Python: Lambda in a sum

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I'm trying to do the following, which is a representative example of what my final goal will be:

 yu = lambda x: 0
 for i in range(0,5):
      yu = lambda x: i + yu(x)

Unfortunately, it returns:

RuntimeError: maximum recursion depth exceeded

when I do:

print yu(0)

The print statement should return 10.

What's the correct way to do this?

 yu = lambda x: i + yu(x)

This makes yu into a function that always calls itself, guaranteeing infinite recursion with no base case.

Why? Well, you've built a closure where yu (and i) are the local variables in the function or module that the for loop is part of. That's not what you want; you want to close over the current values of yu and i, not the outer variables.

I'm not sure why you're even using lambda in the first place. If you want to define a function and give it a name, use def.

Here's an easy solution:

def yu(x): return 0
def make_new_yu(yu, i):
    def new_yu(x): return i + yu(x)
    return new_yu
for i in range(0, 5):
    yu = make_new_yu(yu, i)

By making the wrapping explicit, the correct way to do it becomes the most obvious way to do it.

You can, of course, use a lambda inside make_new_yu without making things more confusing:

def make_new_yu(yu, i):
    return lambda x: i + yu(x)

And you can even make the initial definition a lambda if you want. But if you insist on not having any def statements, you need to force the right values into the closure in some way, e.g., by using the default-value trick. That's much easier to get wrong—and harder to read once you've done it.

If you want an intuitive understanding of the difference, without learning the details: a function body defines a new scope. So, defining the function (by lambda or def) inside that function means your closure is from that new scope.

Lambda Operator, filter, reduce and map, You can assign the function to a variable to give it a name. The following example of a lambda function returns the sum of its two arguments: >>> f = lambda x, y� Considering the answers you try to sum up 0,1,2,3,4. If that was right you could use the following lambda expression: yu = lambda x: x + yu(x+1) if x<4 else x For yu(0) it delivers 10 as a result. The break condition is the value of x which is required to stay smaller than 4 in order to add up.

In the end, you have:

yu = lambda x: i + yu(x)

but yu will be looked up at runtime, not when you constructed the lambda. Do this instead:

for i in range(0,5):
    yu = lambda x, yu=yu: i + yu(x)

This does not return 10, though. It returns 20 instead:

>>> yu = lambda x: 0
>>> for i in range(0,5):
...     yu = lambda x, yu=yu: i + yu(x)
... 
>>> yu(0)
20

because now i is still looked up from the context (and by now the loop has finished so it's 4). Solution? Move i to a keyword argument too:

for i in range(0,5):
    yu = lambda x, yu=yu, i=i: i + yu(x)

Now this works:

>>> yu = lambda x: 0
>>> for i in range(0,5):
...     yu = lambda x, yu=yu, i=i: i + yu(x)
... 
>>> yu(0)
10

Moral of the story? Bind your context properly to the scope of the lambda.

python-sum-lambda-functions.py � GitHub, To demonstrate lambda functions we will loop through the range. # Conventional approach, we need to write different functions for. # sum of range of numbers. Use a conditional expression for the body of the lambda: >>> sum_digits = lambda n: 0 if n == 0 else (n % 10) + sum_digits(n // 10) >>> count_digit = lambda n: 0 if n == 0 else 1 + count_digit(n // 10) Also, if is preferred to use // for the division so that the code will still work in Python 3.

Considering the answers you try to sum up 0,1,2,3,4. If that was right you could use the following lambda expression:

yu = lambda x: x + yu(x+1) if x<4 else x

For yu(0) it delivers 10 as a result. The break condition is the value of x which is required to stay smaller than 4 in order to add up.

Assuming that is what you desired to do, you should leave out the loop for a rather concise statement.

This lambda expression differentiates from the others in resulting in different values (other than 10, when not choosing 0 as the parameter x) depending on the given argument.

Python Lambda Function, Python lambda Sum As you can see from the Python image below, we passed 10 as an argument. 10 + 5 = 15. In this Python lambda Sum example, we are using two arguments to perform addition or find the sum of two numbers. It means we have to assign two values while calling this expression. Python lambda Sum In this example, we are using the lambda expression to add 5 to the given argument value. This code accepts one value because we specified one argument after the lambda keyword. After the colon, it is an expression or the functionality it has to perform when we call this anonymous lambda function.

I believe I didn't fully understand your question, but could anyone check if this is what he meant?

I assumed you wouldn't need an iterator to generate a list of digits

yu = lambda x: x[0] + yu(x[1:]) if x!=[] else 0

Python Anonymous Function, Python anonymous function example tutorial, python lambda example, python So, if we want to make a function which will calculate sum of two number. A Lambda Function in Python programming is an anonymous function or a function having no name. It is a small and restricted function having no more than one line. Just like a normal function, a Lambda function can have multiple arguments with one expression.

facto=lambda f:f if f==0 else f+facto(f-1)
print(facto(4))

How to take the average of a list in Python, Using reduce() and lambda functions. The reduce method can be used to loop through the list and the sum can be computed in the lambda function. The number� 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.

How to Use Python Lambda Functions – Real Python, The function above defines a lambda expression that takes two arguments and returns their sum. Other than providing you with the feedback that Python is� 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. Now that we’ve cleared that up, you can use this info to spark enthralling conversation at office parties.

Python map Examples, Lambda: We use a lambda expression: this one receives a string parameter. It calls the startswith method. And: They are counted by sum. Three of the four strings� Python Lambda Previous Next A lambda function is a small anonymous function. A lambda function can take any number of arguments, but can only have one expression.

1.6 Higher-Order Functions, The first, sum_naturals, computes the sum of natural numbers up to n: In Python, we can create function values on the fly using lambda expressions, which� While a def statement is often more understandable, Python also has a number of features that can be used to replace lambda expressions, including special syntaxes (comprehensions), built-in functions (sum), and standard library functions (in the operators module).

Comments
  • Why are you even using lambda here? You're trying to define a function and give it a name—so why not use def?
  • Final goal is to sum a bunch of functions, starting with an initialized sum function to 0. Coming from MATLAB, it just seemed the natural thing to do: to use lambda functions for this goal. Thank you all for your help!
  • In Python, there is nothing that a lambda can do that a def cannot, except to appear in the middle of an expression (and save the 7 characters of typing return ). And there's a lot that a def can do that a lambda cannot, and they're easier to wrap your head around. Most importantly, def blocks are among the few things that create a new scope in Python (class blocks and modules are others; control statements like for and if are not, and there is no expression like a let that does so either).
  • More importantly, this probably isn't the way you want to solve this problem in the first place. What you're going to end up with is a stack of functions that you call through—which is fine in a language with tail recursion optimization, especially one that can inline through tail calls, but in Python it just means stack limits, hard-to-read tracebacks, and inefficiency.
  • Nope, this returns 20.. you didn't bind i in the context of make_new_yu, and are instead using it as a global.
  • Ah, right, he's also closing over the wrong i, as well as using the wrong yu. Fixed.
  • @abarnert I like your approach since I'm a complete newbie in Python. It works in the interactive prompt. However, when I put the for loop inside a main function, I get an error: UnboundLocalError: local variable 'yu' referenced before assignment. Why is that?
  • @Eli: If you're trying to rebind the global variable yu from inside a function, you have to write global yu before using it in the function. (If you're trying to create a new local variable yu, starting with a copy of the global yu… really, the easiest thing to do there is just not call your local variable yu.)
  • I think something like x=i is also required in the lambda : yu = lambda x=i, yu=yu: x + yu(x).
  • @AshwiniChaudhary: Eh, come again? x is the one positional parameter that you pass in and on. Setting a default value for it makes no sense since it'll never use that default.
  • Ah! yes you're right, i=i is what I should have used(to prevent the closure thing).
  • Really, someone who doesn't know how to understand closures in Python shouldn't be writing code like this. And even someone who does will be better served by a explicit def.