defining a lambda function with two arguments

Question

I have some code as shown below.

import math
square_root = lambda x: math.sqrt(x)
list = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]
map(square_root,list)

Output:

[1.0,
 1.4142135623730951,
 1.7320508075688772,
 2.0,
 2.23606797749979,
 2.449489742783178,
 2.6457513110645907,
 2.8284271247461903,
 3.0,
 3.1622776601683795,
 3.3166247903554,
 3.4641016151377544,
 3.605551275463989,
 3.7416573867739413,
 3.872983346207417,
 4.0]

Now I want to use power instead of square_root

import math
power = lambda x: math.power(x,n)
list = [1,2,3,4,5]
map(power,list,2)

And I get the following error? How do I use two arguments with map?

TypeError Traceback (most recent call last) /home/AD/karthik.sharma/ws_karthik/trunk/ in () ----> 1 map(power,list,2)

TypeError: argument 3 to map() must support iteration

Greg Hewgill · Accepted Answer · 2014-05-13 23:43:39Z

8

One way to do this is the following:

power = lambda x, n: math.pow(x,n)
list = [1,2,3,4,5]
map(power,list,[2]*len(list))

The expression [2]*len(list) creates another list the same length as your existing one, where each element contains the value 2. The map function takes an element from each of its input lists and applies that to your power function.

Another way is:

power = lambda x, n: math.pow(x,n)
list = [1,2,3,4,5]
map(lambda x: power(x, 2),list)

which uses partial application to create a second lambda function that takes only one argument and raises it to the power 2.

Note that you should avoid using the name list as a variable because it is the name of the built-in Python list type.

answered May 13, 2014 at 23:43

Greg Hewgill

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2 Comments

nakedfanatic Over a year ago

You could also use map(power, list, itertools.repeat(2, len(list))) to avoid creating a second list.

SethMMorton Over a year ago

If you wanted to be ultra-fancy, you could use itertools.cycle([2]) instead of [2]*len(list).

R Sahu · Accepted Answer · 2014-05-13 23:46:42Z

3

import math
power = lambda n: lambda x: math.pow(x,n)
list = [1,2,3,4,5]
map(power(2),list)

answered May 13, 2014 at 23:46

R Sahu

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Comments

nakedfanatic · Accepted Answer · 2014-05-13 23:47:59Z

3

Like this:

power = lambda x, n: math.pow(x,n)
list = [1,2,3,4,5]
map(lambda x: power(x, 2), list)

answered May 13, 2014 at 23:47

nakedfanatic

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Comments

rpb · Accepted Answer · 2014-05-13 23:57:29Z

3

List comprehensions are another option:

list = [1,2,3,4,5]
[math.pow(x,2) for x in list]

answered May 13, 2014 at 23:57

rpb

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