T SQL Cursor is only updating row with same value - sql-server

Question

I'm using MS SQL Server and I have the below table 'tmp_AVG_Weekly_Sales':
RowID SKU Shop Week Avg_Value LAMBDA PMF Value
1 ABC 200 2 1 2 0.13 NULL
2 DEF 250 2 2 4 0.018 NULL
3 XYZ 300 3 3 6 0.0024 NULL
I need to work out the Value field based on the below logic - I am using a Cursor and Loop:
DECLARE #CUMULATIVE AS FLOAT = 0;
DECLARE #COUNT AS INT = 0;
DECLARE #LAMBDA AS FLOAT;
DECLARE #RowID AS INT;
DECLARE #PoissonCursor AS CURSOR;
DECLARE #THRESHOLD AS FLOAT = 0.99;
DECLARE #PMF AS FLOAT --= EXP(-#LAMBDA)
SET #PoissonCursor = CURSOR FOR
SELECT RowID
FROM
[tmp_AVG_Weekly_Sales]
OPEN #PoissonCursor;
FETCH NEXT FROM #PoissonCursor INTO #RowID;
WHILE ##FETCH_STATUS = 0
BEGIN
SELECT #LAMBDA = LAMBDA FROM [tmp_AVG_Weekly_Sales] WHERE RowID = #RowID
SELECT #PMF = PMF FROM [tmp_AVG_Weekly_Sales] WHERE RowID = #RowID
WHILE (#CUMULATIVE < #Threshold)
BEGIN
SET #CUMULATIVE += #PMF
SET #COUNT += 1
SET #PMF = #PMF * (#LAMBDA / #COUNT)
END
UPDATE [tmp_AVG_Weekly_Sales] SET [Value] = #COUNT - 1 WHERE RowID = #RowID
FETCH NEXT FROM #PoissonCursor INTO #RowID;
END
However, the above is just populating the Value field with the same value:
RowID SKU Shop Week Avg_Value LAMBDA PMF Value
1 ABC 200 2 1 2 0.13 6
2 DEF 250 2 2 4 0.018 6
3 XYZ 300 3 3 6 0.0024 6
When I am expecting the below:
RowID SKU Shop Week Avg_Value LAMBDA PMF Value
1 ABC 200 2 1 2 0.13 6
2 DEF 250 2 2 4 0.018 9
3 XYZ 300 3 3 6 0.0024 12
Where am I going wrong?

Answer 1

You never reset #CUMULATIVE or increase #Threshold, so the block of SET calls are only executed the first go through and each subsequent UPDATE just uses those original values.