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RandomInt

function RandomInt(aMin, aMax: Integer): Integer;

Example​

procedure ScriptEvent(var Value: variant);
begin
  Value := RandomInt(1, 6); // an inclusive but biased pseudo-random value 1..6
end;

Usage​

RandomInt returns a biased rounded pseudo-random Integer between the two supplied bounds, including both endpoints.

Parameters

NameTypeDescription
aMinIntegerFirst inclusive bound and final offset.
aMaxIntegerSecond inclusive bound. It can be less than aMin, although the names imply otherwise.

Returns

For normal non-overflowing bounds and the default rounding mode, the result lies between the two values inclusive regardless of their order.

Selection guidance

If uniform discrete sampling matters, use a generator designed for integer ranges rather than rounded floating scaling. RandomizeSeed can reproduce a sequence in a controlled single-caller path, but it does not isolate that sequence from other process-wide random calls.

Additional Technical Info​

RandomInt scales Delphi's shared floating Random value across the difference between two Integers, rounds that value, and adds the first bound. Under the default nearest/even rounding mode it can return both endpoints.

It is not a uniform discrete-range generator: the two endpoints receive approximately half the probability of each interior integer. Do not use it for fair dice, unbiased sampling, cryptography or identifiers. The example is fictional and source-reviewed only.

Distribution

For ascending bounds with span N, scaled values lie in [0,N). Nearest rounding maps an interval of width about 0.5 to each endpoint and width about 1 to each interior integer. For RandomInt(1,6), values 1 and 6 are therefore each roughly half as likely as 2, 3, 4 or 5.

The active floating-point rounding mode affects the exact buckets and midpoint behavior. Default Delphi nearest/even treatment is not forced by the wrapper.

Implementation and state

  1. uPSI_vxCommonNumber registers the native function directly.
  2. (aMax - aMin) is calculated first as a 32-bit Integer expression.
  3. The span is multiplied by the next global System.Random Extended value.
  4. Delphi System.Round produces an Int64 offset.
  5. Adding aMin and assigning the function's Integer result completes the call.

The call advances the same global RandSeed used by every Random caller in the process.

Edge cases and errors

  • Equal bounds always return that value but still call and advance System.Random because the multiplication expression is evaluated.
  • Reversed bounds generally produce values across the reversed interval; they are not rejected or normalized.
  • The 32-bit subtraction can overflow/wrap because arithmetic overflow checking is not enabled in the custom unit. A wide interval such as Low(Integer) to High(Integer) does not produce its mathematical span and can yield a completely incorrect range.
  • Product range checking remains enabled in this custom unit; a final out-of-range Integer assignment can raise ERangeError.
  • Shared generator races and calls elsewhere make sequences coupled and non-repeatable in concurrent processing.

External references

Created 2026-07-15