Boston Python Workshop 6/Loops: Difference between revisions
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== <code>while</code> loops == |
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Use a <code>while</code> loop to loop so long as a condition is <code>True</code>. |
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<pre>>>> while i < 10: |
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... print i |
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... i = i + 1 |
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... |
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9</pre> |
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=== <code>break</code> keyword === |
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Use the <code>break</code> keyword to break out of a loop early: |
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<pre> |
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>>> i = 0 |
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>>> while True: |
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... print i |
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... i = i + 1 |
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... if i > 10: |
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... break |
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... |
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10</pre> |
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=== Get user input with <code>raw_input()</code> === |
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<pre> |
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>>> while True: |
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... input = raw_input("Please type something> ") |
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... if input == "Quit": |
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... print "Goodbye!" |
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... break |
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... else: |
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... print "You said: " + input |
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... |
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Please type something> Hello |
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You said: Hello |
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Please type something> How are you? |
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You said: How are you? |
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Please type something> Quit |
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Goodbye! |
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>>> </pre> |
Revision as of 15:08, 30 March 2012
For loops
Use a for
loop to do something to every element in a list.
>>> names = ["Jessica", "Adam", "Liz"] >>> for name in names: ... print name ... Jessica Adam Liz
>>> names = ["Jessica", "Adam", "Liz"] >>> for name in names: ... print "Hello " + name ... Hello Jessica Hello Adam Hello Liz
if
statements inside for
loop
>>> for name in ["Alice", "Bob", "Cassie", "Deb", "Ellen"]: ... if name[0] in "AEIOU": ... print name + " starts with a vowel." ... Alice starts with a vowel. Ellen starts with a vowel.
Sometimes you want to start with a new empty list, and only add to that list if some condition is true:
>>> vowel_names = [] >>> for name in ["Alice", "Bob", "Cassie", "Deb", "Ellen"]: ... if name[0] in "AEIOU": ... vowel_names.append(name) ... >>> print vowel_names ['Alice', 'Ellen']
for
loops inside for
loops
You can put for
loops inside for
loops. The indentation dictates which for
loop a line is in.
>>> letters = ["a", "b", "c"] >>> numbers = [1, 2, 3] >>> for letter in letters: ... for number in numbers: ... print letter * number ... a aa aaa b bb bbb c cc ccc
The order of the for
loops matters. Compare the above example with this one:
>>> for number in numbers: ... for letter in letters: ... print number * letter ... a b c aa bb cc aaa bbb ccc
sorting lists
Use .sort()
to sort a list:
>>> names = ["Eliza", "Joe", "Henry", "Harriet", "Wanda", "Pat"] >>> names.sort() >>> names ['Eliza', 'Harriet', 'Henry', 'Joe', 'Pat', 'Wanda']
Getting the maximum and minimum values from a list
>>> numbers = [0, 3, 10, -1] >>> max(numbers) 10 >>> min(numbers) -1
Generating a list of numbers easily with range()
The range()
function returns a list of numbers. This is handy for when you want to generate a list of numbers on the fly instead of creating the list yourself.
>>> range(5) [0, 1, 2, 3, 4]
Use range
when you want to loop over a bunch of numbers in a list:
>>> numbers = range(5) >>> for number in numbers: ... print number * number ... 0 1 4 9 16
We could rewrite the above example like this:
>>> for number in range(5): ... print number * number ... 0 1 4 9 16
while
loops
Use a while
loop to loop so long as a condition is True
.
>>> while i < 10: ... print i ... i = i + 1 ... 0 1 2 3 4 5 6 7 8 9
break
keyword
Use the break
keyword to break out of a loop early:
>>> i = 0 >>> while True: ... print i ... i = i + 1 ... if i > 10: ... break ... 0 1 2 3 4 5 6 7 8 9 10
Get user input with raw_input()
>>> while True: ... input = raw_input("Please type something> ") ... if input == "Quit": ... print "Goodbye!" ... break ... else: ... print "You said: " + input ... Please type something> Hello You said: Hello Please type something> How are you? You said: How are you? Please type something> Quit Goodbye! >>>