Truth and flow control in Python: Difference between revisions

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==Flow Control==

Now that you know all these great ways to work with booleans, let's use them! Python uses a simple keyword called "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:

Now that you know all these great ways to work with booleans, let's use them!

===If, Elif, and Else==

Python uses a simple keyword called "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

Line 67:

Line 70:

else:

print "Nice to meet you, %s." % name

===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. Python as ! too, but not is often clearer when working with long boolean expressions.

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.

===For concept in lesson: learn(concept)===

The for loop is... (to be continued)

@@ Line 41: / Line 41: @@
 ==Flow Control==
-Now that you know all these great ways to work with booleans, let's use them! Python uses a simple keyword called "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:
+Now that you know all these great ways to work with booleans, let's use them!
+===If, Elif, and Else==
+Python uses a simple keyword called "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
@@ Line 67: / Line 70: @@
   else:
       print "Nice to meet you, %s." % name
+===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. Python as ! too, but not is often clearer when working with long boolean expressions.
+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.
+===For concept in lesson: learn(concept)===
+The for loop is... (to be continued)

Truth and flow control in Python: Difference between revisions

152.43.1.4 (talk)