Integer operators

January 31, 2024

Today we continue our discussion of arithmetic operators, with a topic that is likely not new to you at all: Integer operators.

Integer operators

For two integer values x and y, the integer quotient q = x / y and remainder r = x % y satisfy the following relationships:

x = q*y + r  and  |r| < |y|

This should be pretty obvious and intuitive… If you divide x / y and get a quotiont q and remainder r, then multiplying q * y and adding r should reveal x again.

with x / y truncated towards zero (“truncated division”).

 x     y     x / y     x % y
 5     3       1         2
-5     3      -1        -2
 5    -3      -1         2
-5    -3       1        -2

This should also be unsurprising. If x does not divide evenly into y, then division is truncated toward zero, as in virtually every other language. (Or at least those with the concepts of an integer… Sorry JavaScript.)

The one exception to this rule is that if the dividend x is the most negative value for the int type of x, the quotient q = x / -1 is equal to x (and r = 0) due to two’s-complement integer overflow:

                         x, q
int8                     -128
int16                  -32768
int32             -2147483648
int64    -9223372036854775808

Integer overflow is annoying, and it gives us the one exception to this rule. Why is this?

Let’s have a look…

var x = int8(-128)
fmt.Println(x/-1) // -128, as explained above

What is the “correct” answer? Well, 128, as we can see if we use a larger integer type:

var x = int16(-128)
fmt.Println(x/-1) // 128

But there’s no way to represent the value int8 as an int8 because of integer overflow. The valid values for an int8 are -128 through 127. In fact, if you try to represent 128 as an int8, you’ll get -127, regardless of whether you arrive at that value via division or some other means:

var y = int16(128)
fmt.Println(int8(y)) // -128

So converting from int16(128) to int8 gives us -128.

var z = int8(127)
fmt.Println(z+1) // -128

And adding 1 to int8(127) also gives us -128.

So this special case isn’t quite so special after all.

Quotes from The Go Programming Language Specification Version of August 2, 2023

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