IBM Mainframe Forum

by **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".

by **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.

by **gauravnnl** » Fri Aug 01, 2014 5:41 pm

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

Select all

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.

by **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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