Use one cell to conditionally set the value of another cell

[Koeilik]

Hi. So I have a situation where I would like to manually input data OR have a formula calculate a value for a certain cell.
The situation is as per the table below and is for a travel logbook. The formula in B4 is obviously not correct but describes what I want to do in excel terms.

Start odometer km	End odometer km	Distance traveled
[A2]	[B2]	[C2]	= if(Not(IsBlank(A2)) & Not(IsBlank(B2)), set(C2, B2 - A2))

If there is no value in A2 and B2 then nothing should happen to C2. If there are values in A2 and B2 then C2 should be set to the difference between them. This way Column C can be manually filled in incase the driver forgot to check the odometer readings at the start/end of each trip (as is often the case).

Example of execution below:

Start odometer km	End odometer km	Distance traveled
			{user entered value in C2; A2 & B2 are blank so no calculation is made}
86520	87156	636	{A3 & B3 is not blank; C3 is calculated and set according to B3 - A3}

I thought of leaving Column C without a formula to allow for user input. If values in Columns A and B are inserted, a separate cell (e.g. column D as per tables above) will remotely change the value in column C. Is there a way to achieve this?

 

[6StringJazzer]

You had the right idea as far as the logic, but you can't leave column C blank and still have it calculate a value, unless you use VBA. In C2 use this formula:

=IF(AND(A2<>"",B2<>""),B2-A2,"")

and copy down. When would you want to allow user input there?