Go gotcha: Why doesn't this loop end?

Why does this loop run forever?

var b byte
for b = 250; b <= 255; b++ {
        fmt.Printf("%d %c\n", b, b)
}

Answer

After the b == 255 iteration, b++ is executed. This overflows (since the maximum value for a byte is 255) and results in b == 0. Therefore b <= 255 still holds and the loop restarts from 0.

For unsigned integer values, the operations +, -, *, and << are computed modulo 2ⁿ, where n is the bit width of the unsigned integer's type.

For signed integers, the operations +, -, *, and << may legally overflow and the resulting value exists and is deterministically defined by the signed integer representation, the operation, and its operands. No exception is raised as a result of overflow. The Go Programming Language Specification: Arithmetic operators

If we use the standard loop idiom with a strict inequality, the compiler will catch the bug:

var b byte
for b = 250; b < 256; b++ {
        fmt.Printf("%d %c\n", b, b)
}

../main.go:2:17: constant 256 overflows byte

One solution is to use a wider data type, such as an int:

for i := 250; i < 256; i++ {
        fmt.Printf("%d %c\n", i, i)
}

Another option is to put the termination test at the end of the loop:

for b := byte(250); ; b++ {
        fmt.Printf("%d %c\n", b, b)
        if b == 255 {
                break
        }
}

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