Python - and/or confusion!

[HMW]

Hi!

Check out the snippet below:

print "\nEnter names, \"Q\" to quit: 

string = raw_input()

if string == "Q" or string == "q":
	print "Exiting..."
	sys.exit(0)

while string != "Q" and string != "q":
	vector3.append(string)
	string = raw_input()

As you all can see, in the if-sequence I have an 'or', and it works that way. However; in the while-sequence I have an 'and', otherwise it does NOT work - I am truly confused by this and would appreciate any input.

Thanks for reading,

HMW

[atreyu]

Hey,

Your code works for me. Well, I had to add a double-quote at the end of your first line to close the print statement, and comment out vectore3 call b/c I don't have that defined, but the and/or logic worked.

Maybe post your whole code, and show what error statements python is reporting.

[polypagan]

I am not certain I understand what you are asking. This looks to me like a problem in (positive/negative) Boolean logic:

if (A or B)
then exit.

while (!A and !B)
then do something.

in effect:

if A or B are true, both conditionals cause the program to end. Or, to put it another way, if both A and B are false, something gets done.

What have I missed?

And why are both tests necessary?

[Leibnix]

I think what might be puzzling is the lack of symmetry: why must one use 'or' in one case and 'and' in the other, when both are there to exit the program?

The answer is, the first 'if' exits when the condition is true, where as the while loop does the opposite, it keep going while it's condition is true.

You might change your program to:

print "\nEnter names, \"Q\" to quit: 

string = raw_input()

if string == "Q" or string == "q":
	print "Exiting..."
	sys.exit(0)

while True:
	vector3.append(string)
	string = raw_input()
        if string == "Q" or string == "q":
	        print "Exiting..."
	        sys.exit(0)

You could also simplify:

print "\nEnter names, \"Q\" to quit: 

while True:
	string = raw_input()
        if string == "Q" or string == "q":
	        print "Exiting..."
	        sys.exit(0)
	vector3.append(string)

If this didn't help, please be more specific about your problem.

[unlimitedscolobb]

Check this out: De Morgan's laws - Wikipedia, the free encyclopedia

The essential snippet:

NOT (P AND Q) = (NOT P) OR (NOT Q)
NOT (P OR Q) = (NOT P) AND (NOT Q

In your case P is string == "Q" and Q is string == "q".

[HMW]

Thank you all for your replies. I realize I should have been more specific, you are all great programmers - not mind readers!

The snippet was to illustrate my confusion with "and" versus "or". The code itself is merely me trying to learn Python. I come from a C++ background, so I am simply writing some test programs with different stuff (vectors, if, while, for... you name it) to get the hang of things. I do remember being just as confused with "&&" versus "||" in C++. Maybe this is something I will simply have to learn to live with!

The upside of me being thick with regards to this is that there is a 50% chance. If it's not behaving corectly the first time around, change "and" to "or" or vice versa.

Thank you all again for your replies. This is how I educate myself; try - question - read - try again - learn. So all your input really matters to me.

Thanks for reading this,

HMW

[unlimitedscolobb]

The upside of me being thick with regards to this is that there is a 50% chance. If it's not behaving corectly the first time around, change "and" to "or" or vice versa.

I still insist on you reading this Wikipedia article: De Morgan's laws - Wikipedia, the free encyclopedia . It is most surely going to help you out of the situation you are describing. It's okay if you don't understand everything immediately; just come back and ask more questions; we are here to help.

I remember myself being confused with the and/or stuff; with time you the confusion just disappears (at least in my case it did)

An easy approach to setting up simple conditions would be as follows: formulate the condition in natural language. If you have a "neither ... nor" construction, use and; if you have an "either ... or" construction, use or.

For example, your first condition is "break when the user inputs either Q or q", so it's or. Your second condition is, on the other hand, "run until the input is neither Q, nor q", so it's and.

[HMW]

I still insist on you reading this Wikipedia article: De Morgan's laws - Wikipedia, the free encyclopedia . It is most surely going to help you out of the situation you are describing. It's okay if you don't understand everything immediately; just come back and ask more questions; we are here to help.

I remember myself being confused with the and/or stuff; with time you the confusion just disappears (at least in my case it did)

An easy approach to setting up simple conditions would be as follows: formulate the condition in natural language. If you have a "neither ... nor" construction, use and; if you have an "either ... or" construction, use or.

For example, your first condition is "break when the user inputs either Q or q", so it's or. Your second condition is, on the other hand, "run until the input is neither Q, nor q", so it's and.

Thanks mate. I will definitely check out the link. What you wrote above has already helped me a lot.

Truly appreciate your input,

HMW

[hazel]

Another way of looking at this is to compare "=" and "!=" logic.

"=" logic is very specific: that one value is true, everything else is false. "= or =" is a bit more general, but there is still a reasonable chance of the condition being true or false.

"!=" logic is quite unspecific: any value except that one will be true. And with "!= or !=", the result is guaranteed to be true for all values, including the two specified.

Specifically: (string != "Q") or (string != "q") is true for "Q" (the second condition is true), for "q" (the first condition is true) or anything else (both conditions are true).

A condition that is always true doesn't work.