How to round to nearest int when casting float to int in go

Question

When casting float to int the decimal is discarded. What's a clean way to cast so that it rounds to the nearest whole number instead.

x := int(3.6) should equal 4 instead of 3.

Agace

Possible duplicate of Go: Converting float64 to int with multiplier – Mcdonald

Related issue: math: add a Round function – Irascible

@Akavall edited title, – Agace

int(Round(f)) to round a float to an int. See https://mcmap.net/q/56303/-how-to-round-to-nearest-int-when-casting-float-to-int-in-go. float64(i) to set an int to a float. See https://mcmap.net/q/55267/-idiomatic-type-conversion-in-go. – Dasha

Has this changed? See go.dev/play/p/tEKYVunbNd_x. – Gracious

Answer 1

int(f+0.5) will cause upwards rounding if it's >= .5

This only works well for low precision rounding. E.g.

var f1 = 6.499999999999999
var f2 = 6.4999999999999999
var i1 = int(f1+0.5) // equals 6
var i2 = int(f2+0.5) // equals 7

This can cause non-deterministic behavior. See answer below w/ math.round for a better solution.

Agace

This provides a type conversion; please note that the term cast is not used. – Origami

Also, your type conversion will not work well with negative numbers if you intended the rounding to be to the nearest whole integer. – Origami

Answer 2

You could use int(math.Round(f)) to round to the nearest whole number when converting a float to an int in Go. The decimal is also discarded when a float is set to a byte or a rune. Truncation doesn't happen when it's set to a string or a bool.

package main

import (
  . "fmt"
  . "math"
)

func main() {
  f := 3.6
  c := []interface{}{byte(f), f, int(Round(f)), rune(f), Sprintf("%.f", f), f != 0}
  checkType(c)
}
func checkType(s []interface{}) {
  for k, _ := range s {
     Printf("%T %v\n", s[k], s[k])
  }
}

Round returns the nearest integer, rounding half away from zero. See https://golang.org/pkg/math/#Round. See https://mcmap.net/q/56920/-convert-a-float64-to-an-int-in-go.

f := 3.6 truncates to “uint8 3”, f is “float64 3.6”, int(Round(f)) rounds up to “int 4”, rune(f) truncates to “int32 3”, Sprintf("%.f", f) is “string 3.6” and f != 0 outputs “bool true”.

Dasha

A boolean, numeric, or string type can be set to another type. For[]byte see https://mcmap.net/q/56302/-how-to-convert-uint8-to-string, int see https://mcmap.net/q/55267/-idiomatic-type-conversion-in-go, []rune see https://mcmap.net/q/56310/-golang-how-does-the-rune-function-work, string see https://mcmap.net/q/55268/-how-can-i-convert-string-to-integer-in-golang and for bool see https://mcmap.net/q/56305/-is-there-a-way-to-convert-integers-to-bools-in-go-or-vice-versa. – Dasha