Getting the first character of a line

With a line from the Markdown file represented as a string variable called line_contents, we can now figure out what kind of line this is. There are really only two kinds of lines we care about for this naive Markdown compiler: first-order headings and paragraphs.

If a line starts with a hash (#) symbol, then it’s a first-order heading. If, however, it starts with any other alphanumeric value, it’s a paragraph.

You might think that we would do something like line_contents[0] to get the first character of line_contents, but Rust doesn’t store strings as a sequence of characters in memory. Instead, we have to first convert the variable into a sequence of characters, take the first one from that sequence, and then convert it into something that we can use.

To do this, we will create a new variable called first_char which will (you guessed it) hold the first character of line_contents. The variable type will be a vector of characters, and we will create it like this:

1

let mut first_char: Vec<char> = line_contents.chars().take(1).collect();

Let’s look at the right-hand one piece at a time:

• line_contents.chars() says, “Get the line_contents variable and convert it to a sequence of characters.” (Rust will create an Iterator object of characters)
• .take(1) says, “Now take the first element of that iterable object.” (Rust will convert this Iterator into a Take<char> object, a special kind of iterator)
• .collect() says, “Now convert everything I have retrieved up to this point into a Collection–something that I can subsequently use–of the type matching the left-hand variable (which is Vec<char>)

It’s a lot of steps, but the more you read them from left to right the more they will make sense. Rust is a language that feels like C but behaves like Ruby; there is a lot going on behind the scenes! At the end of the day, the most important part of the above expression is that collect() takes all the work we did of pulling out the first character and collects up the result in a vector for us.

At this point we have line_contents, which holds the entire contents of the line, and first_char, which has the first character of line_contents. Neat!

We’re about to start doing different things based on what the first character of the line is, but before we do, let’s create a new string variable that will hold the valid HTML that the current line we are reading (line_contents) will translate to:

1

let mut output_line = String::new();

Just like when we created a Path variable by using Path::new(), we’re using String::new() to declare a mutable variable named output_line.

Our main output variable is tokens; that’s a vector a strings that will contain one string object for every line that comes through from the file. When we are done processing line_contents based on the value of first_char (whether it is a # or not), we will write valid HTML to output_line and then push output_line into tokens. Then, in the last section of this chapter, we will iterate through the strings in tokens and write them all to an output file.