Suppose you have a long list of elements and you are only interested in every other element of the list. Perhaps you only want to examine the first or last elements, or a specific range of entries in your list. Python has strong indexing built-in capabilities. Here are some examples of how to achieve these scenarios.
Here’s a simple list that will be used throughout the examples:
lst = ['alpha', 'bravo', 'charlie', 'delta', 'echo']To iterate over each element in the list, a for loop like below can be used:
for s in lst:
print s[:1] # print the first letterThe for loop assigns s for each element of lst. This will print:
a
b
c
d
eOften you need both the element and the index of that element. The enumerate keyword performs that task.
for idx, s in enumerate(lst):
print("%s has an index of %d" % (s, idx))The index idx will start with zero and increment for each iteration, while the s will contain the element being processed. The previous snippet will output:
alpha has an index of 0
bravo has an index of 1
charlie has an index of 2
delta has an index of 3
echo has an index of 4If we want to iterate over a range (remembering that Python uses zero-based indexing), use the range keyword.
for i in range(2,4):
print("lst at %d contains %s" % (i, lst[i]))This would output:
lst at 2 contains charlie
lst at 3 contains deltaThe list may also be sliced. The following slice notation goes from element at index 1 to the end with a step of 2. The two for loops give the same result.
for s in lst[1::2]:
print(s)
for i in range(1, len(lst), 2):
print(lst[i])The above snippet outputs:
bravo
deltaIndexing and slicing is a topic of its own.