CODEWITHSUNDEEP
pythonloopsfor-loop
Kernel Pource
Kernel Pource

Reputation: 27

Ranges in for loop off by 1?

I'm working through the book automating the boring stuff with python and came across this code he wrote to find phone numbers within a document.

The format of phone numbers is (12 characters): 123-456-7890

Why does the code check if length of text is 12? but doesn't python count from 0 and so it'll look for a length of 13?

Similarly, he wants to check if the first three digits are numbers but he uses a range(0, 3). Doesn't this check the first 4 digits including a hyphen which is not a number?

Thanks for the help.

def isPhoneNumber(text):
    if len(text) != 12:
        return False
    for i in range(0, 3):
        if not text[i].isdecimal():
            return False
    if text[3] != '-':
        return False
    for i in range(4, 7):
        if not text[i].isdecimal():
            return False
    if text[7] != '-':
        return False
    for i in range(8, 12):
        if not text[i].isdecimal():
           return False
    return True

Upvotes: 0

Views: 217

Answers (4)

Picachieu
Picachieu

Reputation: 3782

You're getting list indexing, the len function, and the range function confused.

List indexing starts at 0 and increases for each element. The 13th element in a list is at index 12, and would be accessed with text[12].

The len function, on the other hand, returns the actual length of the list. The length of a 2-element is, of course, 2, and that's what len would return. In your example, the text is 13 characters long. len returns 13, but the last index of the text would be at text[12].

The range function is inclusive of the starting value and exclusive of the ending value. In your example of range(0, 3), it checks the items at locations 0, 1, and 2, not 0, 1, 2, and 3. If you want to check the first four numbers, use range(0, 4) instead, which will check the elements at list indexes 0, 1, 2, and 3, which are also the first, second, third, and fourth elements in the string.

Upvotes: 0

Russ Brown
Russ Brown

Reputation: 171

The len function returns the number of objects in a list. i.e. 

>>> len([1, 2, 3])
3

range(0, 3) produces 0, 1, 2 and hence only checks the first three numbers.

>>> for i in range(0, 3):
...     print(i)
... 
0
1
2

Upvotes: 0

Bridgette
Bridgette

Reputation: 408

It's best to separate the concepts of indexes (the positions of objects in a list) and length (how many there are). Here's the doc of len(s) in Python.

range(start, stop) in Python includes the start and excludes the stop parameter. In mathematical notation, [start, stop). Therefore range(0,3) will check 0, 1, 2.

Upvotes: 0

Bharel
Bharel

Reputation: 26900

len() returns the actual length of the string. Think of it, a string with length 0 ("") also exists.

The last value of range(start, end) is end-1. The number 2 is in the range of 1-3 and the number 3 in the range of 3-5. Inclusive bottom and exclusive upper bound.

Upvotes: 1

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