Problem Statement

Build an   identity matrix   of a size known at run-time.

An identity matrix is a square matrix of size n × n, where the diagonal elements are all 1s (ones), and all the other elements are all 0s (zeroes).

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{\displaystyle I_{n}={\begin{bmatrix}1&0&0&\cdots &0\0&1&0&\cdots &0\0&0&1&\cdots &0\\vdots &\vdots &\vdots &\ddots &\vdots \0&0&0&\cdots &1\\end{bmatrix}}}

Let's start with the solution:

IDMATRIX
=LAMBDA(n,
    LET(
        ixs, SEQUENCE(n, n, 0, 1),
        x, MOD(ixs, n),
        y, QUOTIENT(ixs, n),

        IF(x = y,
            1,
            0
        )
    )
)