Describe the difference between "pass by value" and "pass by reference" in function parameter passing.?

In computer programming, passing arguments to a function is essential to make programs dynamic, flexible and to enable code reusability. The two commonly used methods to pass arguments to functions are “pass by value” and “pass by reference”.

**Pass by Value**

When arguments are passed by value, the function receives a copy of the value of the actual parameter that is passed. What this means is that the function receives a new different variable in memory that contains the same value that was passed, but the original variable that was passed as an argument is not modified by anything that happens inside the function, because it remains in the scope of the calling function.

Here is an example in Python:

def increase(x):
    x += 1
    print("Inside the function x =", x)

a = 5
increase(a)
print("Outside the function a =", a)

*Output:*

Inside the function x = 6
Outside the function a = 5

In the above code, we have defined a function named "increase" which takes a "x" parameter. When we pass the value "a" to the function "increase", the value of "a" is copied into the variable "x" of the function "increase". Any modification on "x" would not have any effect on the original value of "a". Therefore, we can see that even though "x" was increased inside the function, the value of "a" remains the same outside the function.

**Pass by Reference**

When arguments are passed by reference, the function receives a memory address (i.e., a reference) of the actual parameter that is passed. It means that instead of creating a copy of the value, it creates a pointer to the original memory address location of the variable to be modified. In this way, any modification made in the function affects the original variable, since they refer to the same memory location.

Here is another example in Python:

def change_list(mylist):
    mylist.append([4, 5, 6])
    print("Inside the function mylist =", mylist)

mylist = [1, 2, 3]
change_list(mylist)
print("Outside the function mylist =", mylist)

*Output:*

Inside the function mylist = [1, 2, 3, [4, 5, 6]]
Outside the function mylist = [1, 2, 3, [4, 5, 6]]

In the above code, we have defined a function named "change_list" which takes a "mylist" parameter. We have passed a list "mylist" to the function, and then, we have modified the same list inside the function by appending another list to it. Since "mylist" was passed as a reference, the original list was modified.

In conclusion, "pass by value" means that the function receives a copy of the value of the actual parameter that is passed, while "pass by reference" means that the function receives a memory address of the actual parameter that is passed. If the parameter is passed by reference, it is possible for the function to modify the original variable, but if it is passed by value, the function can not modify the original variable. It is essential to understand the difference between these concepts as it has a significant impact on how functions work and how they can be used.