9.6. Traversal and the for Loop: By Index¶
It is possible to use the range function to systematically
generate the indices of the characters. 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.
Consider the following example.
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, i will be 0 and the “a” will be printed.
Then, i 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. Recall that we can create ranges that count down as well as up so the following code will print the characters from right to left.
Run the above program using CodeLens to trace the values of i and satisfy
yourself that they are correct. In particular, note the start and end of the
range.
Check your understanding
- 0
- The for loop visits each index but the selection only prints some of them.
- 1
- o is at positions 4 and 8
- 2
- Yes, it will print all the characters in even index positions and the o character appears both times in an even location.
- Error, the for statement cannot have an if inside.
- The for statement can have any statements inside, including if as well as for.
strings-11-1: How many times is the letter o printed by the following statements?
s: str
s = "python rocks"
for i in range(len(s)):
if i % 2 == 0:
print(s[i])