Data commit

Task/Combinations/Stata/combinations-1.stata

@@ -0,0 +1,13 @@
+program combin
+tempfile cp
+tempvar k
+gen `k'=1
+quietly save "`cp'"
+rename `1' `1'1
+forv i=2/`2' {
+	joinby `k' using "`cp'"
+	rename `1' `1'`i'
+	quietly drop if `1'`i'<=`1'`=`i'-1'
+}
+sort `1'*
+end

Task/Combinations/Stata/combinations-2.stata

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+. set obs 5
+. gen a=_n
+. combin a 3
+. list
+
+     +--------------+
+     | a1   a2   a3 |
+     |--------------|
+  1. |  1    2    3 |
+  2. |  1    2    4 |
+  3. |  1    2    5 |
+  4. |  1    3    4 |
+  5. |  1    3    5 |
+     |--------------|
+  6. |  1    4    5 |
+  7. |  2    3    4 |
+  8. |  2    3    5 |
+  9. |  2    4    5 |
+ 10. |  3    4    5 |
+     +--------------+

Task/Combinations/Stata/combinations-3.stata

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+function combinations(n,k) {
+	a = J(comb(n,k),k,.)
+	u = 1..k
+	for (i=1; 1; i++) {
+		a[i,.] = u
+		for (j=k; j>0; j--) {
+			if (u[j]-j<n-k) break
+		}
+		if (j<1) return(a)
+		u[j..k] = u[j]+1..u[j]+1+k-j
+	}
+}
+
+combinations(5,3)