Complicated SQL while loop - sql

Question

I am trying to create a while loop in SQL and it seems kind of complex. Here's what I need it to achieve:
Iterate through a single VARCHAR string (ex. '123')
If the nth character is in an even position in the string (ex. 2nd, 4th .... letter in the string), it must be added(SUM) to a base variable (Let's assume #z)
If the nth character is in an odd position in the string (ex. 1st, 3rd .... letter in the string), it must be multiplied by 2. If this newly generated value (Let's assume #y) is less than 10, it must be added(SUM) to the base variable (Still the same assumed #z). If #y is greater than 10, we need to subtract 9 from #y before adding(SUM) it to #z
After iterating through the entire string, this should return a numeric value generated by the above process.
Here is what I've done so far, but I'm stuck now (Needless to say, this code does not work yet, but I think I'm heading in the right direction):
DECLARE #x varchar(20) = '12345'
DECLARE #p int = len(#x)
WHILE #p > 0
SELECT #x =
stuff(#x, #p, 1,
case when CONVERT(INT,substring(#x, #p, 1)) % 2 = 0 then CONVERT(INT, #x) + CONVERT(INT,substring(#x, #p, 1))
end), #p -= 1
RETURN #x;
PS. The input will always be 100% numeric values, but it is formatted as VARCHAR when I recieve it.
UPDATE
The expected result for the sample string is 15

Answer 1

You can do this without using a loop. Here is a solution using Tally Table:
DECLARE #x VARCHAR(20) = '12345'
DECLARE #z INT = 0 -- base value
;WITH E1(N) AS( -- 10 ^ 1 = 10 rows
SELECT 1 FROM(VALUES (1),(1),(1),(1),(1),(1),(1),(1),(1),(1))t(N)
),
E2(N) AS(SELECT 1 FROM E1 a CROSS JOIN E1 b), -- 10 ^ 2 = 100 rows
E4(N) AS(SELECT 1 FROM E2 a CROSS JOIN E2 b), -- 10 ^ 4 = 10,000 rows
CteTally(N) AS(
SELECT TOP(LEN(#x)) ROW_NUMBER() OVER(ORDER BY(SELECT NULL))
FROM E4
),
CteChars(N, num) AS(
SELECT
t.N, CAST(SUBSTRING(#x, t.N, 1) AS INT)
FROM CteTally t
WHERE t.N <= LEN(#x)
)
SELECT
SUM(
CASE
WHEN N % 2 = 0 THEN num
WHEN num * 2 < 10 THEN num * 2
ELSE (num * 2) - 9
END
) + #z
FROM CteChars
The CTEs up to CteTally generates a list of number from 1 to LEN(#x). CteChars breaks #x character by character into separate rows. Then the final SELECT does a SUM based on the conditions.
OUTPUT : 15

Answer 2

Check below if it helps you
DECLARE #x varchar(20) = '12345'
DECLARE #p int = 1
DECLARE #result bigint=0;
DECLARE #tempval int =0;
WHILE #p <= len(#x)
BEGIN
SET #tempval = CONVERT(INT,substring(#x, #p, 1));
if(#p%2 = 1)
BEGIN
SET #tempval = #tempval * 2;
IF(#tempval >= 10) SET #tempval = #tempval - 9;
END
SET #result = #result + #tempval;
SET #p = #p + 1;
END;
PRINT #result;--This is the result
RETURN #x;

Answer 3

DECLARE #x INT = 12345
DECLARE #p int = len(#x)
DECLARE #z INT =0
PRINT #p%2
SET #x=#x/10
PRINT #x
WHILE #p > 0
BEGIN
IF(#p%2 = 0)
BEGIN
SET #z=#z+#x%10
SET #p=#p-1
SET #x=#x/10
END
ELSE
BEGIN
SET #z=#z+(2*(#x%10))
SET #p=#p-1
SET #x=#x/10
IF(#x>=10)
BEGIN
SET #x=(#x/10+#x%10)
END
END
END
SELECT #z

Answer 4

The while loop does not seem necessary here.
This can be achieved with a CTE that will split the string and a case statement:
DECLARE #x varchar(20) = '12345';
with split(id, v) as (
select 0, cast(0 as tinyint)
union all
select id+1, cast(SUBSTRING(#x, id+1, 1) as tinyint)
From split
Where id+1 <= len(#x)
)
Select Result = SUM(
Case When id % 2 = 0 then v
When v < 5 then v*2
Else (v*2)-9
End
)
From split
output = 15