Swift: Byte array to decimal value

Question

In my project, I communicate with a bluetooth device, the bluetooth device must send me a timestamp second, I received in byte:

[2,6,239]

When I convert converted to a string:

let payloadString = payload.map {
            String(format: "%02x", $0)
        }

Output:

["02", "06","ef"]

When I converted from the website 0206ef = 132847 seconds

How can I directly convert my aray [2,6,239] in second (= 132847 seconds)? And if it's complicated then translate my array ["02", "06,"ef"] in second (= 132847 seconds)

Martin R · Accepted Answer · 2018-11-15 08:23:37Z

2

The payload contains the bytes of the binary representation of the value. You convert it back to the value by shifting each byte into its corresponding position:

let payload: [UInt8] = [2, 6, 239]
let value = Int(payload[0]) << 16 + Int(payload[1]) << 8 + Int(payload[2])
print(value) // 132847

The important point is to convert the bytes to integers before shifting, otherwise an overflow error would occur. Alternatively, with multiplication:

let value = (Int(payload[0]) * 256 + Int(payload[1])) * 256 + Int(payload[2])

or

let value = payload.reduce(0) { $0 * 256 + Int($1) }

The last approach works with an arbitrary number of bytes – as long as the result fits into an Int. For 4...8 bytes you better choose UInt64 to avoid overflow errors:

let value = payload.reduce(0) { $0 * 256 + UInt64($1) }

edited Nov 15, 2018 at 8:23

answered Nov 15, 2018 at 7:55

Martin R

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12 Comments

Mickael Belhassen Over a year ago

I do not understand why the shift is not always the same, you write once << 16, then, << 8, then for the last nothing why? Same with the multiplication why the last you do not multiply it?

Mickael Belhassen Over a year ago

It is possible that the device does not send me 3 byte but 4 so how should I do?

Mickael Belhassen Over a year ago

If the device sends me 4byte instead of 3 [2,3,6,222] the code remains the same?

Martin R Over a year ago

@MickaelBelhassen: I added a version with works with a variable number of bytes.

Mickael Belhassen Over a year ago

The latest version is generic? for more bytes?

|

Rizwan · Accepted Answer · 2018-11-15 08:23:22Z

1

payloadString string can be reduced to hexStr and then converted to decimal

var payload = [2,6,239];
let payloadString = payload.map {
    String(format: "%02x", $0)
}

//let hexStr = payloadString.reduce(""){$0 + $1}
let hexStr = payloadString.joined()
if let value = UInt64(hexStr, radix: 16) {
    print(value)//132847
}

edited Nov 15, 2018 at 8:23

answered Nov 15, 2018 at 8:06

Rizwan

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4 Comments

Mickael Belhassen Over a year ago

If the device sends me 4byte instead of 3 [2,3,6,222] the code remains the same?

Rizwan Over a year ago

yes code will remain same for 4byte or more. For [2,3,6,222] output will be 33752798 with same above code.

Martin R Over a year ago

Note that payloadString.reduce(""){$0 + $1} can be simplified to payloadString.joined()

Rizwan Over a year ago

Correctly pointed, that's a better easier way. Updated answer.

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