Reading your first program

Prerequisite: lessons 1–6. Work in organizer unless stated otherwise.

Why this matters

The last lesson used a supplied program. Now we read its instructions. Open src/main.rs, replace its entire contents with the following, save it and run cargo run from organizer:

fn main() {
    println!("My organizer");
    println!("Task: learn Rust");
}

Output:

My organizer
Task: learn Rust

Every symbol has a purpose

fn introduces a function, a named action. main names the function where execution of our ordinary program starts. The empty parentheses contain no declared parameters. Braces delimit a block, a group of actions. Lesson 14 will develop functions with parameters; for now recognise this entry-point form.

println! prints text followed by a line break. The ! indicates a macro, a construct expanded into code during compilation. Think of an instruction-generating template. It is not an ordinary function, and writing your own macro is outside this lesson. The parentheses hold what is supplied to it.

Text in double quotes is a string literal, a value written directly in the source. The quotes delimit it and are not printed. The semicolon ends this statement. Four-space indentation makes nesting readable, but braces determine the actual boundaries.

The two statements run in order and produce two lines. // begins a single-line comment, explanatory text for a reader through the end of that line; it is not executed.

An understandable mistake

Removing a closing quote makes the text boundary unclear to the compiler. Removing ! changes the kind of invocation; it no longer calls the macro we learned. Change one thing at a time and repeat cargo check, introduced in lesson 6.

Recall map

fn main(): entry; braces: block; println!: output; quoted text: literal; semicolon: statement ending; //: explanation for people.

Warm-up

1. Predict the order of the two printed lines before running.
2. Complete: println___(“Text”);
3. Restore a missing closing brace in your copy using the first diagnostic.

Exercise

Required. Print three lines in this order: “Today’s plan”, “Learn Rust”, “Review Cargo commands”. Use three statements. The expected output contains these lines without the quotation marks.

Your own data. Replace the second line with your own task.

Hint and reference answer

Each println! ends a line. Try reconstructing the program using only the recall map. If necessary, practise one line first. Warm-up answers: heading before task; the missing symbol is !; the closing brace ends main’s block.

fn main() {
    println!("Today’s plan");
    println!("Learn Rust");
    println!("Review Cargo commands");
}

Expected output:

Today’s plan
Learn Rust
Review Cargo commands

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