skip the rest of the body of a loop and move on to the next iteration using continue.

Another example:

# assign the i variable
i = 0

# while loop
while i < 6: # initial condition

	i += 1 # reassign i

	if i ==3: # if statement
		continue # continue statement

	print(i) # print statement

In this example, the current iteration of the loop will stop when i == 3 is true. Unlike with break, the loop will not end. Instead when i == 3 is true, the loop will skip over the final nested print statement and return to the beginning of the loop for a new iteration.

Additional Resources#

For more on break and continue:

Comprehension Check#

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