Control flow in Toit

If-statements

If-statements work in a very similar way to other languages. The condition is terminated with a colon, :, and the body is either a one-liner on the same line, or delimited by indentation:

  if i == 42: print "Found the answer!"
  if i < 100:
    print "Found a smallish number!"

Else-clauses work in the same way:

  if i % 3 == 0:
    print "fizz"
  else:
    print i

Cascaded ifs can be written with else if:

  if i % 3 == 0:
    if i % 5 == 0:
      print "fizzbuzz"
    else:
      print "fizz"
  else if i % 5 == 0:
    print "buzz"
  else:
    print i

Sometimes you may prefer the ternary ?: operator instead of small ifs:

  buzz := i % 5 == 0
  if i % 3 == 0:
    print (buzz ? "fizzbuzz" : "fizz")
  else if buzz:
    print "buzz"
  else:
    print i

Loops

Often, loops are used to iterate over a collection. In this case you should usually use the built-in do method of the Collection class. Some collections also have do --reversed for looping from last to first.

  // Loop over all elements in a list:
  list.do: | element |
    print element

  // Loop over elements 10 to 19 of a list, using a slice:
  list[10..20].do: | element |
    print element

  // Loop over all elements, except the first (the one with
  // index zero):
  list[1..].do:
    print it

At an even higher level, the built-in collections have some functional-style methods, like any, every, reduce and map that iterate over collections for you

However, sometimes you have to write your own loops. The rest of this section will explain how that works in Toit.

If you simply want to execute a block of code multiple times you can use repeat which is a method on the int class that takes a Toit block.

Example of a loop that runs a fixed number of times, given by the end integer variable.

  end.repeat: | i | print i

To use steps, use a for loop as in:

  for i := start; i < end; i += step: print i

Use a while loop to execute a code block as long as a condition is true, as in the following example:

main:
  max := 5
  counter := 0

  while counter < max:
    print counter
    counter += 1

Breaking loops

Sometimes, it is necessary to terminate a for loop or a while loop prematurely regardless of the results of the conditional tests. In these cases, you can use the break statement:

main:
  max := 50
  counter := 0

  while counter < max:
    print counter
    counter += 1
    if counter > 20: break

Similarly you can use the continue statement to skip the rest of a loop and go straight to the next iteration:

main:
  sum := 0
  for i := 0; i < 10; i++:
    log sum
    if sum >= 10: continue
    // This line is skipped if sum is already >= 10.
    sum += i

The for and while loops are control structures built into the language, which support break and continue. On the other hand, repeat and do are methods that take blocks. Here you have to add the method name (do or repeat) to the continue statement, to indicate which loop you wish to continue.

main:
  sum := 0
  10.repeat:
    log sum
    if sum > 10:
      continue.repeat  // Skip the rest of the block.
    sum += it

The equivalent break.do and break.repeat are not yet implemented.

Sometimes you can instead use a return statement that exits the method completely:

my_function collection:
  collection.do:
    if it.is_the_one_we_want:
      // Returns from my_function, also breaks out of the
      // loop.
      return it
  return null

If this is not possible, and you need an actual break statement, the do or repeat can be rewritten with a for loop.