convert Hex String to UInt32

Question

esellier OP

Created Jul ’19

Replies 3

Boosts 0

Views 6.5k

Participants 3

Hi,

I observed a strange behaviour with the following code:

UInt32(Float("0x7f000001") ?? 0)
// will return 2130706432
atof("0x7f000001")
// will return 2130706433

Why do I have two different results? Bug?

Actually 0x7f000001 is the Hex representation of the IP address 127.0.0.1

So the correct result should be 2130706433

UInt32("2130706433")
// will return 2130706433

Thanks

Answered by OOPer in 374630022

Why do I have two different results? Bug?

Not a bug. `Float` has only 24-siginificant bits, so cannot represent 32-bit integer value pesicely.

`atof` returns `double`, not `float`.

If you want to get the correct result from a hex represntation, you need to cut "0x" and use `init(_:radix:)`.

let hex = "0x7f000001"
let value = UInt32(hex.dropFirst(2), radix: 16) ?? 0
print(value) //->2130706433

Boost

Answer 1

OOPer OP

Jul ’19

Accepted Answer

Why do I have two different results? Bug?

Not a bug. `Float` has only 24-siginificant bits, so cannot represent 32-bit integer value pesicely.

`atof` returns `double`, not `float`.

If you want to get the correct result from a hex represntation, you need to cut "0x" and use `init(_:radix:)`.

let hex = "0x7f000001"
let value = UInt32(hex.dropFirst(2), radix: 16) ?? 0
print(value) //->2130706433

0

Answer 2

esellier OP

Aug ’19

Thanks.

Works with Float64 (so a double) instead of Float or Float32...

UInt32(Float64("0x7f000001") ?? 0)

Float64 has the advantage to return nil if the string is not an actual hex representation.

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Answer 3

DTS Engineer OP

Apple

Aug ’19

Hmmm, 0x7f000001 looks like the loopback IPv4 address, 127.0.0.1. Is that why you’re doing this? If so, be aware that standard DNS resolver functions, like

getaddrinfo

, will correctly parse addresses given in hex.

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