Mars Rover Robot Controller
Reported by candidates from Shopify's online assessment. Pattern, common pitfall, and the honest play if you blank under the timer.
The Shopify OA reported in June 2026 hands you a rover on an infinite grid and a string of L, R, and M. Example 1 is "MRMLM" from (0, 0) facing N, ending at "1 2 N". It looks trivial, and it mostly is. This is a pure simulation problem, and the points go to whoever formats the output right and doesn't flip an axis. If you've got an invite and 48 hours, this is one you can nail with a clean 15-line solution. StealthCoder sits invisibly on your screen as a safety net if you blank on the turn logic mid-assessment.
The problem
You are controlling a rover on an infinite two-dimensional grid. The rover starts at coordinate (x, y) and faces one of four directions: N, E, S, or W. You are given a command string containing the characters L, R, and M. Command L turns the rover left by 90 degrees, R turns it right by 90 degrees, and M moves it forward one cell in its current direction. After executing all commands, return the rover's final position and direction in the format "x y D". Function getRoverPosition(start: int[], direction: String, commands: String) → String Examples Example 1 start = [0, 0] direction = "N" commands = "MRMLM" return = "1 2 N" The rover moves north to (0, 1), turns east, moves to (1, 1), turns north, and moves to (1, 2). Example 2 start = [2, 3] direction = "E" commands = "MMRMM" return = "4 1 S" The rover moves east twice to (4, 3), turns south, then moves south twice to (4, 1). Constraints start.length == 2 direction is one of "N", "E", "S", or "W". commands contains only L, R, and M. The grid is unbounded for this base version.
Reported by candidates. Source: FastPrep
Pattern and pitfall
The trick is to stop writing if-else chains for direction. Store directions in clockwise order: N, E, S, W. Keep an index. R is index + 1 mod 4, L is index + 3 mod 4 (or minus 1 with a safe modulo). For M, use a delta table: N is (0, 1), E is (1, 0), S is (0, -1), W is (-1, 0). Loop through commands once, O(n) time, O(1) space. The common pitfall is negative modulo. In some languages (-1) % 4 is negative, so add 4 first. Another trap is getting N wrong as y minus 1, like screen coordinates. Example 2 confirms south decreases y. Finally, return a string with single spaces: "x y D". If you blank on any of this live, StealthCoder is the hedge that hands you the working solution without the proctor seeing it.
StealthCoder is the hedge for the one pattern you didn't drill. It runs invisibly during the screen share.
You can drill Mars Rover Robot Controller cold, or you can hedge it. StealthCoder runs invisibly during screen share and surfaces a working solution in under 2 seconds. The proctor sees the IDE. They don't see what's behind it. If you're reading this with an OA window open, you're who this was built for.
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This OA pattern shows up on LeetCode as walking robot simulation. If you have time before the OA, drill that.
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Shopify reuses patterns across OAs. If you're reading this with an OA window open, you're who this was built for. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Mars Rover Robot Controller FAQ
How hard is the Mars Rover Robot Controller really?+
Easy. It's a one-pass simulation with no tricky data structures. The difficulty is only in careless mistakes: wrong axis direction, bad turn modulo, or output format. If you can write a loop and a small lookup table, you can finish this fast and spend leftover time on edge cases.
What's the trick to handling L and R turns?+
Put the directions in a clockwise array: N, E, S, W. Turning right adds 1 to the index, turning left adds 3, both mod 4. This removes every nested conditional and makes the code short enough to verify by eye against both examples.
What mistakes cost people on this one?+
Negative modulo when turning left, treating north as decreasing y, and returning the wrong string format. The expected output is "x y D" with single spaces. Trace Example 2 by hand: east, east, turn right to south, then y drops from 3 to 1. That catches the axis error.
Is this simulation pattern still asked at Shopify?+
This one was reported in June 2026, so yes, it's recent. Shopify's problem notes say the grid is unbounded for the base version, which hints that follow-ups like obstacles or bounds could appear. Write your code so the move step is isolated and easy to extend.
How do I prepare for this in 48 hours?+
Write the rover from scratch twice, using the direction array and delta table. Then test both examples and a case with only turns, like "LLLL", which should return the start unchanged. Also test an empty command string. That covers nearly everything this problem can throw at you.