CODEWITHSUNDEEP
pythonfunctionpython-2.7
Rui Miranda
Rui Miranda

Reputation: 11

Trouble with python functions

I started learning python (2.7), using LearnPythonTheHardWay, a few days ago and now that i'm studying functions i'm kind of lost. Can somebody explain me how do they work (if possibles with one example and another a litle harder)? And if you know some tips that will help me on my journey I would appreciate. Thanks. PS: I don't have any previous knowledge of programming. I'm starting from scratch.

for example(simple one (from the book)):

def secret_formula(started):
  jelly_beans = started * 500
  jars = jelly_beans / 1000
  crates = jars / 100
  return jelly_beans, jars, crates

start_point = 10000
beans, jars, crates = secret_formula(start_point)

print "With a starting point of: %d" % start_point
print "We'd have %d beans, %d jars, and %d crates." % (beans, jars, crates)

How do jelly_beans become beans? What is the (started) for?...

Upvotes: 0

Views: 509

Answers (3)

astrosyam
astrosyam

Reputation: 867

On way to understand functions, is to go back to basic mathematics.

Consider the mathematical expression : 

 f(x) = x + 2

In this case, the f(x) will add 2 to the value of x.

therefore, f(0) = 0 + 2 , gives 2. Similarly for other values of x.

when x = 3....f(3) = 5 
when x = 5....f(5) = 7

Thus, for an input value of x, it will produce an output which is the evaluation of the expression x + 2.

In python, this expression will be something like this :

def f(x):              # here x is the input value
    output = x + 2     #calculates the expression using x
    return(x+2)        #and we return the value

suppose we want to find the value for x = 3. this will be : f(3)

f(3) will now give you 5 as above.

We can save this value in another variable ,

y =f(3)

Here y saves the value returned by our function when you pass 3 to it. Thus y will be 5.

In your example, 

def secret_formula(started):         #here we have **started** instead of x
  jelly_beans = started * 500        #bunch of calculations
  jars = jelly_beans / 1000
  crates = jars / 100
  return jelly_beans, jars, crates   #returns the calculated value

Below that,

start_point = 10000
beans, jars, crates = secret_formula(start_point)  #what will the output be , if I give 1000 to secret_formula .. ie... secret_formula(1000)

Now secret_formula function returns three outputs 

return jelly_beans, jars, crates

We are assigning these outputs to beans, jars, crates in the respective order.

Now beans will have the value that jelly_beans has, so on...

So what happened to jelly_beans? To put it crudely, variables which are used within a function are only available within itself. Consider them as intermediate values which are discarded once used. Do read upon scope and scope rules.

The function will return some values which we now store in other variables.

Functions can be very useful when you have to do something repeatedly. Instead of rewriting the same code again and again, you can just call the function.

Consider this, random scenario :

def printNow():
    print("Hiwatsup, blah blah blah ")
    #some insane calculations
    print("Hiwatsup, blah blah blah ")
    #more random things.

Now, whenever you want to do all these things , you just need to put printNow(). You don't have to retype all of it!

Upvotes: 1

Clay
Clay

Reputation: 4760

On this line:

beans, jars, crates = secret_formula(start_point)

It is assigning the return value of secret_formula to beans.

Inside of secret_formula it is creating the jelly_beans variable that will only exist while it is inside of the secret_formula. After secret_formula runs the result defined by return jelly_beans, jars, crates can be assigned to new variables. In this case it assigns those variables to beans, jars, crates

So, beans is equal to jelly_beans, jars is equal jars and crates is equal to crates.

Upvotes: 2

rofls
rofls

Reputation: 5115

Generally in programming, and otherwise, you should think of functions as taking an input and producing an output. For example:

def square(num):
    return num**2

That returns (important keyword) the value num**2, which is num squared since ** is exponentiation in Python. However, you can have something inside a definition that doesn't return anything (no real output). For example:

def square(num):
    squared = num**2

won't actually return anything. But, it's still a valid function.

If you continue to study python, you will encounter many functions. You will probably continue having trouble understanding difficult ones even as you become more skilled, so don't get too hung up on them. If there is something in particular you don't understand, that would make a good question. For an example of a more complicated function:

def fibonnaci(n=1):
    if n in [1,2]:
        return 1
    else: 
        return fibonnaci(n-1)+fibonnaci(n-2)

Upvotes: 2

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