Loops/Increment loop index within loop body: Difference between revisions

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=={{header|Common Lisp}}==

<lang lisp>

(defun primep (n) ; https://stackoverflow.com/questions/15817350/

(cond ((= 2 n) t) ; Hard-code "2 is a prime"

((= 3 n) t) ; Hard-code "3 is a prime"

((evenp n) nil) ; If we're looking at an even now, it's not a prime

(t ; If it is divisible by an odd number below its square root, it's not prime

(do* ((i 3 (incf i 2))) ; Initialize to 3 and increment by 2 on every loop

((or (> i (isqrt n)) ; Break condition index exceeds its square root

(zerop (mod n i))) ; Break condition it is divisible

(not (zerop (mod n i)))))))) ; Returns not divisible, aka prime

(do ((i 42) ; Initialize index to 42

(c 0)) ; Initialize count of primes to 0

((= c 42)) ; Break condition when there are 42 primes

(incf i) ; Increments index by unity

(if (primep2 i)(progn (incf c) ; If prime increment count of primes

(format t "~&~5<~d~;->~>~20<~:d~>" c i) ; Display count of primes found and the prime

(incf i (decf i))))) ; Increment index to previous index plus the prime

</lang>

{{out}}

<pre>

1 -> 43

2 -> 89

3 -> 179

4 -> 359

5 -> 719

6 -> 1,439

7 -> 2,879

8 -> 5,779

9 -> 11,579

10 -> 23,159

11 -> 46,327

12 -> 92,657

13 -> 185,323

14 -> 370,661

15 -> 741,337

16 -> 1,482,707

17 -> 2,965,421

18 -> 5,930,887

19 -> 11,861,791

20 -> 23,723,597

21 -> 47,447,201

22 -> 94,894,427

23 -> 189,788,857

24 -> 379,577,741

25 -> 759,155,483

26 -> 1,518,310,967

27 -> 3,036,621,941

28 -> 6,073,243,889

29 -> 12,146,487,779

30 -> 24,292,975,649

31 -> 48,585,951,311

32 -> 97,171,902,629

33 -> 194,343,805,267

34 -> 388,687,610,539

35 -> 777,375,221,081

36 -> 1,554,750,442,183

37 -> 3,109,500,884,389

38 -> 6,219,001,768,781

39 -> 12,438,003,537,571

40 -> 24,876,007,075,181

41 -> 49,752,014,150,467

42 -> 99,504,028,301,131