Double-quoted Strings Are Binaries

Introduction

Unlike single-quoted strings, the contents of a double-quoted string (dqs) are stored as a consecutive sequence of bytes in UTF-8 encoding. Clearly, this is more efficient in terms of memory and certain forms of access, but it does have two implications.

• First, because UTF-8 characters can take more than a single byte to represent, the size of the binary is not necessarily the length of the string.

   iex> dqs = "∂x/∂y" 
   "∂x/∂y"
   iex> String.length dqs 
   5
   iex> byte_size dqs
   9
   iex> String.at(dqs, 0) 
   "∂"
   iex> String.codepoints(dqs) 
   ["∂", "x", "/", "∂", "y"] 
   iex> String.split(dqs, "/") 
   ["∂x", "∂y"]
  

• Second, because we’re no longer using lists, we need to learn and work with the binary syntax alongside the list syntax in your code.

Strings and Elixir libraries

When Elixir library documentation uses the word “string” (and most of the time it uses the word “binary”), it means double-quoted strings. The String module defines functions that work with double-quoted strings. Let’s cover some of them below with examples.

• at(str, offset) returns the grapheme at the given offset (starting at 0). Negative offsets count from the end of the string.

   iex> String.at("∂og", 0) 
   "∂"
   iex> String.at("∂og", -1) 
   "g"
  

• capitalize(str) converts str to lowercase, and then capitalizes the first character.

   iex> String.capitalize "école" 
   "École"
  

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