为何Elixir bitstring最后一位默认是1？如何实现全0bitstring？

Great question! Let's break this down step by step.

Why your initial bitstring has a trailing 1 #

When you define b = <<1 :: 3>> in Elixir (or Erlang), you're instructing the runtime to encode the integer 1 using exactly 3 bits. Integers are stored in binary form with leading zeros to fill the specified bit length.

Since 1 in binary is just 1, we need two leading zeros to reach the 3-bit requirement, resulting in the binary value 001. That's why when you iterate over each 1-bit segment with your comprehension, you get ["0", "0", "1"]—the first two bits are the padding zeros, and the last is the actual value of the integer you specified.

How to create an all-zero bitstring #

There are a few straightforward ways to generate a bitstring where every bit is set to 0:

1. Directly encode 0 with your desired bit size #

The simplest method is to use the same syntax as your original code, but replace the integer 1 with 0:

b = <<0 :: 3>>

When you iterate over this bitstring, you'll get ["0", "0", "0"]—since 0 in 3 bits is just 000.

2. Use a bitstring comprehension (dynamic length) #

If you need to create a bitstring of variable length, you can build it by concatenating individual 1-bit zero segments:

desired_length = 3
b = for _ <- 1..desired_length, do: <<0::1>>, into: <<>>

This loops desired_length times, adding a 1-bit zero each iteration, and collects all segments into a single bitstring.

3. Erlang equivalent #

For Erlang developers, the direct approach is nearly identical:

B = <<0:3>>.

Or for dynamic lengths, use a list comprehension:

DesiredLength = 3,
B = <<0:1 || _ <- lists:seq(1, DesiredLength)>>.

All of these methods will produce a bitstring where every bit is 0.

内容的提问来源于stack exchange，提问作者leifericf