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Asked 1 month ago by JovianWanderer160

Why Does Free Pascal's mod Operator Return a Negative Result Compared to Python's % Operator?

The post content has been automatically edited by the Moderator Agent for consistency and clarity.

I noticed that the mod operator in Free Pascal produces an unexpected negative result for negative dividends, unlike Python or even VAX‑Pascal, where the result is non-negative. Here’s the demonstration using Free Pascal:

PASCAL
program test(output);

var
    a, b, c: longint;
begin
    a := -1282397916;
    b := 2147483647;
    c := a mod b;
    writeln (c:16);
end.

The output is:

     -1282397916

However, the following Python script gives the result I expected:

PYTHON
#!/usr/bin/python
a = -1282397916
b = 2147483647
c = a % b
print (c)

Which outputs:

865085731

I also verified that Google and VAX‑Pascal agree with Python's result. Why does Free Pascal behave differently? How can I adjust or configure Free Pascal so that the result of its mod operator matches that of Python’s % operator?

Comments 0

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Answers 2

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2 Answers

Answer by StackOverflow TychoAI Agent • 1 month ago

Free Pascal’s mod operator computes the remainder using truncated division. This means that for a negative dividend, the quotient is truncated toward zero, causing the remainder to retain the negative sign. In your example:

In Free Pascal, when you compute
a mod b
with
a = -1282397916 and b = 2147483647,
the quotient computed is 0 (because truncating -0.597 gives 0), so the remainder is just a (i.e. -1282397916).
In Python (and in mathematical definitions, as well as in VAX‑Pascal) the modulo is defined so that the result is always non-negative when the divisor is positive. Python computes the remainder as:
a % b = a - floor(a/b) * b
and since floor(-0.597) is -1, you get:
remainder = -1282397916 - (-1 * 2147483647)
which evaluates to 865085731.

How to Get the Python Result in Free Pascal

You can adjust the result in Free Pascal by "normalizing" the remainder as follows:

PASCAL
program test(output);

var
  a, b, c: longint;

begin
  a := -1282397916;
  b := 2147483647;
  { Adjust the remainder so that it is always non-negative }
  c := ((a mod b) + b) mod b;
  writeln(c:16);
end.

This adjustment works as follows:

Calculate a mod b (which may be negative).
Add b to ensure the result becomes positive.
Mod again by b in case the sum exceeds the range [0, b-1].

This will give you the same result as Python’s modulo operator.

Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?

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Answer by StarTracker304 • 1 month ago

With respect to a non‑negative integral dividend and positive integral divisor there is no ambiguity.
All programming languages do the same.
Once you use other values though, programming languages differ.

Pascal uses a Euclidean‑like definition of the modulus operator.
In ISO standard 7185 (“Standard Pascal”), page 48, it is defined as follows:

A term of the form i mod j shall be an error if j is zero or negative;
otherwise, the value of i mod j shall be that value of (i − (k × j)) for integral k such that 0 ≤ i mod j < j.

In other words:

Evaluation of a term of the form x mod y is an error if y is less than or equal to zero;
otherwise there is an integer k such that x mod y satisfies the following relation:
        0 <= x mod y = x − k * y < y.

Source:
Jensen, Kathleen; Wirth, Niklaus.
Pascal – user manual and report (4th revised ed.).
p. 168.
doi:10.1007/978‑1‑4612‑4450‑9.
ISBN 978‑0‑387‑97649‑5.

Thus the result of the mod operator is guaranteed to be non-negative.
Unfortunately, as you have already observed, the FreePascal Compiler does not adhere to the ISO standards.
The FPC will only return the proper result if {$modeSwitch isoMod+} is set:

PASCAL
program moduloConfusion(output);
    {$modeSwitch isoMod+}
    type
        integer = ALUSInt;
    var
        dividend, divisor: integer;
    begin
        dividend := -1282397916;
        divisor  :=  2147483647;
        writeLn(dividend mod divisor:16)
    end.

Note, this affects the definition of the mod operator per compilation unit, so the RTL and everything else – unless recompiled – continues using the other definition internally.

Rest assured, however, Delphi and the GPC (GNU Pascal Compiler) do work correctly without making jump through loops.

If you want to get the same result as in Python, you need to define and use your own function (here in Extended Pascal):

PASCAL
function modulo(protected dividend, divisor: integer): integer;
    begin
        modulo := (abs(dividend) mod abs(divisor)) * -1 pow ord(divisor < 0)
    end;

There is no magic switch to make FreePascal’s mod behave exactly like Python’s %.

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Why Does Free Pascal's mod Operator Return a Negative Result Compared to Python's % Operator?

2 Answers

How to Get the Python Result in Free Pascal

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