Learn Python the Hard Way - Read for Free

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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Learn Python the Hard Way - Read for Free

(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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lke a formula.

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