List, for. In list loops, we often need no index. We require just the elements in order. The for-loop is ideal in this situation. It eliminates the confusion of an index variable.
Loop options. We can use methods like enumerate() to get tuples of indexes and elements in a list. And with range() we can act upon indexes, and change how a loop progresses.
For example. This is the simplest list looping syntax. We encounter each of the 4 list elements in the for-loop. We use the identifier "element" in the loop body.
# An input list.
elements = ["spider", "moth", "ant", "lizard"]
# Use simple for-loop.for element in elements:
print(element)spider
moth
ant
lizard
Adjacent elements. Often we need only one element as we loop. But in some cases, we need adjacent elements to compare. Here we access adjacent elements in the list.
Tip Starting at index 1 is key. In the loop body, we access the previous element, at index "i-1", and the current element.
# Input list.
elements = [0, 10, 20, 30]
# Use range.for i in range(1, len(elements)):
# Get two adjacent elements.
a = elements[i - 1]
b = elements[i]
# Print 2 elements.
print(a, b)0 10
10 20
20 30
Enumerate list. Here we use the enumerate method to loop over a list. We unpack a tuple of (index, element) at each iteration of the loop.
colors = ["green", "orange", "yellow"]
# Call enumerate() to access indexes and elements.# ... Unpack each tuple returned from enumerate().
for i, color in enumerate(colors):
print("INDEX:", i)
print("COLOR:", color)INDEX: 0
COLOR: green
INDEX: 1
COLOR: orange
INDEX: 2
COLOR: yellow
Reverse indexes. Suppose we want to loop over a list in reverse, from the last to the first index. We can use a range() call and specify a step of -1, which means "go backwards."
Important The range() call here does not return the indexes, but returns 1 greater than the indexes, so we must subtract 1.
ids = [201, 202, 203]
# Loop over the list indexes in reverse order (from last to first).# ... We go from the length and stop after 1.# So we must subtract 1 to get the indexes.for i in range(len(ids), 0, -1):
print("INDEX:", i - 1)
print("ID:", ids[i - 1])INDEX: 2
ID: 203
INDEX: 1
ID: 202
INDEX: 0
ID: 201
Reversed list, for. Sometimes we do not need the indexes of each element—we just want the elements themselves. Here we call reversed() to act upon a reversed view of the list.
And We then use a for-loop over the reversed view of the list. We print the element values to the console.
ids = [600, 601, 602]
# Loop over list in reverse.
for id in reversed(ids):
print("REVERSED:", id)REVERSED: 602
REVERSED: 601
REVERSED: 600
A summary. Lists are made to be looped over—looping is a fast and natural operation on Python lists. We can loop over indexes, or the elements themselves.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Aug 12, 2022 (edit link).
