Async and Practical

Word Count with Map

Counting words is a classic map exercise: split the text, walk each word, and increment its tally in a map. Default Dart maps keep insertion order, so the summary stays stable.

Program

Play the program to tally six words and build a deterministic summary string.

word_count.dart
Replay: real traced execution (multi-file project)
void main() {
  var text = 'red blue red green blue red';
  var words = text.split(' ');
  var counts = <String, int>{};
  for (var w in words) {
    counts[w] = (counts[w] ?? 0) + 1;
  }
  var entries = counts.entries;
  var parts = entries.map((e) => '${e.key}:${e.value}');
  var summary = parts.join(',');
  print(summary);
}

  1. text ← red blue red green blue red

    1void main() {2  var text = 'red blue red green blue red';3  var words = text.split(' ');
    values this stepred blue red green blue redtext

  2. words ← [red, blue, red, green, blue, red]

    2var text = 'red blue red green blue red';3var words = text.split(' ');4var counts = <String, int>{};
    values this step[red, blue, red, green, blue, red]words

  3. counts ← {}

    3var words = text.split(' ');4var counts = <String, int>{};5for (var w in words) {
    values this step{}counts

  4. counts ← {red: 3, blue: 2, green: 1}

    4var counts = <String, int>{};5for (var w in words) {6  counts[w] = (counts[w] ?? 0) + 1;
    values this step{red: 3, blue: 2, green: 1}counts6 wordsloop

  5. entries ← red, blue, green

    7}8var entries = counts.entries;9var parts = entries.map((e) => '${e.key}:${e.value}');
    values this stepred, blue, greenentries

  6. parts ← [red:3, blue:2, green:1]

    8var entries = counts.entries;9var parts = entries.map((e) => '${e.key}:${e.value}');10var summary = parts.join(',');
    values this step[red:3, blue:2, green:1]parts

  7. summary ← red:3,blue:2,green:1

    9var parts = entries.map((e) => '${e.key}:${e.value}');10var summary = parts.join(',');11print(summary);
    values this stepred:3,blue:2,green:1summary

  8. print(summary);

    10  var summary = parts.join(',');11  print(summary);12}
    outputred:3,blue:2,green:1
    values this stepred:3,blue:2,green:1summary
split `'red blue ... red'.split(' ')` returns `[red, blue, red, green, blue, red]`.
counter map `?? 0` starts unseen words at zero before the program adds one.
insertion order Walking `counts.entries` keeps insertion order, so the joined summary is deterministic.