Adding value plus 1 each time as an update

Question

In Advantage trying to update a value in a temp table, but adding 1 each time. The starting value needs to be one more than the max value from another table, and each time a new row is updated, one is added to that. Ignore all but the declare cursor and @nevid strings at the top. I can get it to populate the evid column in my temp table, but it adds 71 to each record, which is the correct next number, but I need it to be 71,72,73 etc. Where am I going wrong?

DECLARE cur CURSOR;
DECLARE @nevid INTEGER;
DECLARE @startdate string;
DECLARE @starttime string;
DECLARE @expectedenddate string;
DECLARE @expectedendtime string;
DECLARE @enddate string;
DECLARE @endtime string;
@nevid = 
( 
       SELECT (max(evid)+1) 
       FROM   pcplevnt
);

SELECT * 
INTO   #tmpev 
FROM   <table>;open cur 
AS 
  SELECT * 
  FROM   #tmpev;
    WHILE
        FETCH cur do 
        UPDATE #tmpev 
        SET evid = cast(@nevid AS sql_char(4));SET @nevid = @nevid + 1;
    END WHILE;
close cur;

Answer 1

I have build a MVCE to reproduce and fix your problem:

DECLARE cur    CURSOR;
DECLARE @nevid INTEGER;

TRY DROP TABLE #pcplevnt; CATCH ALL END TRY; 
TRY DROP TABLE #tmpev;    CATCH ALL END TRY; 

CREATE TABLE
  #pcplevnt
(
  evid INTEGER
);

DELETE FROM #pcplevnt;
INSERT INTO #pcplevnt (evid) SELECT 111 FROM system.iota;

SET @nevid = (
SELECT
  max(evid) + 1
FROM #pcplevnt
);

CREATE TABLE
  #tmpev
(
  id AUTOINC,
  evid NVARCHAR(4)
);

INSERT INTO
  #tmpev
(
  evid
)
      SELECT '1' FROM system.iota
UNION SELECT '2' FROM system.iota
UNION SELECT '3' FROM system.iota
;

OPEN cur AS SELECT * FROM #tmpev;

WHILE FETCH cur DO
  UPDATE
    #tmpev
  SET
    evid = CAST(@nevid AS SQL_CHAR(4))
  WHERE
    id = cur.id
  ;
  SET @nevid = @nevid + 1;
END WHILE;

CLOSE cur;

select * from #tmpev;

Pay attention to the WHERE condition in the UPDATE inside of the loop:

  UPDATE
    #tmpev
  SET
    evid = CAST(@nevid AS SQL_CHAR(4))
  WHERE
    id = cur.id
  ;
  SET @nevid = @nevid + 1;

I have added a primary key field id that I can compare against inside the loop in order to only update one row of the temp table at once.