The Mathematics of Grid Cell Selection

A random cell picker assigns exactly equal probability to every position in a grid. For a 8×8 grid, each of the 64 cells holds a probability of 1/64 ≈ 1.56%. The notation mirrors spreadsheet convention: column letters identify the horizontal position (A, B, C through H), and row numbers identify the vertical position (1 through 8). This produces a unique coordinate for every cell in the grid.

Filling the Grid

How many picks until every cell has been selected at least once? The coupon collector's problem applies directly. For 64 cells, the expected number of picks is 64 × H(64) ≈ 304, where H is the harmonic number. The heat map in the grid above shows your progress: each cell gradually warms as it accumulates hits, and the gaps reveal which cells are still waiting. The visual IS the statistical demonstration.

Cryptographic Fairness

Each cell is selected by crypto.getRandomValues() with rejection sampling to eliminate modulo bias across all 64 possible positions. A flat random integer is generated in the range [0, 63], then decomposed into row and column indices. Every cell receives exactly equal treatment from the entropy source. The selection runs entirely in your browser.

In the Classroom

Random grid selection solves a common classroom need: choosing a student fairly. Assign each student a cell in the grid (row = desk row, column = desk column). Project /grid/8/8 on screen and pick a cell. The scanning animation builds anticipation, and the heat map guarantees transparency: everyone can see the full history of selections. Over time, the uniform coverage demonstrates that the process is genuinely fair.

For a statistical lesson, have students predict how many picks are needed to cover every cell in a 5×5 grid. Most students guess 25 to 30. The mathematical answer is approximately 95. That gap between intuition and reality is the coupon collector's surprise, visible in real time on the coverage chart. The tool requires no accounts, stores no student data, and works on any device.

Private by Architecture

The server delivers this page. Your device generates every cell selection. Your pick history and heat map data live in localStorage on your machine. Sharing the URL shares the grid configuration. The recipient picks their own independent cells from their own device entropy.