Comparison function in Python using Lambdas

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I am trying to understand lambdas and I get the idea but how do I define multiple conditions for a Point2 [x,y] comparison, so something like:

if x1 < x2: -1
if x1 == x2: 0
if x1 > x2: 1

The code above is equivalent to:

cmp(x1,x2)

or in a (ugly) lambda expression:

lambda x1,x2: 1 if x1>x2 else (0 if x1==x2 else -1)

(works only in Python 2.6 and above).

Normally you should use lambda expressions only for functions like

def fun(...):
    return ...

How to Use Python Lambda Functions – Real Python, How lambdas compare with regular function objects; How to write lambda functions; Which functions in the Python standard library leverage lambdas; When to use  In python you cannot directly compare functions created by lambda expressions: >>> (lambda x: x+2) == (lambda x: x+2) False I made a routine to hash the disassembly. import sys import dis Stack Overflow

my_compare = lambda x1,x2 : cmp(x1, x2)
my_compare( -100, 100 )

Overusing lambda expressions in Python, along with built-in functions like filter() , map() etc. Alternatives to Lambdas Map. The built-in function map () takes a function as a first argument and applies it to each of the elements of its Filter. The built-in function filter (), another classic functional construct, can be converted into a list Reduce. Since Python 3, reduce () has gone

In such a case, lambda expressions aren't usually the best thing. As Jon Skeet mentioned, you're gonna end with multiple if-else expressions:

lambda x1, x2: -1 if x1 < x2 else (0 if x1 == x2 else -1)

For your specific problem:

lambda x1, x2: cmp(x1, x2)

is the way to go.

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EDIT: Updated to be real Python according to PEP308 :) Note that the PEP has interesting information about how this should be parenthesized depending on which version of Python you're using. I won't attempt to reproduce it here - just read the PEP!

How about:

-1 if x1 < x2 else (0 if x1 == x2 else 1)

(That's without any knowledge of Python lambda expressions, but it's a fairly common way of expressing this logic in a single expression, which I guess is what you're after.)

EDIT: Others have suggested using cmp - I've been assuming that the questioner actually wants more complicated logic, such as providing their own comparisons, but wants the general form of "choose from a few conditions in a single expression".

Lambda Functions in Python, Today's piece covers using lambda, map, and filter functions in Python. We'll be covering the basic syntax of each and walking through some  We can also use lambda functions when we have to pass a small function to another function. Don’t worry – we’ll cover this in detail soon when we see how to use lambda functions in Python. Lambda functions were first introduced by Alonzo Church in the 1930s. Mr. Church is well known for lambda calculus and the Church-Turing Thesis.

Simply one line of code to return the biggest of two values:

greater = lambda x,y : x if (x>y) else y   
print(greater(3,5))

Python loop for inside lambda, For example, here's a case-insensitive string comparison: sorted(​student_tuples, key=lambda student: student[2]) # sort by age [('dave', 'B', 10), ('​jane', 'B', 12), ('john', Using those functions, the above examples become simpler and faster:. Summary Lambdas, also known as anonymous functions, are small, restricted functions which do not need a name (i.e., an Every lambda function in Python has 3 essential parts: The lambda keyword. The parameters (or bound variables), and The function body. The syntax for writing a lambda is: lambda

Lambda, Map, and Filter in Python - Better Programming, Starting with 3.7, any function can use asynchronous generator expressions. Comparison operators implement a particular notion of what the value of an object is. The expression lambda parameters: expression yields a function object. Why Use Lambda Functions? The power of lambda is better shown when you use them as an anonymous function inside another function. Say you have a function definition that takes one argument, and that argument will be multiplied with an unknown number:

Sorting HOW TO, Chapter on the Lambda Operator and the functions map, filter and reduce. Lambda functions are mainly used in combination with the functions filter(), map() use Λ for Wilks's lambda in multivariate analysis of variance to compare group​  The lambda part is based on the use of the keyword lambda to define them in Python. Lambda is also the 11th letter of the Greek alphabet. But the main reason that the name is used in Python is because the term lambda is used to describe anonymous functions in calculus .

6. Expressions, A Python lambda is just another method to define a function. The general syntax of a Python lambda is: lambda arguments: expression. Lambda functions can  Map is a Python built-in function that takes in a function and a sequence as arguments and then calls the input function on each item of the sequence. For example, assume we have a list of integers and we want to square each element of the list using the map function. >>> L = [1, 2, 3, 4] >>> list(map(lambda x: x**2, L)) [1, 4, 9, 16]

Comments
  • -1: spending time "understanding" lambdas. They're "def" spelled funny.
  • When you say: if x1>x2 what does it return? It just goes to else?
  • 1 if x1>x2 else (0 if x1==x2 else -1) is the python equivalent of Javas: x1>x2?1:(x1==x2?0:-1). And yes it tries x1>x2 if that is False, it takes the value after else, if True it takes the value before the it (1 in that case)
  • From PEP 308, it looks like it will work in Python 2.5 if you put brackets in: "lambda x1,x2: (1 if x1>x2 else (0 if x1==x2 else -1))" - but I can't easily test that.
  • Thanks Jon, didn't know that.
  • my_compare = cmp; my_compare(-100, 100)
  • Better yet, just: cmp(-100, 100) Code was purely to demonstrate lamda usage.
  • Thanks Jon. I was surprised to see you reply to a python question but knew you would have great lambda knowledge from C# :)
  • Doh - looked at a Python page, but it was giving an example from PHP. Aargh. Trying to do too many things at once.
  • ...and what about if they are equals?
  • then the equal element will get returned , which is intutive I guess