Download For Loops in Python

For Loops in Python
For Loops
Range
Breaks
For Loops in Python
L435/L555
Dept. of Linguistics, Indiana University
Fall 2014
1/9
For Loops
For Loops in Python
For Loops
Range
Breaks
Iteration
For loops allow us to iterate over each element of a set or
sequence
Syntax:
f o r < var > i n < set > :
do . . .
do . . .
2/9
Examples
For Loops in Python
For Loops
Range
I
Iterating over a list:
Breaks
words = [ ’ g o t ’ , ’me ’ , ’ l o o k i n g ’ , ’ so ’ ,
’ c r a z y ’ , ’ r i g h t ’ , ’ now ’ ]
f o r w i n words :
p r i n t (w)
I
Iterating over a string:
phrase = ’ l o o k i n g so c r a z y i n l o v e ’
f o r w i n phrase :
p r i n t (w)
3/9
The Range Iterator
For Loops in Python
For Loops
Range
Iteration
Breaks
Python has a built-in function, range, which allows us to
iterate over the numbers specified in the range.
What do these do?
list
list
list
list
( range ( 0 , 1 0 ) )
( range ( 1 0 ) )
( range ( 1 , 11 , 2 ) )
( range ( 0 , − 10, − 1))
Note: in Python 3, range is an iterator, so it generates
numbers on the fly
I
the list function compresses them into a list
4/9
Range in Loops
For Loops in Python
For Loops
Range
Breaks
f o r number i n range ( 0 , 1 0 ) :
p r i n t ( number )
a = ’ uh oh uh oh uh oh , oh no no ’
f o r number i n range ( len ( a ) ) :
p r i n t ( number , a [ number ] )
5/9
for vs. while
For Loops in Python
For Loops
Range
Breaks
a = ’ uh oh uh oh uh oh , oh no no ’
i = 0
while i < len ( a ) :
print ( i , a [ i ] )
i += 1
f o r i i n range ( len ( a ) ) :
print ( i , a [ i ] )
6/9
Breaking out of Loops
I
break: stops the execution of the loop, independent of
the test
For Loops in Python
For Loops
Range
Breaks
f o r x i n range ( 0 , 1 0 0 0 ) :
i f ( x % 7 ) == 0 :
p r i n t ( ’ f i r s t number d i v i s i b l e by 7 : ’ , x )
break
I
continue: skip the rest of the loop body, but continue
with the loop
f o r x i n range ( 0 , 1 0 0 0 ) :
i f ( x % 7 ) == 0 :
print ( x )
else :
continue
p r i n t ( x , ” i s d i v i s i b l e by 7 ” )
7/9
Loop Else
For Loops in Python
For Loops
Range
Iteration
Breaks
else can follow a loop, then it is executed in case the loop is
NOT exited by break.
f o r x i n range ( 1 0 0 ) :
i f x > 101:
print ( x )
break
else :
p r i n t ( ’ n o t found ’ )
Only if the break condition is met will the else not run
8/9
List comprehensions again
For Loops in Python
For Loops
Range
Breaks
I
List comprehensions
sasha = [1,2,3,4,5]
fierce = [x**2 for x in sasha]
I
List comprehensions with conditions
fierce = [x**2 for x in sasha if x%2==0]
9/9