Nth Prime number



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Nth Prime number

Postby gauravnnl » Tue Jul 15, 2014 3:52 pm

Hi,

Can anyone help me to get the logic to find Nth prime number in cobol. Below is the logic to check if the number is prime or not.But a bit confused while implementing logic to get Nth prime number like if I enter 3 then it should give result as 5 and If i enter 4 then it should give 7 because the prime number series is like- 2,3,5,7,11,13 and so on.

WORKING-STORAGE SECTION.
77 N PIC 9(3).
77 Q PIC 9(3).
77 R PIC 9(3).
77 I PIC 9(3).

PROCEDURE DIVISION.
PARA-A.
DISPLAY ( 1 , 1 ) ERASE.
DISPLAY ( 2 , 1 ) "ENTER AN INTEGER:".
ACCEPT ( 2 , 20 ) N.
PERFORM PARA2 VARYING I FROM 2 BY 1 UNTIL I > N.
STOP RUN.

PARA2.
DIVIDE N BY I GIVING Q REMAINDER R.
IF R==0
DISPLAY(5, 1) "NOT PRIME"
ELSE
DISPLAY(5, 1) "PRIME".
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Re: Nth Prime number

 

Re: Nth Prime number

Postby Robert Sample » Tue Jul 15, 2014 4:31 pm

Your problem is actually two-fold:
1. Find all prime numbers up to a given value
2. Return the nth prime number.

For what you are wanting, a sieve of Eratosthenes would be a better way to go. Use a table and set the non-prime values to zero. When you have generated your prime values, to find the nth prime number, merely iterate through the table, returning the nth non-zero number.
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Re: Nth Prime number

Postby gauravnnl » Fri Aug 01, 2014 5:41 pm

Here is the code...if anyone want to refer--

IDENTIFICATION DIVISION.

PROGRAM-ID.    PRIME.

AUTHOR.        XYZ.

DATE-WRITTEN.  JULY 2014.

ENVIRONMENT DIVISION.
CONFIGURATION SECTION.
SOURCE-COMPUTER. IBM-370 WITH DEBUGGING MODE.

DATA DIVISION.
WORKING-STORAGE SECTION.

01 WS-VAR.
   10 WS-INPUT              PIC 9(04) VALUE ZEROES.

   10 WS-OUT                PIC 9(04) VALUE ZEROES.

   10 WS-TEMP-1             PIC 9(4)  VALUE 0.
   10 WS-TEMP-2             PIC 9(4)  VALUE 0.
   10 WS-TEMP-3             PIC 9(4)  VALUE 0.
   10 WS-TEMP-4             PIC 9(4)  VALUE 0.
   10 WS-TEMP-5             PIC 9(4)  VALUE 0.

PROCEDURE DIVISION.
    ACCEPT WS-INPUT.
    DISPLAY 'INPUT:' WS-INPUT
    MOVE 1 TO WS-TEMP-1
    MOVE 2 TO WS-OUT
    MOVE 2 TO WS-TEMP-2

    IF WS-TEMP-1 = WS-INPUT
      DISPLAY WS-INPUT 'TH PRIME NO IS:' WS-OUT
      GO TO 500-PARA
    ELSE
      COMPUTE WS-OUT = WS-OUT + 1
      PERFORM 100-PARA
    END-IF.

100-PARA.
    PERFORM 200-PARA UNTIL WS-TEMP-2 >= WS-OUT
    PERFORM 300-PARA.

200-PARA.
    DIVIDE WS-OUT BY WS-TEMP-2 GIVING    WS-TEMP-3
                               REMAINDER WS-TEMP-4
    IF WS-TEMP-4 = 0
      COMPUTE WS-OUT = WS-OUT + 1
      MOVE 2 TO WS-TEMP-2
    ELSE
      COMPUTE WS-TEMP-2 = WS-TEMP-2 + 1
    END-IF
      GO TO 100-PARA.

300-PARA.
    COMPUTE WS-TEMP-1 = WS-TEMP-1 + 1
    IF WS-TEMP-1 = WS-INPUT
      DISPLAY WS-INPUT 'TH PRIME NO IS:' WS-OUT
      GO TO 500-PARA
    ELSE
      COMPUTE WS-OUT = WS-OUT + 1
      MOVE 2 TO WS-TEMP-2
      GO TO 100-PARA
    END-IF.

500-PARA.
      DISPLAY WS-OUT
    STOP RUN.
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Re: Nth Prime number

Postby BillyBoyo » Fri Aug 01, 2014 6:50 pm

You really should try this without using GO TO. If you start your programming using GO TO like that, you're likely to continue that way, and your programs will be difficult to understand/maintain.

Another clarity issue is meaningful names for things. Give everything a meaningful name (c rubbish-name good-name ALL) and see how much more information your program immediately offers to the reader.
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