Change numbers if string changes

[HaroonSid]

hi,
i want to change numbers if string change of column b
for example
b1 = a then a1 = 1
b2 = a then a2 = 1
b3 = b then a3 = 2
b4 = b then a4 = 2
b5 = b then a5 = 2
b6 = b then a6 = 2
b7 = c then a7 = 3
b8 = d then a8 = 4
b9 = d then a9 = 4
b10 = d then a10 = 4

thank you

[Glenn Kennedy]

With data starting in ROW 2:

=IF(B2="","",IF(B2=B1,A1,A1+1))

[AlKey]

Try this
Enter formula in A1 and copy down

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v	A	B
1	1	a
2	1	a
3	2	b
4	2	b
5	3	c
6	4	d
7	4	d
8	4	d
9	5	s
10	6	q