Language Constructs

🎶
Oliver’s Notes

Cascading if

I might teach this indentation pattern for if when there’s more than two arms, as an alternative to actually nesting them. (The parse tree is still nested, but the indentation level isn’t.)

if (expr == 1) {
  …
} else {
  if (expr == 2) {
    …
  } else {
    if (expr == 3) {
      …
    } else {
      …
    }
  }
}

⇒

if (expr == 1) {
  …
} else if (expr == 2) {
  …
} else if (expr == 3) {
  …
} else {
  …
}

switch / case

I like to introduce switch as an alternative to “chained” if statements:

if (expr == 1) {
  …
} else if (expr == 2) {
  …
} else if (expr == 3) {
  …
} else {
  …
}

⇒

switch (expr) {
  case 1:
    …
    break;
  case 2:
    …
    break;
  case 3:
    …
    break;
  default:
    …
}

that has the advantages:

  • Communicates the design intent – this is the most important
  • Only evaluates expr once (doesn’t matter if expr is a variable)

Note: Having to use break (in C/C++/Arduino, but lots of other languages too) is a gotcha and an annoyance though.

Note: It took me decades to get straight that switch goes on the outside and case is the thing that goes inside. Then I made the mnemonic that there are a number of different cases, and I haven’t gotten confused since. (It’s so direct and obvious that it’s barely a mnemonic, but I needed it.) I think if I’d thought more about mechanical switches I could have used that as mnemonic too.

The else is optional

In a chained if, the else is optional. In a switch/case, the default is optional.

if (expr == 1) {
  …
} else if (expr == 2) {
  …
} else if (expr == 3) {
  …
}
switch (expr) {
  case 1:
    …
    break;
  case 2:
    …
    break;
  case 3:
    …
    break;
}

Optional break

The break comes in handy when two (or more) cases share the same code:

if (expr == 1) {
  // executed when expr is 1
} else if (expr == 2 || expr == 3) {
  // executed when expr is 2 or 3
}
switch (expr) {
  case 1:
    // executed when expr is 1
    break;
  case 2:
  case 3:
    // executed when expr is 2 or 3
    break;
}