Why Does fmt.Printf Display Incorrect Binary Representations for Signed Integers in Go?

The Discrepancy between Two's Complement and fmt.Printf

Original Question:

When attempting to print the binary representation of a signed integer, fmt.Printf produces output that differs from the expected Two's complement value.

Analysis:

Two's complement is the technique used by computers to internally represent signed integers. For example, -5 would be represented as 1111 1011. However, fmt.Printf, when used with a signed integer, produces an unexpected result. Converting to an unsigned integer, however, results in the correct binary representation.

Explanation:

The behavior observed is not due to the internal representation of the integer. It is instead the result of how fmt.Printf formats binary numbers.

The fmt.integer function, utilized by fmt.Printf, converts negative signed integers to positive values before formatting. This results in the binary value of the absolute value being prepended with a - sign.

Example:

<code class="go">var i int8 = -5
fmt.Printf("%b", i) // Outputs: -101 (Incorrect)</code>

In this example, -5 is converted to 5 in binary, which is then prepended with a ., resulting in -101.

Resolution:

To obtain the correct binary representation of a signed integer, one can convert it to an unsigned integer and then use fmt.Printf to print the binary representation.

<code class="go">var i int8 = -5
var u uint8 = uint8(i)
fmt.Printf("%b", u) // Outputs: 1111 1011 (Correct)</code>

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