Loops/Increment loop index within loop body

Sometimes, one may need (or want) a loop which its iterator (the index variable) is modified within the loop body.

Goal

Demonstrate the best way to accomplish this.

Task

Write a loop which:

starts the index (variable) at 42
(at iteration time) increments the index by unity
if the index is prime:

displays the index and the prime (to the terminal)
increments the index such that the new index is now that prime

terminates the loop when 42 primes are shown

Extra credit: because of the primes get rather large, use commas within the displayed primes to ease comprehension.

Show all output here.

Note

Not all programming languages allow the modification of a loop's index. If that is the case, then use whatever method that is appropriate or idiomatic for that language. Please add a note if the loop's index isn't modifiable.

Related tasks

REXX

<lang rexx>/*REXX pgm displays primes found: starting Z at 42, if Z is a prime, add Z, else add 1.*/ numeric digits 20; d=digits() /*ensure enough decimal digits for Z. */ parse arg limit . /*obtain optional arguments from the CL*/ if limit== | limit=="," then limit=42 /*Not specified? Then use the default.*/ n=0 /*the count of number of primes found. */

    do z=42  until n==limit                     /* ◄──this DO loop's index is modified.*/
    if isPrime(z)  then do;  n=n + 1            /*Z  a prime?  Them bump prime counter.*/
                             say right('n='n, 9)     right(commas(z), d)
                             z=z + z - 1        /*also, bump the  DO  loop index  Z.   */
                        end
    end   /*z*/                                 /* [↑] a small tribute to Douglas Adams*/

exit /*stick a fork in it, we're all done. */ /*──────────────────────────────────────────────────────────────────────────────────────*/ commas: parse arg _; do j=length(_)-3 to 1 by -3; _=insert(',', _, j); end; return _ /*──────────────────────────────────────────────────────────────────────────────────────*/ isPrime: procedure; parse arg #; if wordpos(#, '2 3 5 7')\==0 then return 1

                                        if # // 2==0 | # // 3    ==0  then return 0
          do j=5  by 6  until j*j>#;    if # // j==0 | # // (J+2)==0  then return 0
          end   /*j*/
        return 1                                /*Exceeded sqrt(#)?  Then  #  is prime.*/</lang>

output:

      n=1                   43
      n=2                   89
      n=3                  179
      n=4                  359
      n=5                  719
      n=6                1,439
      n=7                2,879
      n=8                5,779
      n=9               11,579
     n=10               23,159
     n=11               46,327
     n=12               92,657
     n=13              185,323
     n=14              370,661
     n=15              741,337
     n=16            1,482,707
     n=17            2,965,421
     n=18            5,930,887
     n=19           11,861,791
     n=20           23,723,597
     n=21           47,447,201
     n=22           94,894,427
     n=23          189,788,857
     n=24          379,577,741
     n=25          759,155,483
     n=26        1,518,310,967
     n=27        3,036,621,941
     n=28        6,073,243,889
     n=29       12,146,487,779
     n=30       24,292,975,649
     n=31       48,585,951,311
     n=32       97,171,902,629
     n=33      194,343,805,267
     n=34      388,687,610,539
     n=35      777,375,221,081
     n=36    1,554,750,442,183
     n=37    3,109,500,884,389
     n=38    6,219,001,768,781
     n=39   12,438,003,537,571
     n=40   24,876,007,075,181
     n=41   49,752,014,150,467
     n=42   99,504,028,301,131