Divide a String Into Groups of Size k

Divide a String Into Groups of Size k

What is this problem about?

The Divide a String Into Groups of Size k coding problem asks you to take a string and split it into several groups, each of length $k$. If the last group doesn't have enough characters to reach length $k$, you should pad it with a specified fill character. This is a common string formatting task that ensures all segments of a string meet a uniform size requirement.

Why is this asked in interviews?

Companies like Google and Bloomberg use this "Easy" question to verify basic string manipulation skills and loop control. It evaluations whether you can handle string boundaries and implement simple padding logic. While straightforward, it’s a good test of a candidate's ability to write clean, bug-free code for a task that mimics real-world data chunking or encryption block preparation.

Algorithmic pattern used

This is a Simulation or String Manipulation problem.

1. Initialize an empty list to store the groups.
2. Iterate through the string in steps of $k$.
3. For each step, extract a substring of length $k$.
4. If the extracted substring is shorter than $k$ (which only happens for the last group), append the fill character until its length is exactly $k$.
5. Return the list of groups.

Example explanation

String: "abcdefgh", $k = 3$, fill = 'x'.

1. First group: "abc".
2. Second group: "def".
3. Third group: "gh" (only 2 characters). Pad with one 'x' to get "ghx". Result: ["abc", "def", "ghx"].

Common mistakes candidates make

• Off-by-one errors: Incorrectly calculating the end index of the substring, leading to IndexOutOfBounds exceptions.
• Padding Logic: Forgetting to pad the last group or padding it incorrectly.
• Inefficiency: Using repeated string concatenation in a loop instead of a StringBuilder or list of strings (though for short strings, the impact is minimal).

Interview preparation tip

Always clarify the behavior for the last segment. If the string is already a multiple of $k$, do you add an extra padded segment? (Usually no). Practice using the substring(start, end) method in your preferred language to become fluent in slicing data.