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19.10.2016

Learn Python the Hard Way ­ Read for Free

Exerc se 21: Funct ons Can Return Someth ng You 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 = and a new Python word return 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:

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19.10.2016

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Learn Python the Hard Way ­ Read for Free

def add(a, b):

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print "ADDING %d + %d" % (a, b)

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return a + b

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def subtract(a, b):

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print "SUBTRACTING %d - %d" % (a, b)

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return a - b

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def multiply(a, b):

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print "MULTIPLYING %d * %d" % (a, b)

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return a * b

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def divide(a, b):

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print "DIVIDING %d / %d" % (a, b)

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return a / b

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print "Let's do some math with just functions!"

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age = add(30, 5)

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height = subtract(78, 4)

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weight = multiply(90, 2)

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iq = divide(100, 2)

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print "Age: %d, Height: %d, Weight: %d, IQ: %d" % (age, height, weight, iq)

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# A puzzle for the extra credit, type it in anyway.

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print "Here is a puzzle."

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what = add(age, subtract(height, multiply(weight, divide(iq, 2))))

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print "That becomes: ", what, "Can you do it by hand?"

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19.10.2016

Learn Python the Hard Way ­ Read for Free

We are now do ng our own math funct ons for add , subtract , multiply , and divide . The mportant th ng to not ce s the last l ne where we say 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 . You m ght say th s as, "I add a and b then return them."

4

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.

a

and

b

.

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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19.10.2016

Learn Python the Hard Way ­ Read for Free

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

raw_input()

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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