Substring search in Python: Counting ‘bob’ in a string

[Splendid Digital Solutions]

Great question! The reason why we use i+3 and not i+2 when checking the substring is because of how Python string slicing works, especially when specifying ranges in slices.

String slicing in Python:

When you slice a string like s[start:end], the start index is inclusive (the slice starts at start), but the end index is exclusive (the slice stops just before end).

Why i+3:

• If we want to check for a substring of exactly 3 characters starting at position i, we need to include the indices i, i+1, and i+2.
• To achieve this, we specify the slice as s[i:i+3], because:
• i is the starting index.
• i+3 ensures that the substring includes characters at positions i, i+1, and i+2 (but excludes the character at index i+3).

Example:

Let’s take s = 'azcbobobegghakl'.

• When i = 3, the slice s[3:6] means:
• Start at index 3 (which is 'b').
• Stop just before index 6 (which is 'o' at index 4, and 'b' at index 5).
• The slice s[3:6] gives 'bob' (which is what we want to check).

What happens if we use i+2 instead?

If we wrote s[i:i+2], we would only get a substring of 2 characters, which is not enough to check for 'bob'.

• For example, s[3:5] would give 'bo', which is not equal to 'bob'.
• This is because the slice would only include characters at positions i and i+1 (but not i+2).

Conclusion:

We use i+3 to ensure that we are checking a substring of exactly 3 characters (s[i:i+3]), which includes the characters at indices i, i+1, and i+2.

So, the correct way to check for 'bob' is with:

if s[i:i+3] == 'bob': # Checks the substring of length 3

Source: Created with help of AI tool

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