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pierian-data
GitHub Repository: pierian-data/complete-python-3-bootcamp
 Path: blob/master/09-Empty-Section-Skip/02-Reduce.ipynb
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Kernel: Python 3

Content Copyright by Pierian Data

reduce()

Many times students have difficulty understanding reduce() so pay careful attention to this lecture. The function reduce(function, sequence) continually applies the function to the sequence. It then returns a single value.

If seq = [ s1, s2, s3, ... , sn ], calling reduce(function, sequence) works like this:

  • At first the first two elements of seq will be applied to function, i.e. func(s1,s2)

  • The list on which reduce() works looks now like this: [ function(s1, s2), s3, ... , sn ]

  • In the next step the function will be applied on the previous result and the third element of the list, i.e. function(function(s1, s2),s3)

  • The list looks like this now: [ function(function(s1, s2),s3), ... , sn ]

  • It continues like this until just one element is left and return this element as the result of reduce()

Let's see an example:

from functools import reduce

lst =[47,11,42,13]
reduce(lambda x,y: x+y,lst)
113

Lets look at a diagram to get a better understanding of what is going on here:

from IPython.display import Image
Image('http://www.python-course.eu/images/reduce_diagram.png')
Image in a Jupyter notebook

Note how we keep reducing the sequence until a single final value is obtained. Lets see another example:

#Find the maximum of a sequence (This already exists as max())
max_find = lambda a,b: a if (a > b) else b

#Find max
reduce(max_find,lst)
47

Hopefully you can see how useful reduce can be in various situations. Keep it in mind as you think about your code projects!