Consider this minimal example:
def arfunc(x,*a):
sa=a[1]
return sa
b=arfunc(1.,(5.,2.))
print(b)
I need to pass an array whose length may vary to a function and then work with individual array elements within that function. If I exchange a[1] by a[0] in the function, the entire list is returned. Why does this not work, and how do I make it work?
A function taking variable arguments (*args) expects them as several arguments:
b=arfunc(1., 5., 2.)
print(b)
If you want to use the function as you did, remove the asterisk:
def arfunc(x, a):
...
a = a[0] inside the function body.* symbol in function definition allows a function to have a variadic type. The variable a will become a collection of all of the positional arguments passed after the first one: x.
There is no need to use the * symbol to let a have a variable length. Lists already have that property in Python.
you missed one thing that is if we have to pass the variable length argument(called Arbitrary Arguments in python) with a function you need to add *(asterisk)
def arfunc(x,*a): # here *a can hold argument that may vary in numbers.
sa=a[1]
return sa
b=arfunc(1.,*(5.,2.)) # here you also have to provide a arbitary arguments like this.
print(b)
For better understanding refer below written code:
def arfunc(x,*a):
sa=a[1]
return sa
d = 1.
e = (5.,2.)
b=arfunc(d,*e)
print(b)
You can pass list or list of list.
Eg
List -
[1,2,3,4]
List of lists -
[1,2,3[4,5,6]]
If thats what you are trying to do.
The star '*' operator is used to unpack arguments from a list. It is similar to the varargs in C/C++ or Java when a function expects a random number of arguments. You cannot change the original list / array in this case.
However, if you need to pass an array on that you will change inside the function, and then returned it back, I recommend you to pass and array or list to the function without the unpacking operator *. Example:
def try_to_change_list_contents(the_list):
print('got', the_list)
the_list.append('four')
print('changed to', the_list)
outer_list = ['one', 'two', 'three']
print('before, outer_list =', outer_list)
try_to_change_list_contents(outer_list)
print('after, outer_list =', outer_list)
You can take a look at this post too: topic
(5.,2.)is not an array, it is a tuple.sa=a[1]line, and you will see thata == ((5., 2.),)- a tuple of one element which is also a tuple.