I want to print out the elements in the list in reverse order recursively.
def f3(alist):
if alist == []:
print()
else:
print(alist[-1])
f3(alist[:-1])
I know it is working well, but I don't know the difference between
return f3(alist[:-1])
and
f3(alist[:-1])
Actually both are working well.
My inputs are like these.
f3([1,2,3])
f3([])
f3([3,2,1])
There is a difference between the two although it isn't noticeable in this program. Look at the following example where all I am doing is passing a value as an argument and incrementing it thereby making it return the value once it hits 10 or greater:
from sys import exit
a = 0
def func(a):
a += 1
if a >= 10:
return a
exit(1)
else:
# Modifications are made to the following line
return func(a)
g = func(3)
print(g)
Here the output is 10
Now if I re-write the code the second way without the "return" keyword like this :
from sys import exit
a = 0
def func(a):
a += 1
if a >= 10:
return a
exit(1)
else:
# Modifications are made to the following line
func(a)
g = func(3)
print(g)
The output is "None".
This is because we are not returning any value to handle.
In short, return is like internal communication within the function that can be possibly handled versus just running the function again.
Related
Like in R:
a <- 2
or even better
a ← 2
which should translate to
a = 2
and if possible respect method overloading.
= is overloaded (not in the multiple dispatch sense) a lot in Julia.
It binds a new variable. As in a = 3. You won't be able to use ← instead of = in this context, because you can't overload binding in Julia.
It gets lowered to setindex!. As in, a[i] = b gets lowered to setindex!(a, b, i). Unfortunately, setindex! takes 3 variables while ← can only take 2 variables. So you can't overload = with 3 variables.
But, you can use only 2 variables and overload a[:] = b, for example. So, you can define ←(a,b) = (a[:] = b) or ←(a,b) = setindex!(a,b,:).
a .= b gets lowered to (Base.broadcast!)(Base.identity, a, b). You can overload this by defining ←(a,b) = (a .= b) or ←(a,b) = (Base.broadcast!)(Base.identity, a, b).
So, there are two potentially nice ways of using ←. Good luck ;)
Btw, if you really want to use ← to do binding (like in 1.), the only way to do it is using macros. But then, you will have to write a macro in front of every single assignment, which doesn't look very good.
Also, if you want to explore how operators get lowered in Julia, do f(a,b) = (a .= b), for example, and then #code_lowered f(x,y).
No. = is not an operator in Julia, and cannot be assigned to another symbol.
Disclaimer: You are fully responsible if you will try my (still beginner's) experiments bellow! :P
MacroHelper is module ( big thanks to #Alexander_Morley and #DanGetz for help ) I plan to play with in future and we could probably try it here :
julia> module MacroHelper
# modified from the julia source ./test/parse.jl
function parseall(str)
pos = start(str)
exs = []
while !done(str, pos)
ex, pos = parse(str, pos) # returns next starting point as well as expr
ex.head == :toplevel ? append!(exs, ex.args) : push!(exs, ex)
end
if length(exs) == 0
throw(ParseError("end of input"))
elseif length(exs) == 1
return exs[1]
else
return Expr(:block, exs...) # convert the array of expressions
# back to a single expression
end
end
end;
With module above you could define simple test "language":
julia> module TstLang
export #tst_cmd
import MacroHelper
macro tst_cmd(a)
b = replace("$a", "←", "=") # just simply replacing ←
# in real life you would probably like
# to escape comments, strings etc
return MacroHelper.parseall(b)
end
end;
And by using it you could probably get what you want:
julia> using TstLang
julia> tst```
a ← 3
println(a)
a +← a + 3 # maybe not wanted? :P
```
3
9
What about performance?
julia> function test1()
a = 3
a += a + 3
end;
julia> function test2()
tst```
a ← 3
a +← a + 3
```
end;
julia> test1(); #time test1();
0.000002 seconds (4 allocations: 160 bytes)
julia> test2(); #time test2();
0.000002 seconds (4 allocations: 160 bytes)
If you like to see syntax highlight (for example in atom editor) then you need to use it differently:
function test3()
#tst_cmd begin
a ← 3
a ← a + a + 3 # parser don't allow you to use "+←" here!
end
end;
We could hope that future Julia IDEs could syntax highlight cmd macros too. :)
What could be problem with "solution" above? I am not so experienced julian so many things. :P (in this moment something about "macro hygiene" and "global scope" comes to mind...)
But what you want is IMHO good for some domain specific languages and not to redefine basic of language! It is because readability very counts and if everybody will redefine everything then it will end in Tower of Babel...
I’m trying to get a better understanding of recursion using the merge sort algorithm using Python 2.7. I have written a small snippet of code to break down a list recursively. The code seems to work fine except for the last step. For the base case, the program is supposed to return a list of size 1. However, it is returning the value “none”. Where am I going wrong?
mylist = [14,88,2,14,9,123,1,5]
def concour(thelist):
mid = len(thelist) / 2
LeftSide = thelist[:mid]
print LeftSide,'length is ', len(LeftSide)
if len(LeftSide) == 1: #the base case here
print LeftSide
return LeftSide
else:
concour(LeftSide) #recursive call
print concour(mylist)
"""
[14, 88, 2, 14] length is 4
[14, 88] length is 2
[14] length is 1
[14]
None
"""
You're missing your return statement in the recursive call.
Say I have a string.
"poop"
I want to change "poop" to "peep".
In fact, I also want all of the o's in poop to change to e's for any word I put in.
Here's my attempt to do the above.
def getword():
x = (input("Please enter a word."))
return x
def main():
y = getword()
for i in range (len(y)):
if y[i] == "o":
y = y[:i] + "e"
print (y)
main()
As you can see, when you run it, it doesn't amount to what I want. Here is my expected output.
Enter a word.
>>> brother
brether
Something like this. I need to do it using slicing. I just don't know how.
Please keep your answer simple, since I'm somewhat new to Python. Thanks!
This uses slicing (but keep in mind that slicing is not the best way to do it):
def f(s):
for x in range(len(s)):
if s[x] == 'o':
s = s[:x]+'e'+s[x+1:]
return s
Strings in python are non-mutable, which means that you can't just swap out letters in a string, you would need to create a whole new string and concatenate letters on one-by-one
def getword():
x = (input("Please enter a word."))
return x
def main():
y = getword()
output = ''
for i in range(len(y)):
if y[i] == "o":
output = output + 'e'
else:
output = output + y[i]
print(output)
main()
I'll help you this once, but you should know that stack overflow is not a homework help site. You should be figuring these things out on your own to get the full educational experience.
EDIT
Using slicing, I suppose you could do:
def getword():
x = (input("Please enter a word."))
return x
def main():
y = getword()
output = '' # String variable to hold the output string. Starts empty
slice_start = 0 # Keeps track of what we have already added to the output. Starts at 0
for i in range(len(y) - 1): # Scan through all but the last character
if y[i] == "o": # If character is 'o'
output = output + y[slice_start:i] + 'e' # then add all the previous characters to the output string, and an e character to replace the o
slice_start = i + 1 # Increment the index to start the slice at to be the letter immediately after the 'o'
output = output + y[slice_start:-1] # Add the rest of the characters to output string from the last occurrence of an 'o' to the end of the string
if y[-1] == 'o': # We still haven't checked the last character, so check if its an 'o'
output = output + 'e' # If it is, add an 'e' instead to output
else:
output = output + y[-1] # Otherwise just add the character as-is
print(output)
main()
Comments should explain what is going on. I'm not sure if this is the most efficient or best way to do it (which really shouldn't matter, since slicing is a terribly inefficient way to do this anyways), just the first thing I hacked together that uses slicing.
EDIT Yeah... Ourous's solution is much more elegant
Can slicing even be used in this situation??
The only probable solution I think would work, as MirekE stated, is y.replace("o","e").
I need to write a recursive function that returns the remainder of two numbers. Here's what I wrote:
def remainder(a,b):
if a==b:
return 0
else:
k=a-b
return a-b + remainder(b,a-k)
If we test remainder(5,3) the function will return 2 and it's correct but if we test remainder(15,3),
we'll get 12 and its false. I just don't know how to debug it.
You are missing a case: (when a < b)
def remainder(a,b):
if a<b: #trivial: remainder = a - b*0 = a
return a
else: #reduce the problem to a simple one
return remainder(a-b, b)
Test:
print remainder(15,3)
Output:
0
Here if you are lazy and don't want to write more than 2 lines:
def remainder(a,b):
return a if a < b else remainder(a-b, b)
It should be something like this :
def remainder(a,b):
if a<b:
return a
else:
return remainder(a-b,b)
You can do:
def remainder(a, b):
if a < b:
return a
return remainder(a - b, b)
Examples:
>>> remainder(15, 3)
0
>>> remainder(14, 3)
2
>>> remainder(13, 3)
1
If a < b then it means we're done: we know the result of the computation and we can return a. Otherwise we need to subtract b from a and we call remainder again recursively. This can then repeatedly continue until a is smaller than b. Once that happens the recursion stops (i.e., it won't call remainder again) but we're able to return the result.
I find functools.partial to be extremely useful, but I would like to be able to freeze arguments out of order (the argument you want to freeze is not always the first one) and I'd like to be able to apply it to several methods on a class at once, to make a proxy object that has the same methods as the underlying object except with some of its methods parameters being frozen (think of it as generalizing partial to apply to classes). And I'd prefer to do this without editing the original object, just like partial doesn't change its original function.
I've managed to scrap together a version of functools.partial called 'bind' that lets me specify parameters out of order by passing them by keyword argument. That part works:
>>> def foo(x, y):
... print x, y
...
>>> bar = bind(foo, y=3)
>>> bar(2)
2 3
But my proxy class does not work, and I'm not sure why:
>>> class Foo(object):
... def bar(self, x, y):
... print x, y
...
>>> a = Foo()
>>> b = PureProxy(a, bar=bind(Foo.bar, y=3))
>>> b.bar(2)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: bar() takes exactly 3 arguments (2 given)
I'm probably doing this all sorts of wrong because I'm just going by what I've pieced together from random documentation, blogs, and running dir() on all the pieces. Suggestions both on how to make this work and better ways to implement it would be appreciated ;) One detail I'm unsure about is how this should all interact with descriptors. Code follows.
from types import MethodType
class PureProxy(object):
def __init__(self, underlying, **substitutions):
self.underlying = underlying
for name in substitutions:
subst_attr = substitutions[name]
if hasattr(subst_attr, "underlying"):
setattr(self, name, MethodType(subst_attr, self, PureProxy))
def __getattribute__(self, name):
return getattr(object.__getattribute__(self, "underlying"), name)
def bind(f, *args, **kwargs):
""" Lets you freeze arguments of a function be certain values. Unlike
functools.partial, you can freeze arguments by name, which has the bonus
of letting you freeze them out of order. args will be treated just like
partial, but kwargs will properly take into account if you are specifying
a regular argument by name. """
argspec = inspect.getargspec(f)
argdict = copy(kwargs)
if hasattr(f, "im_func"):
f = f.im_func
args_idx = 0
for arg in argspec.args:
if args_idx >= len(args):
break
argdict[arg] = args[args_idx]
args_idx += 1
num_plugged = args_idx
def new_func(*inner_args, **inner_kwargs):
args_idx = 0
for arg in argspec.args[num_plugged:]:
if arg in argdict:
continue
if args_idx >= len(inner_args):
# We can't raise an error here because some remaining arguments
# may have been passed in by keyword.
break
argdict[arg] = inner_args[args_idx]
args_idx += 1
f(**dict(argdict, **inner_kwargs))
new_func.underlying = f
return new_func
Update: In case anyone can benefit, here's the final implementation I went with:
from types import MethodType
class PureProxy(object):
""" Intended usage:
>>> class Foo(object):
... def bar(self, x, y):
... print x, y
...
>>> a = Foo()
>>> b = PureProxy(a, bar=FreezeArgs(y=3))
>>> b.bar(1)
1 3
"""
def __init__(self, underlying, **substitutions):
self.underlying = underlying
for name in substitutions:
subst_attr = substitutions[name]
if isinstance(subst_attr, FreezeArgs):
underlying_func = getattr(underlying, name)
new_method_func = bind(underlying_func, *subst_attr.args, **subst_attr.kwargs)
setattr(self, name, MethodType(new_method_func, self, PureProxy))
def __getattr__(self, name):
return getattr(self.underlying, name)
class FreezeArgs(object):
def __init__(self, *args, **kwargs):
self.args = args
self.kwargs = kwargs
def bind(f, *args, **kwargs):
""" Lets you freeze arguments of a function be certain values. Unlike
functools.partial, you can freeze arguments by name, which has the bonus
of letting you freeze them out of order. args will be treated just like
partial, but kwargs will properly take into account if you are specifying
a regular argument by name. """
argspec = inspect.getargspec(f)
argdict = copy(kwargs)
if hasattr(f, "im_func"):
f = f.im_func
args_idx = 0
for arg in argspec.args:
if args_idx >= len(args):
break
argdict[arg] = args[args_idx]
args_idx += 1
num_plugged = args_idx
def new_func(*inner_args, **inner_kwargs):
args_idx = 0
for arg in argspec.args[num_plugged:]:
if arg in argdict:
continue
if args_idx >= len(inner_args):
# We can't raise an error here because some remaining arguments
# may have been passed in by keyword.
break
argdict[arg] = inner_args[args_idx]
args_idx += 1
f(**dict(argdict, **inner_kwargs))
return new_func
You're "binding too deep": change def __getattribute__(self, name): to def __getattr__(self, name): in class PureProxy. __getattribute__ intercepts every attribute access and so bypasses everything that you've set with setattr(self, name, ... making those setattr bereft of any effect, which obviously's not what you want; __getattr__ is called only for access to attributes not otherwise defined so those setattr calls become "operative" & useful.
In the body of that override, you can and should also change object.__getattribute__(self, "underlying") to self.underlying (since you're not overriding __getattribute__ any more). There are other changes I'd suggest (enumerate in lieu of the low-level logic you're using for counters, etc) but they wouldn't change the semantics.
With the change I suggest, your sample code works (you'll have to keep testing with more subtle cases of course). BTW, the way I debugged this was simply to stick in print statements in the appropriate places (a jurassic=era approach but still my favorite;-).