example of a lambda function

```
#How to define and use lambda functions
add = lambda a, b: a + b
print(add(6, 9))
```

In Python, a `Lambda Function`

is a single-lined function that is defined without an explicit name. They are also known as** anonymous functions**.

While the `def `

keyword is used to define named functions, the `lambda`

keyword is used to define anonymous functions i.e lambda functions.

The lambda functions are more lightweight and concise compared to named functions, this makes them ideal for simple operations that do not require complex logic.

To define this type of functions, we use lambda keyword with arguments and the expression as shown below:

`lambda arguments : expression`

The comma-separated arbitrary arguments are separated from the function expression/body by a colon. We can only use one expression in the functions body.

Note: lambda functions can have any number of arguments but only a single expression.

Consider the following function:

```
def func(x):
return x
```

The equivalent lambda function to the above function is as follows:

`lambda x : x`

We do not need to explicitly use the `return`

statement with lambda functions because it automatically returns the value of the expression after evaluation. Other examples:

a lambda function to add two numbers:

`lambda a, b : a + b`

The above lambda function takes two arguments

and**a**

and returns the sum of those two arguments** b**` (a + b)`

.

a lambda function to tell whether a number is odd or even:

`lambda num : "Odd" if num % 2 else "Even"`

The above lambda function will return 'Odd' if the number is odd , else it will return 'Even'.

## Calling lambda functions

A lambda functions can be called directly by enclosing it with parentheses, For example:

```
#returns the input value
print((lambda x : x)(9))
#returns the sum of the two arguments
print((lambda a, b : a + b)(10, 20))
#Checks whether a number is even or odd
print((lambda x : 'Odd' if x % 2 else 'Even')(50))
```

However, in order to make the lambda functions persist and be usable multiple times, we can assign them to a variable, then use the variable name to call the function. Example:

```
add = lambda a, b : "%s + %s = %s"%(a, b, a + b)
print(add(1, 2))
print(add(6, 7))
print(add(100, 200))
```

This is not unique to lambda functions, we can also assign variable names to normal functions, however, the normal function retains it's declaration name because the name is associated with the function object itself.

```
def add(a, b):
return a + b
demo = add
print(demo(1, 2))
print(demo(100, 200))
#The function retains declaration name
print(demo)
```

## Uses of lambda functions

Lambda functions are in some cases more cryptic than the 'normal' functions because you have to compress all the function logic into one expression. Additionally, anything that you can achieve with `lambda `

functions, you can also effectively accomplish it with the named functions. There are some instances, however, when `lambda `

functions are more suited than the regular functions.

### For simple transformation

The `lambda `

functions can be more suitable if all we want is to transform an object in a simple way.

```
x = 5
y = 10
print(x + (lambda a : a *a)(y) ) #print sum of x and the square of y
my_list = [8, 7, 2, 9, 5]
print((lambda l : sorted(l))(my_list)[0]) #get the first element of the sorted a sorted list.
```

### As arguments to higher-order functions

In Python and other functional languages, it is common to pass functions as arguments to other functions. In such cases, `lambda `

functions are often used because they can be defined inline. Some builtin such functions includes:

#### The` map()`

functions:

The `map()`

function is used to perform an action on each item in a collection.

```
L = list(map(lambda a : a + 1,[10, 20, 30, 40])) # add 1 to each number in a list
print(L)
names = [('John', 'Doe'), ('Jane', 'Doe'), ('Will', 'Smith')] # return a fullname given a tuples of first and last names
names = list(map(lambda name: name[0] + ' ' + name[1], names ))
print(names)
```

#### The` filter()`

functions

This function is used to filter values in given sequence by their boolean values, based on a given function. Examples:

```
my_list = [1, 5, 3, 11, 0, 2, 9, 4, 15,8, 10, 35, 67, 25, 30]
evens = list(filter(lambda a: not a % 2, my_list)) #return only the even numbers in my_list
print(evens)
multiples_of_5 = list(filter(lambda a: a and not a % 5, my_list)) #returns only the multiples of 5 in my_list
print(multiples_of_5)
```

#### The `functools.reduce() `

function

This function applies a function of two arguments cumulatively on a sequence of elements from left to right, reducing the sequence to a single value.

```
from functools import reduce
my_list = [1, 2, 3, 4, 5]
sum = reduce(lambda a, b : a + b, my_list)#returns the sum of all the numbers in my_list
print(sum)
prod = reduce(lambda a, b : a * b, my_list) #returns the product of all the numbers in my_list
print(prod)
```

### As key Functions

Lambda expressions can be used as keys in functions and methods that allow customization using functions as keys.

For example in the builtin `sorted()`

function and the `sort() `

method of lists, a `lambda `

expression can be used to define the sorting criteria.

```
my_list = [('John', 30), ('Joe', 20), ('Jane', 25)]
my_sorted_list = sorted(my_list, key = lambda a : a[1] )
print(my_sorted_list)
```

In the above example, we use the `lambda `

expression to indicate to the `sorted() `

function that it should sort the list elements(which are tuples) based on their second elements(at index `1`

). If no key was given the function would have sorted the list element alphabetically, based on their first element, therefore, the sorted list would have been `[('Jane', 25), ' ('John', 30), ('Joe', 20)]`

.

Other builtin functions that allows key argument includes `max() `

and `min()`

.

```
data = [{'name': 'John', 'age': 30 }, {'name': 'Alex', 'age': 24}, {'name': 'Sam', 'age': 28}]
max_age = max(data, key=lambda x:x['age'])
min_age = min(data, key=lambda x:x['age'])
print("The oldest person is", max_age['name'])
print("The youngest person is", min_age['name'])
```