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CS1110

Lecture 11: Iterating over Sequences

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Prelim 1

The exam is in one week. It

covers material in lecture up

through today and in lab up

through this week¡¯s lab.

Slides by D. Gries, L. Lee, S. Marschner, W. White

Processing sequences: The map Function

General form: map(?function?, ?sequence?)

if x is a sequence of n items and 

f is a function with one parameter:

which could be

a list or a string

if x is a sequence of n items and 

m is a method with no parameters:

map(m, x)

map(m, x)

map(f, x)

calls the function once

for each list item

[f(x[0]), f(x[1]), ¡­, f(x[n¨C1])]

[x[0].m(), x[1].m(), ¡­, x[n¨C1].m()]

Mapping a function over a list

x

id1

id1

0 1.2

1 3.9

2 ¨C4.8

3

0.1

x = [1.2, 3.9, ¨C4.8, 0.1]

y = map(round, x)

a function taking one argument

round(1.2)

round(3.9)

round(¨C4.8)

round(0.1)

>>> y

[1.0, 4.0, ¨C5.0, 0.0]

y

id2

id2

0 1.0

1 4.0

2 ¨C5.0

3

0.0

Mapping a function over a list

x

id1

x = ['a', 'list', 'of', 'things']

y = map(len, x)

y

id2

id1

0

1

2

3

id2

¡®a¡¯

len(¡®a¡¯)

¡®list¡¯

len(¡®list¡¯)

¡®of¡¯

len(¡®of¡¯)

¡®things¡¯

len(¡®things¡¯)

>>> y

[1, 4, 2, 6]

0

1

2

3

1

4

2

6

Mapping a method over a list

x

id1

id1

0 ¡®Lee¡¯

1 ¡®Python¡¯

2 ¡®CS1110¡¯

3

¡®A+¡¯

x = ['Lee', 'Python', 'CS1110', 'A+']

y = map(str.lower, x)

the method lower in the class str:

a method with no parameters*

x[0].lower()

x[1].lower()

x[2].lower()

x[3].lower()

>>> y

['lee', 'python', 'cs1110', 'a+']

y

id2

id2

0 ¡®lee¡¯

1 ¡®python¡¯

2 ¡®cs1110¡¯

3

¡®a+¡¯

*In this case you could also

use string.lower instead,

which is the function lower

in the module string.

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