Three remarks about iteration over sequences

Correctly iterating over sequences

When iterating over a sequence, you may be tempted to use something like:

for i in range(len(sequence)):
    element = sequence[i]
    operate on element

This is particularly common for people who have some experience with different programming languages, like C++. In Python this is wrong! Instead, you should simply iterate over sequence elements disregarding their indices:

for element in sequence:
    operate on element

This is much more elegant and works also with sequences that are not indexed with numbers, like dictionaries or sets.

Item numbering in a loop

Sometimes there is a need to iterate through the sequence of a set and number them simultaneously. This can be done as follows:

number = 0
for element in sequence:
    loop block
    number += 1

However, this is not the best solution. It is much better to use a function enumerate(sequence) that will generate successive sequence elements and their numbers in each run of the loop:

for number, element in enumerate(sequence):
    print("Element no", number, "in the sequence is", element)

Iterating over several sequences simultaneously

It is also possible to iterate over several sequences simultaneously using the function zip(sequence1, sequence2...). The number of sequences for simultaneous iteration is arbitrary. The condition is that all these sequences must be of the same length (in fact, it is not really necessary: only as many elements as there are in the shortest sequence will be taken into account). This iteration takes the form:

for element1, element2, element3 in zip(sequence1, sequence2, sequence3):
    loop block

For example:

first_names = ["Harry", "Frodo", "James"]
last_names = ["Potter", "Baggins", "Bond"]

for first_name, last_name in zip(first_names, last_names):
    print(first_name, last_name)

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