Unable to run given code in Python.

Have code here (with its code here) that have difficulty in not only understanding, but if try to put at pythontutor.com, then cannot run too, as shown in the attachment.
So, obviously cannot run it too.

Am also going through the 3rd edition of the book titled: Learn_Python_Programming_An_in_depth_introduction_to_the_fundamentals, by: Fabrizio Romano, Heinrich Kruger; in order to understand where am making mistake in putting up the iterator.
But, seems have stuck in starting from the start, as till now have reached page #65, with silly doubts there too.


Didn't attract any response here.

So, came here, though have problem both in code running and understanding.

My understanding of the code is that the

lambda A: sum(2*(A[a]==a)-(A[A[a]]==a) for a in A)

For the array 'A', the index variable 'a' takes sequentially all values in it.
The condition 'A[a]==a' is met when the element at i-th position is the same as the i-th element.

Say, for the below cases:

1.  0123 : identity permutation,
2.  0213 : here A[0]=0, A[3]=3,
3.  0132 : here A[0]=0, A[1]=1,
4.  1023 : here A[2]=2, A[3]=3,
5.  1203 : here A[3]=3,
6.  1320 : here A[2]=2,
7.  2103 : here A[1]=1, A[3]=3,
8.  2013 : here A[3]=3,
9.  3120 : here A[1]=1, A[2]=2,
10. 3102 : here A[1]=1,

Though am confused on the interpretation, as well as the need for the condition:

A[A[a]]==a

Please post the code in question. Be sure to use [code] [/code] tags to preserve indentation.

Originally Posted by Turbocapitalist Please post the code in question. Be sure to use [code] [/code] tags to preserve indentation.

Am following the code as given in my first message, i.e. here.

That states:

lambda A:sum(2*(A[a]==a)-(A[A[a]]==a)for a in A) 

from itertools import permutations
for *x, in permutations([0,1,2,3]):
   print(x,f(x))

which am confused as the array A is not specified.

Also, the above code doesn't run.

=================================================
P.S. : modification

Sorry, am not conversant still with Python, and on copying the code verbatim, i.e. including header too; it works.
The code is:

f=\
lambda A:sum(2*(A[a]==a)-(A[A[a]]==a)for a in A) 

from itertools import permutations
for *x, in permutations([0,1,2,3]):
   print(x,f(x))

Am confused still as what does the first line of code mean even. Seems the first line of code defines a function that takes the definition of 'A',
as given on the second line. But, not know of this syntax.

Nor, am sure how the value corresponding to each permutation of the array is being computed, so unable to understand the running of the code too.

The output is:

[0, 1, 2, 3] 4
[0, 1, 3, 2] 0
[0, 2, 1, 3] 0
[0, 2, 3, 1] 1
[0, 3, 1, 2] 1
[0, 3, 2, 1] 0
[1, 0, 2, 3] 0
[1, 0, 3, 2] -4
[1, 2, 0, 3] 1
[1, 2, 3, 0] 0
[1, 3, 0, 2] 0
[1, 3, 2, 0] 1
[2, 0, 1, 3] 1
[2, 0, 3, 1] 0
[2, 1, 0, 3] 0
[2, 1, 3, 0] 1
[2, 3, 0, 1] -4
[2, 3, 1, 0] 0
[3, 0, 1, 2] 0
[3, 0, 2, 1] 1
[3, 1, 0, 2] 1
[3, 1, 2, 0] 0
[3, 2, 0, 1] 0
[3, 2, 1, 0] -4

As a start, forget lambdas, and do some basic practices, e.g.

list=[1,2,4,8]
for a in list:
    print(a)

list=[1,2,4,8]
for i in range(len(list)):
    print(i,list[i])

Originally Posted by NevemTeve As a start, forget lambdas, and do some basic practices, e.g. list=[1,2,4,8] for a in list: print(a) list=[1,2,4,8] for i in range(len(list)): print(i,list[i])

Request why the earlier syntax of

def function_name(parameters):

is not used here, instead the function name 'f' is followed by an '=' and a statement-continuation- character '\'.

-----

How that links up with the lambda function on the next line, is not an issue; at least for now.

Look carefully: it is not a working example, it is a counterexample: a piece of code, that doesn't work. The error message is below:

NameError: name 'f' is not defined

Originally Posted by NevemTeve Look carefully: it is not a working example, it is a counterexample: a piece of code, that doesn't work. The error message is below: NameError: name 'f' is not defined

Please see my second message's modification at the end, as it works, on adding the header in the stated code.

from itertools import permutations

#First

f = lambda A:sum(2*(A[a]==a)-(A[A[a]]==a)for a in A) 

for *x, in permutations([0,1,2,3]):
   print(x,f(x))
   
#Second

def f(A):
    A = sum(2*(A[a]==a)-(A[A[a]]==a)for a in A)
    return A
    
for *x, in permutations([0,1,2,3]):
   print(x,f(x))

Look at:

>>> help(sum)
>>> help(permutations)
>>> help(print)

Originally Posted by ajiten Request why the earlier syntax of def function_name(parameters): is not used here, instead the function name 'f' is followed by an '=' and a statement-continuation- character '\'.

They use whatever they want, if syntactically correct. how do you think we can tell you why they implemented a functionality this way or that way?
Otherwise if you start to learn python start it with something easier, this (what you posted) is an advanced topic.
Additionally you can [obviously] read the official documentation and the huge amount of tutorials, examples, sample programs on the net. It is extremely well documented.
For example regarding lambda: https://realpython.com/python-lambda/

You didn't get any answer on the other host, because what you posted there is incomplete.