MAINT: Eliminate ambiguous order of evaluation in the ratios of random variates. (#29598)

zuhu2195 · jorenham · web-flow · commit 7f9339b1bcb2 · 2025-08-23T07:15:45.000-04:00
In C, in an expression such as f1()/f2(), the order of evaluation of f1()
and f2() is unspecified.  When such a ratio was used in the C functions
random_f and random_standard_cauchy, it meant the streams of random
variates for the f and standard_cauchy distributions depended on how the
compiler chose to order the evaluation.  This could result in different
streams of variates when numpy is compiled with different compilers.

By evaluating the numerator and denominator in separate statements, the
ambiguity is eliminated.


Co-authored-by: Joren Hammudoglu &lt;jhammudoglu@gmail.com&gt;
diff --git a/numpy/random/src/distributions/distributions.c b/numpy/random/src/distributions/distributions.c
@@ -468,12 +468,15 @@ double random_chisquare(bitgen_t *bitgen_state, double df) {
 }
 
 double random_f(bitgen_t *bitgen_state, double dfnum, double dfden) {
-  return ((random_chisquare(bitgen_state, dfnum) * dfden) /
-          (random_chisquare(bitgen_state, dfden) * dfnum));
+  double subexpr1 = random_chisquare(bitgen_state, dfnum) * dfden;
+  double subexpr2 = random_chisquare(bitgen_state, dfden) * dfnum;
+  return subexpr1 / subexpr2;
 }
 
 double random_standard_cauchy(bitgen_t *bitgen_state) {
-  return random_standard_normal(bitgen_state) / random_standard_normal(bitgen_state);
+  double subexpr1 = random_standard_normal(bitgen_state);
+  double subexpr2 = random_standard_normal(bitgen_state);
+  return subexpr1 / subexpr2;
 }
 
 double random_pareto(bitgen_t *bitgen_state, double a) {

Original file line number	Diff line number	Diff line change
`@@ -468,12 +468,15 @@ double random_chisquare(bitgen_t *bitgen_state, double df) {`
`468`	`468`	`}`
`469`	`469`
`470`	`470`	`double random_f(bitgen_t *bitgen_state, double dfnum, double dfden) {`
`471`		`- return ((random_chisquare(bitgen_state, dfnum) * dfden) /`
`472`		`- (random_chisquare(bitgen_state, dfden) * dfnum));`
	`471`	`+ double subexpr1 = random_chisquare(bitgen_state, dfnum) * dfden;`
	`472`	`+ double subexpr2 = random_chisquare(bitgen_state, dfden) * dfnum;`
	`473`	`+ return subexpr1 / subexpr2;`
`473`	`474`	`}`
`474`	`475`
`475`	`476`	`double random_standard_cauchy(bitgen_t *bitgen_state) {`
`476`		`- return random_standard_normal(bitgen_state) / random_standard_normal(bitgen_state);`
	`477`	`+ double subexpr1 = random_standard_normal(bitgen_state);`
	`478`	`+ double subexpr2 = random_standard_normal(bitgen_state);`
	`479`	`+ return subexpr1 / subexpr2;`
`477`	`480`	`}`
`478`	`481`
`479`	`482`	`double random_pareto(bitgen_t *bitgen_state, double a) {`