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function code snippet that has function calls I have never seen before. How does it work.

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  function code snippet that has function calls I have never seen before. How does it work. Ronald Cosentino <ronjc.2001@gmail.com> - 2015-10-03 10:40 -0700
    RE: function code snippet that has function calls I have never seen before. How does it work. "Joseph Lee" <joseph.lee22590@gmail.com> - 2015-10-03 11:03 -0700
    Re: function code snippet that has function calls I have never seen before. How does it work. Ervin Hegedüs <airween@gmail.com> - 2015-10-03 20:09 +0200
    Re: function code snippet that has function calls I have never seen before. How does it work. Denis McMahon <denismfmcmahon@gmail.com> - 2015-10-03 19:59 +0000
    Re: function code snippet that has function calls I have never seen before. How does it work. Steven D'Aprano <steve@pearwood.info> - 2015-10-05 00:42 +1100

#97383 — function code snippet that has function calls I have never seen before. How does it work.

def funA(x,y,z):
    return (x+y) * z
def funB(x,y):
    return(x-y)
print(funA(4,funB(2,3), funB(3,2)))

the answer is 3. I don't know how it works.

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#97384

Hi Ronald,
Answers inline.

-----Original Message-----
From: Python-list
[mailto:python-list-bounces+joseph.lee22590=gmail.com@python.org] On Behalf
Of Ronald Cosentino
Sent: Saturday, October 3, 2015 10:41 AM
To: python-list@python.org
Subject: function code snippet that has function calls I have never seen
before. How does it work.

def funA(x,y,z):
    return (x+y) * z
def funB(x,y):
    return(x-y)
print(funA(4,funB(2,3), funB(3,2)))

the answer is 3. I don't know how it works.

JL: Okay, let's step through the print routine.

1. Before print does its job, the function result will be gathered.
2. The argument to print is result of funA, which itself takes result of two
calls to FunB.
3. First, results of funB calls will be gathered (subtracting 3 from 2 and 2
from 3, respectively). So it becomes:
print(funA(4, (2-3), (3-2)))
4. Then funA will continue to perform its job, returning the result for
output. Thus the final expression that print will print is:
print((4+-1) * 1)

When looking at a function that takes result of another function, it is
important to look at what the inner callee does (in this case, look at what
funB does first).
Cheers,
Joseph



-- 
https://mail.python.org/mailman/listinfo/python-list

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#97385

hi,

On Sat, Oct 03, 2015 at 10:40:57AM -0700, Ronald Cosentino wrote:
> def funA(x,y,z):
>     return (x+y) * z
> def funB(x,y):
>     return(x-y)
> print(funA(4,funB(2,3), funB(3,2)))
> 
> the answer is 3. I don't know how it works.

it's simple:

- there is a "composition of functions", generally
  f(g()) (function in argument list of another function)
- first, Python evaulates the arguments first, from left to right 
- in this point, you'll get -1 for 2nd arg, and 1 for 3rd arg
- then your funcA() will be called with these arguents:
  4, -1, 1
- funcA() calculates this:
  (x+y)*z, in this case (4+(-1))*1


which is 3...


a.


-- 
I � UTF-8

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#97386

On Sat, 03 Oct 2015 10:40:57 -0700, Ronald Cosentino wrote:

> def funA(x,y,z):
>     return (x+y) * z
> def funB(x,y):
>     return(x-y)
> print(funA(4,funB(2,3), funB(3,2)))
> 
> the answer is 3. I don't know how it works.

def funA(x, y, z):
    return (x+y) * z

def funB(x, y):
    return (x-y)

# this line
# print(funA(4,funB(2,3), funB(3,2)))
# can be written as the following 4 lines:

a = funB(2, 3) # 2 - 3 -> -1

b = funB(3, 2) # 3 - 2 -> 1

c = funA(4, a, b) # (4 + -1) * 1 -> 3

print(c) # 3

-- 
Denis McMahon, denismfmcmahon@gmail.com

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#97391

On Sun, 4 Oct 2015 04:40 am, Ronald Cosentino wrote:

> def funA(x,y,z):
>     return (x+y) * z
> def funB(x,y):
>     return(x-y)
> print(funA(4,funB(2,3), funB(3,2)))
> 
> the answer is 3. I don't know how it works.


Break it up and consider it a little at a time, starting with the three
values given to funA:

* calculate 4 (too easy, it's already done)

* calculate funB(2, 3)
  => funB(2, 3) returns 2-3, which gives -1

* calculate funB(3,2)
  => funB(3, 2) returns 3-2, which gives 1


Then pass those three values to the funA function:

* calculate funA(4, -1, 1)
  => which returns (4 + -1)*1, which gives 3


and finally pass that value to print:

* print(3)


which prints 3.




-- 
Steven

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