swift - Why are doubles printed differently in dictionaries?

Question

let dic : [Double : Double] = [1.1 : 2.3, 2.3 : 1.1, 1.2 : 2.3]

print(dic)// [2.2999999999999998: 1.1000000000000001, 1.2: 2.2999999999999998, 1.1000000000000001: 2.2999999999999998]



let double : Double = 2.3
let anotherdouble : Double = 1.1

print(double) // 2.3
print(anotherdouble) // 1.1

I don't get that why is the compiler printing values from dictionaries differently? I'm on Swift 3, Xcode 8. Is this a bug or some weird way of optimizing stuff or something?

EDIT

What's even more weird is that :

Some values go over, some go below, some stay as they are! 1.1 is less than 1.1000000000000001 while 2.3 is more than 2.2999999999999998, 1.2 is just 1.2

Answer 1

As already mentioned in the comments, a Double cannot store the value 1.1 exactly. Swift uses (like many other languages) binary floating point numbers according to the IEEE 754 standard.

The closest number to 1.1 that can be represented as a Double is

1.100000000000000088817841970012523233890533447265625

and the closest number to 2.3 that can be represented as a Double is

2.29999999999999982236431605997495353221893310546875

Printing that number means that it is converted to a string with a decimal representation again, and that is done with different precision, depending on how you print the number.

From the source code at HashedCollections.swift.gyb one can see that the description method of Dictionary uses debugPrint() for both keys and values, and debugPrint(x) prints the value of x.debugDescription (if x conforms to CustomDebugStringConvertible).

On the other hand, print(x) calls x.description if x conforms to CustomStringConvertible.

So what you see is the different output of description and debugDescription of Double:

print(1.1.description) // 1.1
print(1.1.debugDescription) // 1.1000000000000001

From the Swift source code one can see that both use the swift_floatingPointToString() function in Stubs.cpp, with the Debug parameter set to false and true, respectively. This parameter controls the precision of the number to string conversion:

int Precision = std::numeric_limits<T>::digits10;
if (Debug) {
  Precision = std::numeric_limits<T>::max_digits10;
}

For the meaning of those constants, see std::numeric_limits:

• digits10 – number of decimal digits that can be represented without change,
• max_digits10 – number of decimal digits necessary to differentiate all values of this type.

So description creates a string with less decimal digits. That string can be converted to a Double and back to a string giving the same result. debugDescription creates a string with more decimal digits, so that any two different floating point values will produce a different output.