Arguments and Parameters

In the previous example, addnum() collected its numbers from the user inside the function. A user defined function can instead receive values at the moment it is called. This is where two closely related terms come in:

• An argument is a value passed to the function during the function call.
• A parameter is the name in the function header that receives the corresponding argument.

Passing an argument to a function

The task: display the sum of the first n natural numbers, where n is passed as an argument. Requirements — (1) n is passed as an argument, (2) calculate the sum of the first n natural numbers, (3) display the sum.

# Sum of the first n natural numbers

# function header — n is the PARAMETER
def sumSquares(n):
    sum = 0
    for i in range(1, n + 1):
        sum = sum + i
    print("The sum of first", n, "natural numbers is:", sum)

num = int(input("Enter the value for n: "))
# num is the ARGUMENT, referring to the value the user typed
sumSquares(num)      # function call

Suppose the user enters 5. Then num refers to the value 5 and is used as the argument in the call sumSquares(num). When the call transfers control to def sumSquares(n):, the parameter n also refers to that same value 5 — argument and parameter are two names bound to one value (this is exactly what Figure 7.3 illustrates).

Argument and parameter share the same identity — id()

Since num and n refer to the same value, they must have the same identity. Python's built-in id() function reveals the identity of the object a name refers to, so we can verify this directly.

The task: a function accepts an integer and increments it by 5, displaying the id of the argument before the call, and the id of the parameter before and after the increment — to check whether the parameter gets a new memory location.

# Add 5 to a user-input number, watching the ids

def incrValue(num):
    # id of the parameter BEFORE the increment
    print("Parameter num has value:", num, "\nid =", id(num))
    num = num + 5
    # id of the parameter AFTER the increment
    print("num incremented by 5 is", num, "\nNow id is", id(num))

number = int(input("Enter a number: "))
print("id of argument number is:", id(number))   # id of the argument
incrValue(number)

A sample run (the actual id values differ from machine to machine and run to run):

Enter a number: 8
id of argument number is: 1712903328
Parameter num has value: 8
id = 1712903328
num incremented by 5 is 13
Now id is 1712903408

Read the output carefully:

• Before the increment, the argument number and the parameter num print the same id — they refer to the same object holding 8.
• After num = num + 5, the parameter's id changes: num now refers to a new object holding 13, while number outside still refers to 8. Figure 7.4 pictures exactly this before/after situation.

Argument and parameter may share a name

Nothing stops the argument and the parameter from having the same name. The task: a user defined function myMean() calculates the mean of floating point values stored in a list. Requirements — (1) one parameter, a list of floating point values; (2) mean = sum of all the numbers divided by how many there are.

# Function to calculate the mean of values in a list

def myMean(myList):
    total = 0
    count = 0
    for i in myList:
        total = total + i     # add each element to total
        count = count + 1     # count the elements
    mean = total / count
    print("The calculated mean is:", mean)

myList = [1.3, 2.4, 3.5, 6.9]
myMean(myList)    # the argument is also called myList

The calculated mean is: 3.[PII:card]

One more numeric example — factorial

The task: a user defined function calcFact() calculates and displays the factorial of a number passed as an argument.

# Function to calculate a factorial

def calcFact(num):
    fact = 1
    for i in range(num, 0, -1):
        fact = fact * i
    print("Factorial of", num, "is", fact)

num = int(input("Enter the number: "))
calcFact(num)

Enter the number: 5
Factorial of 5 is 120

Note that because multiplication is commutative, 5! = 54321 gives the same result as 12345 — so the loop may run downwards (as here) or upwards.

(A) Strings as parameters

The arguments passed so far were all numeric, but functions accept string arguments just as readily. The task: a function accepts a first name and a last name, concatenates them into a full name, and displays Hello followed by the full name (input Gyan and Vardhan should print Hello Gyan Vardhan). Requirements — two parameters for the two names; concatenate with the + operator, keeping a space between them; display the result.

# Function to display a full name

def fullname(first, last):
    # + concatenates strings; " " puts a space between the names
    fullname = first + " " + last
    print("Hello", fullname)

first = input("Enter first name: ")
last = input("Enter last name: ")
fullname(first, last)    # function call

Enter first name: Gyan
Enter last name: Vardhan
Hello Gyan Vardhan

(B) Default parameters

Python allows a parameter to carry a default value — a pre-decided value that the parameter takes when the function call does not supply its corresponding argument.

The task: accept the numerator and denominator of a fraction and call a user defined function mixedFraction() when the fraction is not a proper fraction. The default value of the denominator is 1. The function displays a mixed fraction only if the fraction does not evaluate to a whole number.

# Display a mixed fraction for an improper fraction

def mixedFraction(num, deno = 1):     # deno has default value 1
    remainder = num % deno
    # does the fraction evaluate to a whole number?
    if remainder != 0:
        quotient = int(num / deno)
        print("The mixed fraction =", quotient, "(", remainder, "/", deno, ")")
    else:
        print("The given fraction evaluates to a whole number")

num = int(input("Enter the numerator: "))
deno = int(input("Enter the denominator: "))
print("You entered:", num, "/", deno)
if num > deno:                        # improper fraction?
    mixedFraction(num, deno)          # function call
else:
    print("It is a proper fraction")

Enter the numerator: 17
Enter the denominator: 2
You entered: 17 / 2
The mixed fraction = 8 ( 1 / 2 )
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