Async and Practical

Histogram Counts as Bars

Tallying a small sample into a Map<String, int> and rendering each count as a string of # characters builds a tiny histogram. The bar width is just '#' * count, which makes the proportions visible at a glance.

Program

Play the program to count six samples and render them as compact bars.

histogram.dart
Replay: real traced execution (multi-file project)
void main() {
  var samples = ['red', 'blue', 'red', 'green', 'red', 'blue'];
  var counts = <String, int>{};
  for (var s in samples) {
    counts[s] = (counts[s] ?? 0) + 1;
  }
  var bars = counts.entries
      .map((e) => '${e.key}: ${'#' * e.value}')
      .toList();
  print(bars.join(' | '));
}

  1. samples ← [red, blue, red, green, red, blue]

    1void main() {2  var samples = ['red', 'blue', 'red', 'green', 'red', 'blue'];3  var counts = <String, int>{};
    values this step[red, blue, red, green, red, blue]samples

  2. counts ← {}

    2var samples = ['red', 'blue', 'red', 'green', 'red', 'blue'];3var counts = <String, int>{};4for (var s in samples) {
    values this step{}counts

  3. loop ← tally 6 samples

    3var counts = <String, int>{};4for (var s in samples) {5  counts[s] = (counts[s] ?? 0) + 1;
    values this steptally 6 samplesloop

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

    4for (var s in samples) {5  counts[s] = (counts[s] ?? 0) + 1;6}
    values this step{red: 3, blue: 2, green: 1}counts

  5. bars ← red ### | blue ## | green #

    8    .map((e) => '${e.key}: ${'#' * e.value}')9    .toList();10print(bars.join(' | '));
    values this stepred ### | blue ## | green #bars

  6. print(bars.join(' | '));

    9      .toList();10  print(bars.join(' | '));11}
    outputred: ### | blue: ## | green: #
    values this stepred ### | blue ## | green #bars
counter map `counts[s] = (counts[s] ?? 0) + 1` defaults missing keys to zero before adding one.
string repetition `'#' * count` repeats `#` once per occurrence, turning a tally into a tiny bar.
insertion order Default Dart maps keep insertion order, so the bars render in first-seen order.