Python lambda functions with filter, map & reduce function

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Python lambda functions with filter, map & reduce function
3.5.1 Lambda functions
A small anonymous (unnamed) function can be created with the lambda keyword.
lambda expressions allow a function to be created and passed in one line of code.
It can be used as an argument to other functions. It is restricted to a single expression
lambda arguments : expression
It can take any number of arguments,but have a single expression only.
3.5.1.1 Example - lambda function to find sum of square of two numbers
z = lambda x, y : x*x + y*y
print(z(10,20)) # call to function
Output
500
Here x*x + y*y expression is implemented as lambda function.
after keyword lambda, x & y are the parameters and after : expression to evaluate is
x*x+ y*y
3.5.1.2 Example - lambda function to add two numbers
def sum(a,b):
return a+b
is similar to
result= lambda a, b: (a + b)
lambda function allows you to pass functions as parameters.

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3.5.2 map function in python
The map() function in Python takes in a function and a list as argument. The function is
called with a lambda function with list as parameter.
It returns an output as list which contains all the lambda modified items returned by
that function for each item.
3.5.2.1 Example - Find square of each number from number list using map
def square(x):
return x*x
squarelist = map(square, [1, 2, 3, 4, 5])
for x in squarelist:
print(x,end=',')
Output
1,4,9,16,25,
Here map function passes list elements as parameters to function. Same function can be
implemented using lambda function as below,
Example -
squarelist = map(lambda x: x*x, [1, 2, 3, 4, 5])
for x in squarelist:
print(x)
3.5.2.2 Example - lambda function to find length of strings
playernames = ["John", "Martin", "SachinTendulkar"]
playernamelength = map(lambda string: len(string), playernames)
for x in playernamelength:
print(x, end="")
Output
4 6 15
Lambda functions allow you to create “inline” functions which are useful for functional
programming.

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3.5.3 filter function in python
filter() function in Python takes in a function with list as arguments.
It filters out all the elements of a list, for which the function returns True.
3.5.3.1 Example - lambda function to find player names whose name length is 4
playernames = ["John", "Martin", "SachinTendulkar","Ravi"]
result = filter(lambda string: len(string)==4, playernames)
for x in result:
print(x)
Output
John
Ravi
3.5.4 reduce function in python
reduce() function in Python takes in a function with a list as argument.
It returns output as a new reduced list. It performs a repetitive operation over the
pairs of the list.
3.5.4.1 Example - lambda function to find sum of list numbers.
from functools import reduce
numlist = [12, 22, 10, 32, 52, 32]
sum = reduce((lambda x, y: x + y), numlist)
print (sum)
Output
160
Here it performs repetitive add operations on consecutive pairs. it performs (12+22) then
((12+22)+10) as for other elements of the list.

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3.6 Programs on User Defined Functions
3.6.1 Python User defined function to convert temperature from c to f
Steps
Define function convert_temp & implement formula f=1.8*c +32
Read temperature in c
Call function convert_temp & pass parameter c
Print result as temperature in fahrenheit
Code
def convert_temp(c):
f= 1.8*c + 32
return f
c= float(input("Enter Temperature in C: "))
f= convert_temp(c)
print("Temperature in Fahrenheit: ",f)
Output
Enter Temperature in C: 37
Temperature in Fahrenheit: 98.60
In above code, we are taking the input from user, user enters the temperature in Celsius
and the function convert_temp converts the entered value into Fahrenheit using the
conversion formula 1.8*c + 32
3.6.2 Python User defined function to find max, min, average of given list
In this program, function takes a list as input and return number as output.
Steps
Define max_list, min_list, avg_list functions, each takes list as parameter

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Initialize list lst with 10 numbers
Call these functions and print max, min and avg values
Code
def max_list(lst):
maxnum=0
for x in lst:
if x>maxnum:
maxnum=x
return maxnum
def min_list(lst):
minnum=1000 # max value
for x in lst:
if x<minnum:
minnum=x
return minnum
def avg_list(lst):
return sum(lst) / len(lst)
# main function
lst = [52, 29, 155, 341, 357, 230, 282, 899]
maxnum = max_list(lst)
minnum = min_list(lst)
avgnum = avg_list(lst)
print("Max of the list =", maxnum)
print("Min of the list =", minnum)
print("Avg of the list =", avgnum)
Output
Max of the list = 899
Min of the list = 29
Avg of the list = 293.125
3.6.3 Python User defined function to print the Fibonacci series
In this program, function takes a number as input and return list as output.
In mathematics, the Fibonacci numbers are the numbers in the sequence such as for each
number is the sum of its previous two numbers.

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Following is the example of Fibonacci series up to 6 Terms
1, 1, 2, 3, 5, 8
The sequence Fn of Fibonacci numbers is recursively represented as Fn-1 + Fn-2.
Implement function to return Fibonacci series.
Code
def fiboseries(n): # return Fibonacci series up to n
result = []
a= 1
b=0
cnt=1
c=0
while cnt <=n:
c=a+b
result.append(c)
a=b
b=c
cnt=cnt+1
return result
terms=int(input("Enter No. of Terms: "))
result=fiboseries(terms)
print(result)
Output
Enter No. of Terms: 8
[1, 1, 2, 3, 5, 8, 13, 21]
Enter No. of Terms: 10
[1, 1, 2, 3, 5, 8, 13, 21, 34, 55]
In code above, fiboseries function is implemented which takes numbers of terms as
parameters and returns the fibonacci series as an output list.
Initialize the empty list, find each fibonacci term and append to the list. Then return list
as output
3.6.4 Python User defined function to reverse a String without using Recursion
In this program, function takes a string as input and returns reverse string as output.

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Steps
Implement reverse_string function , Use string slicing to reverse the string.
Take a string as Input
Call reverse_function & pass string
Print the reversed string.
Code
def reverse_string(str):
return str[::-1]
nm=input("Enter a string: ")
result=reverse_string(nm)
print("Reverse of the string is: ", result)
Output
Enter a string: Soft
Reverse of the string is: tfoS
In this code, one line reverse_string is implemented which takes string as parameter and
returns string slice as whole from last character. -1 is a negative index which gives the last
character. Result variable holds the return value of the function and prints it on screen.
3.6.5 Python User defined function to find the Sum of Digits in a Number
Steps
Implement function sumdigit takes number as parameter
Input a Number n
Call function sumdigit
Print result
Code
def sumdigit(n):
sumnum = 0
while (n != 0):

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sumnum = sumnum + int(n % 10)
n = int(n/10)
return sumnum
# main function
n = int(input("Enter number: "))
result=sumdigit(n)
print("Result= ",result)
Output
Enter number: 5965
Result= 25
Enter number: 1245
Result= 12
In this code, Input number n, pass to sumdigit function, sum of digit of given number is
computed using mod operator (%). Mod a given number by 10 and add to the sum variable.
Then divide the number n by 10 to reduce the number. If number n is non zero then repeat
the same steps.
3.6.6 Python Program to Reverse a String using Recursion
Steps
Implement recursive function fibo which takes n terms
Input number of terms n
Call function fibo & pass tems
Print result
Code
def fibo(n):
if n <= 1:
return 1

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else:
return(fibo(n-1) + fibo(n-2))
terms = int(input("Enter number of terms: "))
print("Fibonacci sequence:")
for i in range(terms):
print(fibo(i),end="")
Output
Enter number of terms: 8
Fibonacci sequence:
1 1 2 3 5 8 13 21
In above code, fibo function is implemented using a recursive function, here n<=1 is
terminating condition, if true the return 1 and stops the recursive function. If false return
sum of call of previous two numbers. fibo function is called from the main function for
each term.
3.6.7 Python Program to Find the Power of a Number using recursion
InpuSteps
t base and exp value
call recursive function findpower with parameter base & exp
return base number when the power is 1
If power is not 1, return base*findpower(base,exp-1)
Print the result.
Code
def findpower(base,exp):
if(exp==1):
return(base)
else:
return(base*findpower(base,exp-1))
base=int(input("Enter base: "))
exp=int(input("Enter exp: "))
print("Result: ",findpower(base,exp))

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Output
Enter base: 7
Enter exp: 3
Result: 343
In this code, findpower is a recursive function which takes base and exp as numeric parameters.
Here expt==1 is the terminating condition, if this true then return the base value. If terminating
condition is not true then multiply base value with decrement exp value and call same function