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function
  practice  
  assignment  
  solution  



A function is a block of organized, reusable code that is used to perform a single, related action. Functions provide better modularity for your application and a high degree of code reusing.

Function blocks begin with the keyword def followed by the function name and parentheses ( ( ) ).
Any input parameters or arguments should be placed within these parentheses. You can also define parameters inside these parentheses.
The first statement of a function can be an optional statement - the documentation string of the function or docstring.
The code block within every function starts with a colon (:) and is indented.
The statement return [expression] exits a function, optionally passing back an expression to the caller. A return statement with no arguments is the same as return None.
 
def printme( str ):
"This prints a passed string into this function"
print (str)
return


 

Calling a Function

Defining a function only gives it a name, specifies the parameters that are to be included in the function and structures the blocks of code.

Once the basic structure of a function is finalized, you can execute it by calling it from another function or directly from the Python prompt. Following is the example to call printme() function −

# Function definition is here
"This prints a passed string into this function"
print (str)
return
 
# Now you can call printme function
printme("I'm first call to user defined function!")
printme("Again second call to the same function")

 

 

Pass by reference vs value

All parameters (arguments) in the Python language are passed by reference. It means if you change what a parameter refers to within a function, the change also reflects back in the calling function. For example −

# Function definition is here
def changeme( mylist ):
"This changes a passed list into this function"
print ("Values inside the function before change: ", mylist)
mylist[2]=50
print ("Values inside the function after change: ", mylist)
return
 
# Now you can call changeme function
mylist = [10,20,30]
changeme( mylist )
print ("Values outside the function: ", mylist)
 
Here, we are maintaining reference of the passed object and appending values in the same object. So, this would produce the following result −
Values inside the function before change: [10, 20, 30]
Values inside the function after change: [10, 20, 50]
Values outside the function: [10, 20, 50]

 

There is one more example where argument is being passed by reference and the reference is being overwritten inside the called function.

# Function definition is here
def changeme( mylist ):
"This changes a passed list into this function"
mylist = [1,2,3,4] # This would assi new reference in mylist
print ("Values inside the function: ", mylist)
return
 
# Now you can call changeme function
mylist = [10,20,30]
changeme( mylist )
print ("Values outside the function: ", mylist)
 
The parameter mylist is local to the function changeme. Changing mylist within the function does not affect mylist. The function accomplishes nothing and finally this would produce the following result:
Values inside the function: [1, 2, 3, 4]
Values outside the function: [10, 20, 30]

 

call by value

call by reference

In call by value, a copy of actual arguments is passed to formal arguments of the called function and any change made to the formal arguments in the called function have no effect on the values of actual arguments in the calling function.

In call by reference, the location (address) of actual arguments is passed to formal arguments of the called function. This means by accessing the addresses of actual arguments we can alter them within from the called function.

In call by value, actual arguments will remain safe, they cannot be modified accidentally.

In call by reference, alteration to actual arguments is possible within from called function; therefore the code must handle arguments carefully else you get unexpected results.

 

Example using Call by Value

#include <stdio.h>
void swapByValue(int, int); /* Prototype */
int main() /* Main function */
{
int n1 = 10, n2 = 20;
/* actual arguments will be as it is */
swapByValue(n1, n2);
printf("n1: %d, n2: %d\n", n1, n2);
}
void swapByValue(int a, int b)
{
int t;
t = a; a = b; b = t;
}
OUTPUT
======
n1: 10, n2: 20

 

Example using Call by Reference

#include <stdio.h>
void swapByReference(int*, int*); /* Prototype */
int main() /* Main function */
{
int n1 = 10, n2 = 20;
/* actual arguments will be altered */
swapByReference(&n1, &n2);
printf("n1: %d, n2: %d\n", n1, n2);
}
void swapByReference(int *a, int *b)
{
int t;
t = *a; *a = *b; *b = t;
}
OUTPUT
======
n1: 20, n2: 10

 

 

Function Arguments

You can call a function by using the following types of formal arguments:

Required arguments
Keyword arguments
Default arguments
Variable-length arguments

 

 

Required arguments

Required arguments are the arguments passed to a function in correct positional order. Here, the number of arguments in the function call should match exactly with the function definition.
To call the function printme(), you definitely need to pass one argument, otherwise it gives a syntax error as follows −

# Function definition is here
def printme( str ):
"This prints a passed string into this function"
print (str)
return
 
# Now you can call printme function
printme()
 
When the above code is executed, it produces the following result:
Traceback (most recent call last):
File "test.py", line 11, in <module>
printme();
TypeError: printme() takes exactly 1 argument (0 given)

 

 

Keyword arguments

Keyword arguments are related to the function calls. When you use keyword arguments in a function call, the caller identifies the arguments by the parameter name.
This allows you to skip arguments or place them out of order because the Python interpreter is able to use the keywords provided to match the values with parameters. You can also make keyword calls to the printme() function in the following ways −

# Function definition is here
def printme( str ):
"This prints a passed string into this function"
print (str)
return
 
# Now you can call printme function
printme( str = "My string")


When the above code is executed, it produces the following result −
My string

The following example gives more clear picture. Note that the order of parameters does not matter.

# Function definition is here
def printinfo( name, age ):
"This prints a passed info into this function"
print ("Name: ", name)
print ("Age ", age)
return
 
# Now you can call printinfo function
printinfo( age=50, name="miki" )
 
When the above code is executed, it produces the following result −
Name: miki
Age 50

 

 

Default arguments

A default argument is an argument that assumes a default value if a value is not provided in the function call for that argument. The following example gives an idea on default arguments, it prints default age if it is not passed −

# Function definition is here
def printinfo( name, age = 35 ):
"This prints a passed info into this function"
print ("Name: ", name)
print ("Age ", age)
return
 
# Now you can call printinfo function
printinfo( age=50, name="miki" )
printinfo( name="miki" )
 
When the above code is executed, it produces the following result −
Name: miki
Age 50
Name: miki
Age 35

 

 

Variable-length arguments

You may need to process a function for more arguments than you specified while defining the function. These arguments are called variable-length arguments and are not named in the function definition, unlike required and default arguments.
 
Syntax for a function with non-keyword variable arguments is this −
def functionname([formal_args,] *var_args_tuple ):
"function_docstring"
function_suite
return [expression]

 
An asterisk (*) is placed before the variable name that holds the values of all non keyword variable arguments. This tuple remains empty if no additional arguments are specified during the function call. Following is a simple example −

# Function definition is here
def printinfo( arg1, *vartuple ):
"This prints a variable passed arguments"
print ("Output is: ")
print (arg1)
for var in vartuple:
print (var)
return

# Now you can call printinfo function
printinfo( 10 )
printinfo( 70, 60, 50 )

 
When the above code is executed, it produces the following result −
Output is:
10
Output is:
70
60
50

 

 

Anonymous Functions

These functions are called anonymous because they are not declared in the standard manner by using the def keyword. You can use the lambda keyword to create small anonymous functions.
 
Lambda forms can take any number of arguments but return just one value in the form of an expression. They cannot contain commands or multiple expressions.
An anonymous function cannot be a direct call to print because lambda requires an expression
Lambda functions have their own local namespace and cannot access variables other than those in their parameter list and those in the global namespace.
Although it appears that lambda's are a one-line version of a function, they are not equivalent to inline statements in C or C++, whose purpose is by passing function stack allocation during invocation for performance reasons.

 

Syntax
The syntax of lambda functions contains only a single statement, which is as follows −
lambda [arg1 [,arg2,.....argn]]:expression
 
Following is the example to show how lambda form of function works −
# Function definition is here
sum = lambda arg1, arg2: arg1 + arg2

 

# Now you can call sum as a function
print ("Value of total : ", sum( 10, 20 ))
print ("Value of total : ", sum( 20, 20 ))
 
When the above code is executed, it produces the following result −
Value of total : 30
Value of total : 40

 

 

Return Statement

The statement return [expression] exits a function, optionally passing back an expression to the caller. A return statement with no arguments is the same as return None.

All the above examples are not returning any value. You can return a value from a function as follows −

# Function definition is here
def sum( arg1, arg2 ):
# Add both the parameters and return them."
total = arg1 + arg2
print ("Inside the function : ", total)
return total

# Now you can call sum function
total = sum( 10, 20 )
print ("Outside the function : ", total )
 
When the above code is executed, it produces the following result −
Inside the function : 30
Outside the function : 30

 

 

Scope of Variables

All variables in a program may not be accessible at all locations in that program. This depends on where you have declared a variable.

The scope of a variable determines the portion of the program where you can access a particular identifier.

There are two basic scopes of variables in Python −
Global variables
Local variables

 

 

Global vs. Local variables

Variables that are defined inside a function body have a local scope, and those defined outside have a global scope.
This means that local variables can be accessed only inside the function in which they are declared, whereas global variables can be accessed throughout the program body by all functions. When you call a function, the variables declared inside it are brought into scope. Following is a simple example −

total = 0 # This is global variable.
# Function definition is here
def sum( arg1, arg2 ):
# Add both the parameters and return them."
total = arg1 + arg2; # Here total is local variable.
print ("Inside the function local total : ", total)
return total


# Now you can call sum function
sum( 10, 20 )
print ("Outside the function global total : ", total )
 
When the above code is executed, it produces the following result −
Inside the function local total : 30
Outside the function global total : 0


The way Python uses global and local variables is maverick. While in many or most other programming languages variables are treated as global if not otherwise declared, Python deals with variables the other way around. They are local, if not otherwise declared. The driving reason behind this approach is that global variables are generally bad practice and should be avoided. In most cases where you are tempted to use a global variable, it is better to utilize a parameter for getting a value into a function or return value to get it out. Like in many other program structures, Python also imposes good programming habit by design.
 
Variables in Python are implicitly declared by defining them, i.e. the first time you assign a value to a variable, this variable is declared and has automatically the data type of the object which has to be assigned to it.

 

The following example shows a wild combination of local and global variables and function parameters:

def foo(x, y):
global a
a = 42
x,y = y,x
b = 33
b = 17
c = 100
print(a,b,x,y)

 
a,b,x,y = 1,15,3,4
foo(17,4)
print(a,b,x,y)

The output looks like this:
42 17 4 17
42 15 3 4

 

 

 

 
         
 
 
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