I have 2 columns, a and b, as follows -

a1 0
a2 1
a3 text
a4 10
a5 more text
a6 4
a7 3
a8 23
a9 1
a10 3


b1
b2
b3
b4 12
b5
b6
b7
b8 25
b9
b10


What I want to do is replace the contents of A with the contents of B if it
exists, otherwise leave it as-is.

eg.
a1 0
a2 1
a3 text
a4 12
a5 more text
a6 4
a7 3
a8 25
a9 1
a10 3


How can I do this?

Is there a way for a formula to write to a different cell? eg. C1 --
=if(b1<"","",a1=b1)

ps. the columns are 60600 rows long, and the numbers in B are at irregular
intervals.