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While trying to understand logical 'or'/'and', I encountered another problem (I'm writing Python code here, but my question is about the logic, not about any given programming language). I have som...
#1: Initial revision
by
Karl Knechtel
·
2024-11-13T22:11:03Z (about 1 month ago)
Understanding "de Morgan's laws"
While trying to [understand logical 'or'/'and'](https://software.codidact.com/posts/292968), I encountered another problem (I'm writing Python code here, but my question is about the logic, not about any given programming language). I have some code like:
if x or y:
pass
else:
do_something()
(Of course, following the other Q&A, I made sure that the real `x` and `y` make sense independently as conditions.)
I wanted to focus on the `else` case, because it's actually interesting. I figured out that I could *negate* the condition to flip the cases, and then the `else: pass` part would be unnecessary:
```python
if not(x or y):
do_something()
```
However, *other obvious attempts did not work*:
```python
if not x or y:
do_something()
if not x or not y:
do_something()
if (not x) or (not y): # not even with more parentheses!
do_something()
```
I asked about this elsewhere and was told it had something to do with "de Morgan's laws". What does this mean? Can I use these "laws" to understand how to write the code correctly? How exactly do they apply?