Return different result based on cell contents?

[Eat More Bacon]

If I use index match to refer to column A, how can I return column B if there is a value in column B, but return column C if there is no value in column B?

 

[p45cal]

Your version of Excel?

 

[p45cal]

Loads of ways, among them:
in pre Office365 (long winded):

=IF(INDEX($B$1:$B$3,MATCH(G10,$A$1:$A$3,0))="",INDEX($C$1:$C$3,MATCH(G10,$A$1:$A$3,0)),INDEX($B$1:$B$3,MATCH(G10,$A$1:$A$3,0)))

This might work pre Office365 (I can't test) but may need to be array-entered (Ctrl+Shift+Enter rather than just Enter when committing the formula to the sheet):

=INDEX(IF($B$1:$B$3="",$C$1:$C$3,$B$1:$B$3),MATCH(G10,$A$1:$A$3,0))

In Offices365:

=LET(a,MATCH(G10,$A$1:$A$3,0),b,INDEX($B$1:$B$3,a),IF(b="",INDEX($C$1:$C$3,a),b))

Pre Office365 without index/match:

=IF(VLOOKUP(G10,$A$1:$C$3,2,0)="",VLOOKUP(G10,$A$1:$C$3,3,0),VLOOKUP(G10,$A$1:$C$3,2,0))

where G10 contains what you're looking up.
I'm sure there'll be more elegant ways.

 

[Eat More Bacon]

I'm using Excel 2007.
Thanks for your tips. i notice there is one variable I forgot to mention.
G10 (my lookup) can be a value in either column A or B.
If it is a value in column B, I want to return C.
If it is a value in column A, I want to return Column B, unless Column B is empty, then I want to return Column C.

 

[p45cal]

Not sure if I've got the logic right, so check:

=IFERROR(IF(ISBLANK(VLOOKUP(G10,$A$1:$B$4,2,0)),VLOOKUP(G10,$A$1:$C$4,3,0),VLOOKUP(G10,$A$1:$B$4,2,0)),VLOOKUP(G10,$B$1:$C$4,2,0))

 

[Eat More Bacon]

Thanks