collections.D

The collections.D function returns a slice of N sequentially ordered unique random integers in the half-open interval [0, HIGH) using the provided SEED value. This function implements J. S. Vitter’s Method D¹ for sequential random sampling, a fast and efficient algorithm for this task.

See this article for a detailed explanation.

Examples

{{ collections.D 6 7 42 }} → [4, 9, 10, 20, 22, 24, 41]

The example above generates the same random numbers each time it is called. To generate a different set of 7 random numbers in the same range, change the seed value.

{{ collections.D 2 7 42 }} → [3, 11, 19, 25, 32, 33, 38]

A common use case is the selection of random pages from a page collection. For example, to render a list of 5 random pages using the day of the year as the seed value:

<ul>
  {{ $p := site.RegularPages }}
  {{ range collections.D time.Now.YearDay 5 ($p | len) }}
    {{ with (index $p .) }}
      <li><a href="{{ .RelPermalink }}">{{ .LinkTitle }}</a></li>
    {{ end }}
  {{ end }}
</ul>

The construct above is significantly faster than using the collections.Shuffle function.

Seed value

Choosing an appropriate seed value depends on your objective.