DSXchange

Can you please explain it clearly

My question is it shoudl diplay 1 for first occurence of 'd' and 'c' and when ever 'd' comes after 'c' we have to increment the value, for example d--1 d--1 d--1 d--1 d--1 c--1 c--1 c--1 c--1 c--1 when 'd' comes after 'c' we have incrment d--2 d--2 d--2 c--2 c--2 Again here 'd comes after 'c' d--3 d...

give me the logic to generating sequence number without using surrogate key generator.

input data:
d
d
d
d
c
c
d
d
d
c
d
c
d
d
c
c

output;
d--1
d--1
d--1
d--1
c--1
c--1
d--2
d--2
d--2
c--2
d--3
c--3
d--4
d--4
c--4
c--4