Fix a cell value from another changing cell value

[Simon Ford]

How can I fix a cell value from another cell whose value will change e.g.

A1 = 300

B1 = 500

A1 TO CHANGE TO 500

B1 THEN CHANGES TO 0

A1 TO STAY AS 500 (FIXED)

 

[santeria]

What determines the Values of A1 and B1?
Is that from another cell, or ... ??

 

[Simon Ford]

A1 would be a fixed starting value until B1 temporarily populates (B1 will usually read 0). The value for B1 is updated from a LOOKUP table when conditions are TRUE, but will revert to 0 when they are not. At anytime that the value for B1 becomes greater than 0, A1 must take this as its new fixed value (and not revert to 0 or its original value).

 

[santeria]

Okay, not real clear on it, but sounds like an IF formula for A1 would work if B1 is determined by a lookup...
Of course, some combinations would be possible, if B1 has a reciprocal connection to A1, and not A1 just having the Linear connection to B1...