Truth and flow control in Python
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[edit] Boolean Expressions
Python has a Boolean type. The two values for this type are True and False. Note the capitalization: the first letter must be capitalized. Most text editors, including IDLE, will highlight True and False when properly capitalized.
[edit] The bool() Function
If you need to convert something into a boolean value, use the bool() function. For example:
>>> bool(1) True >>> bool(0) False >>> bool(500) True >>> bool(-1) True
In Python, the integer 0 evaluates to False. All other integers will evaluate to True. As a general rule, try to avoid using floating point numbers in boolean expressions: use the int() operator to turn them into integers first.
Strings behave similarly: an empty string evaluates to False, all other strings evaluate to True.
>>> bool("Hello World!")
True
>>> bool("")
False
>>> bool("True")
True
>>> bool("False")
True
>>> bool("0")
True
Note that what is in the string does not matter: It can be "0", "False" or "ReallyReallyFalse": Everything that is not simply an empty string will evaluate to True.
This means we can write such useful constructs as if ( string ) which will execute a block of code only if the string is non-empty.
[edit] Comparison Operators
Python has a number of built-in comparison operators. They are very versatile and can often be used across data types.
[edit] Equality
The Python operator for Equality is ==. Let's try it out:
>>> 1==1 True >>> "1"=="1" True >>> 2+4 == 24 False >>> "2"+"4" == "24" True
[edit] Conjunction
The Python operator for conjunction is and.
>>> True and False False >>> True and True True >>> False and False False >>> False and True False
The and operator works by evaluating until it runs into something false, and return the value that evaluated to false. If everything is True, it returns the last thing it evaluated.
>>> "a" and "b" 'b' >>> "" and "b" '' >>> True and 7 7 >>> True and [] and "a" []
This means that in a boolean context the result is evaluated to True or False in the way we would expect, using the actual value means that we can benefit from useful side effects in other contexts.
[edit] Disjunction
The or operator works similar to the and operator. It returns the first value to be true, or the final value if there is no true value.
>>> False or ('a', 'b') or 'c'
('a', 'b')
>>> False or True
True
>>> '' or False
False
>>> '' or False or ()
()
- Todo: Add other operators (not, is, in...)
[edit] Flow Control
Now that you know all these great ways to work with booleans, let's use them!
[edit] If, Elif, and Else
Python the keyword "if" to determine if it should execute a section of code. You can tell which lines are in this section by the level of indentation. The general format of if goes like this:
if [Boolean Expression]:
some code
rest of code
This allows for very simple logic. What if you want to have two options, similar to "case" in other languages? Instead of Case, python uses "elif".
if [Boolean Expression]:
code
elif [Boolean Expression]:
More code
else:
Even more code
rest of program
If the boolean expression for the first "if" is True, nothing in the elif section will execute, nor will anything in the else section. If there are multiple things that could evaluate to True, use multiple if statements.
That code sample also introduced the "else" statement. Else gets executed if the "if" statement's boolean expression, and the boolean expressions for all elifs (if there are any), evaluate to False.
So, how about some actual code? Here's a simple program; try entering this into IDLE and running it. Don't forget to define the function.
name = raw_input("What is your name?")
if name == "":
print "Hey! You didn't enter a name!"
elif name == "spam" or name == "eggs":
print "A moose once bit my sister!"
else:
print "Nice to meet you, %s." % name
[edit] While
While is our first example of a loop. While evaluates a boolean expression, and if that expression is True, it runs some section of code. It then re-evaluates that boolean expression: if it still evaluates to True, then it runs again. It keeps running - possibly forever! - until that loop evaluates to False, at which point the program continues with whatever comes after the while loop.
Let's take our previous example. That program ran once, and even if nothing was entered, it was done. Let's suppose we instead want a program that "won't take no for an answer": We'll use a while loop to make the prompt show up again and again until we get some response:
name = raw_input("What is your name?")
while name == "":
print "Hey! You didn't enter a name!"
name = raw_input("Okay, REALLY tell me your name this time:")
if name == "spam" or name == "eggs":
print "A moose once bit my sister!"
else:
print "Nice to meet you, %s" % name
That's even better. However, there are a couple of things I don't like about it. First, I don't like that first boolean expression. After all, we know that:
bool("")==False
Therefore, wouldn't it make more sense just to test if name is False than to test if it's True that it's equal to ""?
while bool(name) == False:
And do we really want to test if it's True than to Test that it's True that it's equal to False?
while not bool(name):
That's right! Python has a "not" keyword, which acts a lot like ! in other languages.
We're missing one last thing: whatever comes after the "while" is automatically evaluated as a boolean expression. That means that in many cases (this one included) we can leave out the "bool()." (There will still be cases where it's needed, though, so remember that it exists!)
while not name: # This is exactly the same as 'while name == "":', but a lot clearer.
Okay, good? Cool. It's time to add a new feature: The for loop.
[edit] For concept in lesson: learn(concept)
If you thought the Python idioms for the while loop were cool, the for loop is even better. A for loop takes this basic form syntax-wise:
for [variable name] in [sequence]:
code
The for loop will execute enough times that each element of "sequence" is assigned to "variable" once. For example:
for i in [1,2,3]: print i, # 1 2 3
Of course, any data type could be within the sequence. While we're at it, let's use a descriptive variable name:
for letter in ['a','b','c']: print letter # output is a b c
As you can see, then, for loops make it easy to iterate over a sequence, or in simpler terms, to do something with each entry in a list. For loops are also excellent for times when you need to run a while loop a fixed number of times. Let's compare doint that with a for loop and a while loop:
c = 0
while c <10:
someFunction(c)
# code, code code
c +=1
While this is legal Python code, it's not really descriptive. Furthermore, when working with large code blocks, it's easy to forget to add 1 to the variable (in this case 'c') at the end. Fortunately, we can do the same thing with a for loop, in a clearer fashion.
for i in range(10):
someFunction(i)
# More code as needed
Note how we don't have to worry about declaring i, or automatically incrementing it. It's also significantly faster, although most of the time your code will run fast enough anyway.
Note that while we used 'i' as the variable name, you could use anything else. As a general rule, when using the variable as a counter, simply use 'i'; this convention is widely followed and helps other programmers know exactly why you chose a for loop. Otherwise, use a descriptive variable name. For example:
aListOfDates = someFunction()
for date in aListOfDates:
doSomethingWith(date)
The main reason we use while loops is sometimes we need to repeatedly prompt a user to get a valid input. Simply put, use while when you don't know or can't calculate how many times a loop will run. If you can, use for.
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