c# barcode generator library CONDITIONALS, LOOPS, AND SOME OTHER STATEMENTS in Font

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CHAPTER 5 CONDITIONALS, LOOPS, AND SOME OTHER STATEMENTS
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strings[index] = '[censored]' index += 1 This also seems a bit awkward, although acceptable. Another solution is to use the built-in function enumerate: for index, string in enumerate(strings): if 'xxx' in string: strings[index] = '[censored]' This function lets you iterate over index-value pairs, where the indices are supplied automatically.
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Reversed and Sorted Iteration
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Let s look at another couple of useful functions: reversed and sorted. They re similar to the list methods reverse and sort (with sorted taking arguments similar to those taken by sort), but they work on any sequence or iterable object, and instead of modifying the object in place, they return reversed and sorted versions: >>> sorted([4, 3, 6, 8, 3]) [3, 3, 4, 6, 8] >>> sorted('Hello, world!') [' ', '!', ',', 'H', 'd', 'e', 'l', 'l', 'l', 'o', 'o', 'r', 'w'] >>> list(reversed('Hello, world!')) ['!', 'd', 'l', 'r', 'o', 'w', ' ', ',', 'o', 'l', 'l', 'e', 'H'] >>> ''.join(reversed('Hello, world!')) '!dlrow ,olleH' Note that although sorted returns a list, reversed returns a more mysterious iterable object. You don t need to worry about what this really means; you can use it in for loops or methods such as join without any problems. You just can t index or slice it, or call list methods on it directly. In order to perform those tasks, you need to convert the returned object, using the list type, as shown in the previous example.
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Breaking Out of Loops
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Usually, a loop simply executes a block until its condition becomes false, or until it has used up all sequence elements. But sometimes you may want to interrupt the loop, to start a new iteration (one round of executing the block), or to simply end the loop.
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break
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To end (break out of) a loop, you use break. Let s say you wanted to find the largest square (the result of an integer multiplied by itself) below 100. Then you start at 100 and iterate
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CHAPTER 5 CONDITIONALS, LOOPS, AND SOME OTHER STATEMENTS
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downwards to 0. When you ve found a square, there s no need to continue, so you simply break out of the loop: from math import sqrt for n in range(99, 0, -1): root = sqrt(n) if root == int(root): print n break If you run this program, it will print out 81 and stop. Notice that I ve added a third argument to range that s the step, the difference between every pair of adjacent numbers in the sequence. It can be used to iterate downwards as I did here, with a negative step value, and it can be used to skip numbers: >>> range(0, 10, 2) [0, 2, 4, 6, 8]
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continue
The continue statement is used less often than break. It causes the current iteration to end, and to jump to the beginning of the next. It basically means skip the rest of the loop body, but don t end the loop. This can be useful if you have a large and complicated loop body and several possible reasons for skipping it. In that case, you can use continue, as follows: for x in seq: if condition1: continue if condition2: continue if condition3: continue do_something() do_something_else() do_another_thing() etc() In many cases, however, simply using an if statement is just as good: for x in seq: if not (condition1 or condition2 or condition3): do_something() do_something_else() do_another_thing() etc() Even though continue can be a useful tool, it is not essential. The break statement, however, is something you should get used to, because it is used quite often in concert with while True, as explained in the next section.
CHAPTER 5 CONDITIONALS, LOOPS, AND SOME OTHER STATEMENTS
The while True/break Idiom
The for and while loops in Python are quite flexible, but every once in a while, you may encounter a problem that makes you wish you had more functionality. For example, let s say you want to do something when a user enters words at a prompt, and you want to end the loop when no word is provided. One way of doing that would be like this: word = 'dummy' while word: word = raw_input('Please enter a word: ') # do something with the word: print 'The word was ' + word Here is an example of a session: Please enter The word was Please enter The word was Please enter a word: first first a word: second second a word:
This works just as desired. (Presumably, you would do something more useful with the word than print it out, though.) However, as you can see, this code is a bit ugly. To enter the loop in the first place, you need to assign a dummy (unused) value to word. Dummy values like this are usually a sign that you aren t doing things quite right. Let s try to get rid of it: word = raw_input('Please enter a word: ') while word: # do something with the word: print 'The word was ' + word word = raw_input('Please enter a word: ') Here the dummy is gone, but I have repeated code (which is also a bad thing): I need to use the same assignment and call to raw_input in two places. How can I avoid that I can use the while True/break idiom: while True: word = raw_input('Please enter a word: ') if not word: break # do something with the word: print 'The word was ' + word The while True part gives you a loop that will never terminate by itself. Instead, you put the condition in an if statement inside the loop, which calls break when the condition is fulfilled. Thus, you can terminate the loop anywhere inside the loop instead of only at the beginning (as with a normal while loop). The if/break line splits the loop naturally in two parts: the first takes care of setting things up (the part that would be duplicated with a normal while loop), and the other part makes use of the initialization from the first part, provided that the loop condition is true. Although you should be wary of using break too often in your code (because it can make your loops harder to read, especially if you put more than one break in a single loop), this
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