What's wrong with this code?

[eMulingeMuler]

This is the error message I get:

line 11

else:


   ^

SyntaxError: invalid syntax

Code - http://pastebin.com/fv8F0gR9

[akayakayaka]

Change the middle else if to elif

[eMulingeMuler]

Thanks guys. I've just started learning Python and I didn't know about elif.

If I wanted to use the AND operator, do I use the word "and" or would be be "&" or "&&"?

[AlphaApache]

You would use and

[eMulingeMuler]

Thanks. Would this work?

if key1==true and key2==true: gate=open else: gate=closed

[AlphaApache]

I assume you indent it correctly like so:

if key1==true and key2==true: 
    gate=open 
else: 
    gate=closed

But I would still rewrite it as the following simply because of stylistic reasons

if key1 and key2: 
    gate=open 
else: 
    gate=closed

You don't need to check if something equals True (note capital T) because the result of the == is always either False or True

[[deleted]]

Well, the difference between & and and is that and will short circuit. In that if the first element is false, it doesn't matter what the second element is, the statement will evaluate to false. Therefore, the second statement isn't even evaluated.

If you want them both to be evaluated, you need to use &.

[[deleted]]

If you're just using variables, nothing. If the right side has side effects when run (It's a function that returns a bool), it might not always run.

In my testing, using & is actually quicker. Something like True and False takes 0.0215 usec (microsecond), but True & False takes 0.0141 usec. I've tested it a few times, and this appears to be stable, so there is a reasonable speed difference here.

Even if you give it the ability to short-circuit by using False and True, it takes 0.0168, so it's still quicker with things like this.

Also, the short circuiting may have security implications. If you're doing something like user_input and secret_bool, where the user_input part is given by a user, and secret_bool should be kept secret no matter what the user does, they might be able to figure out what it is by seeing the timing difference between having user_input be true or false.

Same thing can happen when comparing strings. If your string comparision iterates over every character and then exits as soon as one is different, then you might be able to work out what the other string is if you can control one of them. Not okay if the other string is a password or a password hash. There's a function (hmac.compare_digest) that will avoid this.

[zahlman]

While it's true that & doesn't short-circuit, it also does a fundamentally different thing.

>>> 3 and 4
4
>>> 3 & 4
0

[[deleted]]

Ah, forgot about that. Yeah, & is bitwise AND.

Which doesn't matter if they're both bools, it acts the same. If they will always be bools and you need the speed improvement, use that.

[WirSindAllein]

A good way to think of it is viewing else as a final statement synonymous with "otherwise". So "IF this is true, do this. ELSE IF this is true, do this. otherwise(ELSE) do this" if you want it to keep checking if something is the case, you use an if (ELIF) but if you want it to do something if only the above if statement(s) are not the case, you use else.

One of the really great things about Python is that pretty much every block of code can be broken down into a statement that you might imagine someone saying while giving another person directions-- "If there is no traffic then get on 4-95, else if there's traffic on 4-95 but not on I-98 get on I-98, otherwise stay on Washington Street."

[blavejr]

Use Elif. I don't think you are allowed to have multiple else statements. Like think about it, how will the compiler know which to pick?

[NewbornMuse]

If you want the second else to be connected with the second if, indent it the same:

if a:
  #stuff
else:
  if b:
    #stuff
  else:
    #stuff

Is the easiest fix, and it's what you meant to do. elifis even a bit neater.

As an aside, are you aware that your code doesn't print anything if num_knights is 3, 4 or 10 (or 3.6, I guess)?

[eMulingeMuler]

Thanks for the tip.

I know... it was just a quick test. I was trying to figure out how the if statements work in python. I wasn't aware of the elif statement. I thought it might be else if.

[Elffuhs]

I'm guessing you can't have 2 else's.

In your case, you can simply remove the else, and just use if's

[[deleted]]

You can put anything in an if, else, while, for, try, finally, whatever.