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19.10.2016
Lear n Python the Har d Way - Read for Fr ee
Exercse Exercse 21: Functons Can Return Somethng You have have been usng the
=
character to name varables and set them to numbers or
strngs. We're now gong to blow your mnd agan by showng you how to use new Python word
return
=
and a
to set varables to be a value from a function. There wll be
one thng to pay close attenton to, but frst type ths n:
https://l ear npythonthehar dway.or g/book/ex21.htm l
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(a, b):
1
'ADDING %d + %d'
2
a
3
(a, b)
b
4
(a, b):
5
'SUBTRACTING %d - %d'
6
a
7
(a, b)
b
8
(a, b):
9
'MULTIPLYING %d * %d'
10
a
11
(a, b)
b
12
(a, b):
13
'DIVIDING %d / %d'
14
a
15
(a, b)
b
16 17
'Let's do some math with just functions!'
18 19 20
age
add(30, 5)
21
height
subtract(78, 4)
22
weight
multiply(90, 2)
23
iq
divide(100, 2)
24
'Age: %d, Height: %d, Weight: %d, IQ: %d'
25
(age, height, weight, iq)
26 27 28
# A puzzle for the extra credit, type it in anyway.
'Here is a puzzle.'
29 30 31
what
add(age, subtract(height, multiply(weight, divide(iq, 2))))
32 33
'That becomes: ', what, 'Can you do it by hand?'
https://learnpythonthehardway.org/book/ex21.html
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We are now dong our own math functons for divide
add
,
subtract
,
multiply
. The mportant thng to notce s the last lne where we say
, and
return a + b
(n
add
). What ths does s the followng:
1
Our functon s called wth two arguments:
2
We prnt out what our functon s dong, n ths case 'ADDING.'
3
Then we tell Python to do somethng knd of backward: we return the addton of a + b
4
. You mght say ths as, 'I add
a
a
and
and
b
b
.
then return them.'
Python adds the two numbers. Then when the functon ends, any lne that runs t wll be able to assgn ths
a + b
result to a varable.
As wth many other thngs n ths book, you should take ths real slow, break t down, and try to trace what's gong on. To help there are extra credt to solve a puzzle and learn somethng cool.
What You Should See $ python ex21.py Let's do some math with just functions! ADDING 30 + 5 SUBTRACTING 78 - 4 MULTIPLYING 90 * 2 DIVIDING 100 / 2 Age: 35, Height: 74, Weight: 180, IQ: 50 Here is a puzzle. DIVIDING 50 / 2 MULTIPLYING 180 * 25 SUBTRACTING 74 - 4500 ADDING 35 + -4426 That becomes:
-4391 Can you do it by hand?
Study Drlls https://learnpythonthehardway.org/book/ex21.html
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1
If you aren't really sure what
return
does, try wrtng a few of your own
functons and have them return some values. You can return anythng that you can put to the rght of an 2
=
.
At the end of the scrpt s a puzzle. I'm takng the return value of one functon and using t as the argument of another functon. I'm dong ths n a chan so that I'm
knd of creatng a formula usng the functons. It looks really werd, but f you run the scrpt you can see the results. What you should do s try to fgure out the normal formula that would recreate ths same set of operatons. 3
Once you have the formula worked out for the puzzle, get n there and see what happens when you modfy the parts of the functons. Try to change t on purpose to make another value.
4
Do the nverse. Wrte a smple formula and use the functons n the same way to calculate t.
Ths exercse mght really whack your bran out, but take t slow and easy and treat t lke a lttle game. Fgurng out puzzles lke ths s what makes programmng fun, so I'll be gvng you more lttle problems lke ths as we go.
Common Student Questons Why does Python prnt the formula or the functons 'backward'?
It's not really backward, t's 'nsde out.' When you start breakng down the functon nto separate formulas and functon calls you'll see how t works. Try to understand what I mean by 'nsde out' rather than 'backward.'
How can I use
to enter my own values?
Remember int(raw_input()) ? The problem wth that s then you can't enter floatng pont, so also try usng float(raw_input()) nstead.
What do you mean by 'wrte out a formula'?
Try 24 + 34 / 100 - 1023 as a start. Convert that to use the functons. Now come up wth your own smlar math equaton and use varables so t's more https://learnpythonthehardway.org/book/ex21.html
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