Add some exercises about functions!

README.md

@@ -22,6 +22,15 @@ If you need more help or would like to compare solutions, you can ask in [#rust
 - ["variables3.rs"](http://play.rust-lang.org/?code=%2F%2F+Make+me+compile%21+Scroll+down+for+hints+%3A%29%0A%0Afn+main%28%29+%7B%0A++++let+x+%3D+3%3B%0A++++println%21%28%22Number+%7B%7D%22%2C+x%29%3B%0A++++x+%3D+5%3B%0A++++println%21%28%22Number+%7B%7D%22%2C+x%29%3B%0A%7D%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%2F%2F+In+Rust%2C+variable+bindings+are+immutable+by+default.+But+here+we%27re+trying%0A%2F%2F+to+reassign+a+different+value+to+x%21+There%27s+a+keyword+we+can+use+to+make%0A%2F%2F+a+variable+binding+mutable+instead.%0A)
 - ["variables4.rs"](http://play.rust-lang.org/?code=%2F%2F+Make+me+compile%21+Scroll+down+for+hints+%3A%29%0A%0Afn+main%28%29+%7B%0A++++let+x%3A+i32%3B%0A++++println%21%28%22Number+%7B%7D%22%2C+x%29%3B%0A%7D%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%2F%2F+Oops%21+In+this+exercise%2C+we+have+a+variable+binding+that+we%27ve+created+on%0A%2F%2F+line+4%2C+and+we%27re+trying+to+use+it+on+line+5%2C+but+we+haven%27t+given+it+a%0A%2F%2F+value.+We+can%27t+print+out+something+that+isn%27t+there%3B+try+giving+x+a+value%21%0A%2F%2F+This+is+an+error+that+can+cause+bugs+that%27s+very+easy+to+make+in+any%0A%2F%2F+programming+language+--+thankfully+the+Rust+compiler+has+caught+this+for+us%21%0A)
 
+### Functions
+
+[Relevant chapter in The Rust Programming Language](https://doc.rust-lang.org/stable/book/functions.html)
+
+- ["functions1.rs"](http://play.rust-lang.org/?code=%2F%2F+Make+me+compile%21+Scroll+down+for+hints+%3A%29%0A%0Afn+main%28%29+%7B%0A++++call_me%28%29%3B%0A%7D%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%2F%2F+This+main+function+is+calling+a+function+that+it+expects+to+exist%2C+but+the%0A%2F%2F+function+doesn%27t+exist.+It+expects+this+function+to+have+the+name+%60call_me%60.%0A%2F%2F+It+expects+this+function+to+not+take+any+arguments+and+not+return+a+value.%0A%2F%2F+Sounds+a+lot+like+%60main%60%2C+doesn%27t+it%3F%0A)
+- ["functions2.rs"](http://play.rust-lang.org/?code=%2F%2F+Make+me+compile%21+Scroll+down+for+hints+%3A%29%0A%0Afn+main%28%29+%7B%0A++++call_me%283%29%3B%0A%7D%0A%0Afn+call_me%28num%29+%7B%0A++++for+i+in+0..num+%7B%0A++++++++println%21%28%22Ring%21+Call+number+%7B%7D%22%2C+i+%2B+1%29%3B%0A++++%7D%0A%7D%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%2F%2F+Rust+requires+that+all+parts+of+a+function%27s+signature+have+type+annotations%2C%0A%2F%2F+but+%60call_me%60+is+missing+the+type+annotation+of+%60num%60.%0A)
+- ["functions3.rs"](http://play.rust-lang.org/?code=%2F%2F+Make+me+compile%21+Scroll+down+for+hints+%3A%29%0A%0Afn+main%28%29+%7B%0A++++call_me%28%29%3B%0A%7D%0A%0Afn+call_me%28num%3A+i32%29+%7B%0A++++for+i+in+0..num+%7B%0A++++++++println%21%28%22Ring%21+Call+number+%7B%7D%22%2C+i+%2B+1%29%3B%0A++++%7D%0A%7D%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%2F%2F+This+time%2C+the+function+*declaration*+is+okay%2C+but+there%27s+something+wrong%0A%2F%2F+with+the+place+where+we%27re+calling+the+function.%0A)
+- ["functions4.rs"](http://play.rust-lang.org/?code=%2F%2F+Make+me+compile%21+Scroll+down+for+hints+%3A%29%0A%0A%2F%2F+This+store+is+having+a+sale+where+if+the+price+is+an+even+number%2C+you+get%0A%2F%2F+10+%28money+unit%29+off%2C+but+if+it%27s+an+odd+number%2C+it%27s+3+%28money+unit%29+less.%0A%0Afn+main%28%29+%7B%0A++++let+original_price+%3D+51%3B%0A++++println%21%28%22Your+sale+price+is+%7B%7D%22%2C+sale_price%28original_price%29%29%3B%0A%7D%0A%0Afn+sale_price%28price%3A+i32%29+-%3E+%7B%0A++++if+is_even%28price%29+%7B%0A++++++++price+-+10%0A++++%7D+else+%7B%0A++++++++price+-+3%0A++++%7D%0A%7D%0A%0Afn+is_even%28num%3A+i32%29+-%3E+bool+%7B%0A++++num+%25+2+%3D%3D+0%0A%7D%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%2F%2F+The+error+message+points+to+line+10+and+says+it+expects+a+type+after+the%0A%2F%2F+%60-%3E%60.+This+is+where+the+function%27s+return+type+should+be--+take+a+look+at%0A%2F%2F+the+%60is_even%60+function+for+an+example%21%0A)
+
 ### Uncategorized
 
 A few exercises based on things I've encountered or had trouble with getting used to.

functions/functions1.rs

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+// Make me compile! Scroll down for hints :)
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+fn main() {
+    call_me();
+}
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+// This main function is calling a function that it expects to exist, but the
+// function doesn't exist. It expects this function to have the name `call_me`.
+// It expects this function to not take any arguments and not return a value.
+// Sounds a lot like `main`, doesn't it?

functions/functions2.rs

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+// Make me compile! Scroll down for hints :)
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+fn main() {
+    call_me(3);
+}
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+fn call_me(num) {
+    for i in 0..num {
+        println!("Ring! Call number {}", i + 1);
+    }
+}
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+// Rust requires that all parts of a function's signature have type annotations,
+// but `call_me` is missing the type annotation of `num`.

functions/functions3.rs

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+// Make me compile! Scroll down for hints :)
+
+fn main() {
+    call_me();
+}
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+fn call_me(num: i32) {
+    for i in 0..num {
+        println!("Ring! Call number {}", i + 1);
+    }
+}
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+// This time, the function *declaration* is okay, but there's something wrong
+// with the place where we're calling the function.

functions/functions4.rs

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+// Make me compile! Scroll down for hints :)
+
+// This store is having a sale where if the price is an even number, you get
+// 10 (money unit) off, but if it's an odd number, it's 3 (money unit) less.
+
+fn main() {
+    let original_price = 51;
+    println!("Your sale price is {}", sale_price(original_price));
+}
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+fn sale_price(price: i32) -> {
+    if is_even(price) {
+        price - 10
+    } else {
+        price - 3
+    }
+}
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+fn is_even(num: i32) -> bool {
+    num % 2 == 0
+}
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+// The error message points to line 10 and says it expects a type after the
+// `->`. This is where the function's return type should be-- take a look at
+// the `is_even` function for an example!