# `=` — Classify / Equal

Function. Ranks: `∞` monadic, `0 0` dyadic

`=Y` self-classifies the major cells of `Y`. The result has a row for
each distinct cell, in order of first occurrence, and a column for each
cell of `Y`. An entry is `$t` where its column’s cell equals its row’s
cell.

``` bpl
="aba"            ⍝ [$t $f $t⋄$f $t $f]
```

`X=Y` tests equality, pervasively. NaN equals nothing, as IEEE 754
defines.

``` bpl
1 2 3=2            ⍝ $f $t $f
0.3=0.1+0.2        ⍝ $t
"abc"='b'          ⍝ $f $t $f
```

See [tolerance](../evaluation.qmd#equality-and-ordering) and whole-array
[match `≡`](equal-underbar.qmd).

## Inverse

On Booleans, `=` is its own inverse with either argument fixed.

``` bpl
1 0=⁻¹1 1   ⍝ $t $f
```