CONDITIONALS, LOOPS, AND SOME OTHER STATEMENTS in Font

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CHAPTER 5 CONDITIONALS, LOOPS, AND SOME OTHER STATEMENTS
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There is a useful relative of the if statement, which works more or less like this (pseudocode): if not condition: crash program Now, why on earth would you want something like that Simply because it s better that your program crashes when an error condition emerges than at a much later time. Basically, you can require that certain things be true (for example, when checking required properties of parameters to your functions or as an aid during initial testing and debugging). The keyword used in the statement is assert: >>> age = 10 >>> assert 0 < age < 100 >>> age = -1 >>> assert 0 < age < 100 Traceback (most recent call last): File "<stdin>", line 1, in AssertionError It can be useful to put the assert statement in your program as a checkpoint, if you know something must be true for your program to work correctly. A string may be added after the condition, to explain the assertion: >>> age = -1 >>> assert 0 < age < 100, 'The age must be realistic' Traceback (most recent call last): File "<stdin>", line 1, in AssertionError: The age must be realistic
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Now you know how to do something if a condition is true (or false), but how do you do something several times For example, you might want to create a program that reminds you to pay the rent every month, but with the tools we have looked at until now, you would need to write the program like this (pseudocode): send mail wait one month send mail wait one month send mail wait one month (...and so on)
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CHAPTER 5 CONDITIONALS, LOOPS, AND SOME OTHER STATEMENTS
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But what if you wanted it to continue doing this until you stopped it Basically, you want something like this (again, pseudocode): while we aren't stopped: send mail wait one month Or, let s take a simpler example. Let s say that you want to print out all the numbers from 1 to 100. Again, you could do it the stupid way: print print print ... print print 1 2 3 99 100
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But you didn t start using Python because you wanted to do stupid things, right
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while Loops
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In order to avoid the cumbersome code of the preceding example, it would be useful to be able to do something like this: x = 1 while x <= 100: print x x += 1 Now, how do you do that in Python You guessed it you do it just like that. Not that complicated, is it You could also use a loop to ensure that the user enters a name, as follows: name = '' while not name: name = raw_input('Please enter your name: ') print 'Hello, %s!' % name Try running this, and then just pressing the Enter key when asked to enter your name. You ll see that the question appears again, because name is still an empty string, which evaluates to false.
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Tip What would happen if you entered just a space character as your name Try it. It is accepted because a string with one space character is not empty, and therefore not false. This is definitely a flaw in our little program, but easily corrected: just change while not name to while not name or name.isspace(), or perhaps, while not name.strip().
CHAPTER 5 CONDITIONALS, LOOPS, AND SOME OTHER STATEMENTS
for Loops
The while statement is very flexible. It can be used to repeat a block of code while any condition is true. While this may be very nice in general, sometimes you may want something tailored to your specific needs. One such need is to perform a block of code for each element of a set (or, actually, sequence or other iterable object) of values.
Note Basically, an iterable object is any object that you can iterate over (that is, use in a for loop). You
learn more about iterables and iterators in 9, but for now, you can simply think of them as sequences.
You can do this with the for statement: words = ['this', 'is', 'an', 'ex', 'parrot'] for word in words: print word or numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] for number in numbers: print number Because iterating (another word for looping) over a range of numbers is a common thing to do, Python has a built-in function to make ranges for you: >>> range(0, 10) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] Ranges work like slices. They include the first limit (in this case 0), but not the last (in this case 10). Quite often, you want the ranges to start at 0, and this is actually assumed if you supply only one limit (which will then be the last): >>> range(10) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] The following program writes out the numbers from 1 to 100: for number in range(1,101): print number Notice that this is much more compact than the while loop I used earlier.
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