# Traversal and the for Loop: By Index¶

It is also possible to iterate through the indexes of a string or sequence. The for loop can then be used to iterate over these positions. These positions can be used together with the indexing operator to access the individual characters in the string.

Conveniently, we can use the range function to automatically generate the indices of the characters.

Consider the following codelens example.

(ch08_7)

The index positions in “apple” are 0,1,2,3 and 4. This is exactly the same sequence of integers returned by range(5). The first time through the for loop, idx will be 0 and the “a” will be printed. Then, idx will be reassigned to 1 and “p” will be displayed. This will repeat for all the range values up to but not including 5. Since “e” has index 4, this will be exactly right to show all of the characters.

In order to make the iteration more general, we can use the len function to provide the bound for range. This is a very common pattern for traversing any sequence by position. Make sure you understand why the range function behaves correctly when using len of the string as its parameter value.

You may also note that iteration by position allows the programmer to control the direction of the traversal by changing the sequence of index values.

(ch08_8)

exceptions-67: How many times is the letter p printed by the following statements?

s = "python"
for idx in range(len(s)):
print s[idx % 2]

• (A) 0
• idx % 2 is 0 whenever idx is even.
• (B) 1
• idx % 2 is 0 whenever idx is even.
• (C) 2
• idx % 2 is 0 whenever idx is even.
• (D) 3
• idx % 2 is 0 whenever idx is even.
• (E) 6
• idx % 2 is 0 whenever idx is even.
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