|
|
On this page: for ... in over list, string, dictionary. dict.items(), range(), nested for loops.
for Loops Over a List
As mentioned briefly in Tutorial 17, the for ... in loop is adaptive: you can use it with any sequence type (list, string, tuple) plus dictionaries. The most prototypical use of for ... in loop is over a list. Below, it is used to add up the numbers in a list:
| |
>>> total = 0
>>> for n in [5.99, 12.99, 20.99, 129.99]:
total += n
print 'Total is now', total
Total is now 5.99
Total is now 18.98
Total is now 39.97
Total is now 169.96
>>> total
169.96
| |
for Loops Over a String
When used on a string, the looping has to go through every something. Well, a string is a sequence of characters, so the iteration is done on every character:
| |
>>> for i in 'penguin':
print i
p
e
n
g
u
i
n
| |
for Loops Over a Dictionary
Naturally, we'd also want to be able to loop over a dictionary. When for ... in is used on a dictionary, looping is done over its keys, and not over the values or the key:value pairs:
| |
>>> simpsons = {'Homer':36, 'Marge':35, 'Bart':10, 'Lisa':8}
>>> for s in simpsons:
print s
Homer
Lisa
Marge
Bart
| |
Of course, the values are retrievable via the keys. Hence, printing both the key and the value looks like:
| |
>>> for s in simpsons:
print s, simpsons[s]
Homer 36
Lisa 8
Marge 35
Bart 10
| |
An alternative is to explicitly instruct the for loop to iterate over the key:value pairs. The .items() method on a dictionary induces a list of (key, value) tuples. The for loop then can iterate over this list, and the bound variable should also be the tuple type:
| |
>>> simpsons.items()
[('Homer', 36), ('Lisa', 8), ('Marge', 35), ('Bart', 10)]
>>> for (k,v) in simpsons.items():
print k, v
Homer 36
Lisa 8
Marge 35
Bart 10
| |
Note that the iteration is in no particular order, although it stays consistent. That's because dictionaries are inherently orderless. If you want to go through the dictionary in a particular order (say, alphabetical or numerical), you will have to sort. See Sorting for details.
Indexing With range() Function
range() is a function that's often used with a for loop. range(x,y) creates a list starting with integer x and ending BEFORE y. The starting point x can be omitted, in which case the list starts with 0:
| |
>>> range(2,8)
[2, 3, 4, 5, 6, 7]
>>> range(4)
[0, 1, 2, 3]
| |
The reason why range() is used often with a for loop is the following. In the 'penguin' example above, what if you want to also print out the index of each character? The way the loop is written, i is bound to the character itself, and there's nothing that references the location of an individual character. range() solves this. First, range() together with len('penguin') produces a list of indexes for the word:
| |
>>> len('penguin')
7
>>> range(len('penguin'))
[0, 1, 2, 3, 4, 5, 6]
| |
Now, you can iterate through this list of indexes, and it is easy enough to get to the character once you have its index:
| |
>>> for i in range(len('penguin')):
print i, 'penguin'[i]
0 p
1 e
2 n
3 g
4 u
5 i
6 n
| |
Nested for Loops
If you think a simple for loop is easy, nested for loops can be a bit difficult to wrap your head around. Below, the inner loop repeats 4 times inside another loop, which itself repeats 4 times:
| |
>>> for i in 'abcd':
for j in 'abcd':
print i, j
print ''
a a
a b
a c
a d
b a
b b
b c
b d
c a
c b
c c
c d
d a
d b
d c
d d
| |
Why stop at two? Try a 3-level for loop from Anne Dawson's page. Search for "nested for loop".
|