CODEWITHSUNDEEP
c#typescasting
Tuncaf
Tuncaf

Reputation: 231

What is the difference between casting long.MaxValue to int and casting float.MaxValue to int?

I'm trying to understand difference between some data types and conversion.

public static void ExplicitTypeConversion2()
    {
        long longValue=long.MaxValue;
        float floatValue = float.MaxValue;
        int integerValue = (int) longValue;
        int integerValue2 = (int)floatValue;
        Console.WriteLine(integerValue);
        Console.WriteLine(integerValue2);            
    }

When I run that code block, it outputs:

-1
-2147483648

I know that if the value you want to assign to an integer is bigger than that integer can keep, it returns the minimum value of integer (-2147483648).

As far as I know, long.MaxValue is much bigger than the maximum value of an integer, but if I cast long.MaxValue to int, it returns -1.

What is the difference these two casting? I think the first one also suppose to return -2147483648 instead of -1.

Upvotes: 12

Views: 5954

Answers (3)

Tetsuya Yamamoto
Tetsuya Yamamoto

Reputation: 24957

Let me explain:

long longValue=long.MaxValue;
float floatValue = float.MaxValue;
int integerValue = (int) longValue;
int integerValue2 = (int)floatValue;

The maximum value of long is 9,223,372,036,854,775,807 or 0x7FFFFFFFFFFFFFFF, thus 2's complement after reducing it into 0xFFFFFFFF will return 0x00000001 with minus sign bit, represented as -1 in decimal.

On the other side, the maximum value of float is 3.40282347E+38, thus casting it to int rounded the value to 3E+38 and using 2's complement after reducing it we get the hex value of 0x80000000 with minus sign bit, there is -2147483648 in decimal.

All of this case applies on signed integers, the result will be different on unsigned ones.

Reference:

https://msdn.microsoft.com/en-us/library/system.int64.maxvalue(v=vs.110).aspx

https://msdn.microsoft.com/en-us/library/system.single.maxvalue(v=vs.110).aspx

Upvotes: 3

AhmadWabbi
AhmadWabbi

Reputation: 2197

The binary value of long.MaxValue is 0111...111111(a zero followed by 63 ones). When you cast to int, you keep the lowest 32 bits 111...11111. This is -1 in decimal, as int is signed and two's complement applies.

Upvotes: 6

user555045
user555045

Reputation: 64903

if the value you want to assign to an integer, bigger than that integer can keep, returns minimum value of integer

That's not a rule. The relevant rules are

For integer types in an unchecked context (ie the default):

If the source type is larger than the destination type, then the source value is truncated by discarding its “extra” most significant bits. The result is then treated as a value of the destination type.

For float->int in an unchecked context:

The value is rounded towards zero to the nearest integral value. If this integral value is within the range of the destination type, then this value is the result of the conversion. Otherwise, the result of the conversion is an unspecified value of the destination type.

Chopping off 32 leading bits of off 0x7fffffffffffffff gives 0xffffffff aka -1.

You were never promised you would get int.MinValue for that out of range float->int cast, but you do anyway because it's easy to implement: x64's conversion instruction cvtss2si makes 0x80000000 for out of range results and similarly fistp (the old x87 conversion instruction used by the 32bit JIT) stores "the integer indefinite value" which is 0x80000000.

Upvotes: 16

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