---
name: "fpclassify"
section: "3tcl"
platform: "linux"
package: "tcl9.0-doc"
version: "9.0.1+dfsg-2"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/fpclassify/3tcl"
---

# fpclassify(3tcl)

Floating point number classification of Tcl values

```

```

## NAME

fpclassify - Floating point number classification of Tcl values

## SYNOPSIS

package require **tcl 9.0**

**fpclassify** *value*

```

```

## DESCRIPTION

The **fpclassify** command takes a floating point number, *value*, and returns one of the following strings that describe it:

- **zero**

  *value* is a floating point zero.

- **subnormal**

  *value* is the result of a gradual underflow.

- **normal**

  *value* is an ordinary floating-point number (not zero, subnormal, infinite, nor NaN).

- **infinite**

  *value* is a floating-point infinity.

- **nan**

  *value* is Not-a-Number.

The **fpclassify** command throws an error if value is not a floating-point value and cannot be converted to one.

## EXAMPLE

This shows how to check whether the result of a computation is numerically safe or not. (Note however that it does not guard against numerical errors; just against representational problems.)

```
set value [command-that-computes-a-value]
switch [fpclassify $value] {
    normal - zero {
        puts "Result is $value"
    }
    infinite {
        puts "Result is infinite"
    }
    subnormal {
        puts "Result is $value - WARNING! precision lost"
    }
    nan {
        puts "Computation completely failed"
    }
}
```

## SEE ALSO

[expr(3tcl)](/expr/3tcl.md), [mathfunc(3tcl)](/mathfunc/3tcl.md)

## KEYWORDS

floating point

## STANDARDS

This command depends on the **fpclassify**() C macro conforming to “ISO C99” (i.e., to ISO/IEC 9899:1999).

## COPYRIGHT

```
Copyright © 2018 Kevin B. Kenny <kennykb@acm.org>. All rights reserved
```