While trying to calculate pIC50 values I found a difference in behaviour from the Math Formula to the Expression node. I found a solution, but thought I share it here in case someone else runs into the same issue.
In the Math Formula node, I used to calculate pIC50 (the negative logarithm of molar IC50 values) with the following formula (starting from IC50 values in nanomolar range):
-log($Standard Value$*pow(10,-9)) where standard value is the column with my nanomolar IC50 values, which need to be multiplied with 10^-9 to convert to molar values. For a value of 64.5 this results in 7.19, which is the expected behaviour.
Using the corresponding formula in the Expression node - log10($["Standard Value"] * pow(10,-9)) however returns -infinity, due to a rounding issue I believe. Only by scrolling down in the description of the pow function, one can see the solution to this changed behaviour in the examples:
-
pow(2, 3)returns 8 -
pow(2.0, 3)returns 8.0
So pow(10,-9) returns 0, leading to the infinity result of the full formula. The solution is to use pow(10.0,-9) or (as suggested by K-AI): use 1e-9 for clarity and proper syntax.
For the developers: Not sure if this is a bug or probably expected behaviour. But I think returning 0 (integer?) while calculating a number to the power of a negative number is not intuitive, and it would be great if this would be treated as a float number instead.
(used KNIME version: 5.5.0, Windows)