How to test if a sequence starts with values in another sequence? - python

How to simply test, if a sequence starts with certain values in Python3?
There's of course the solution of writing a custom function, example below. But does Python itself offer some solution? Something more pythonic, an understandable one-liner would be good.
def startswith(sequence, prefix):
"""return true if sequence starts with items in prefix"""
if len(prefix) > len(sequence):
return False
for i in range(len(prefix)):
if sequence[i] != prefix[i]:
return False
return True

len(prefix) <= len(sequence) and all(i==j for i, j in zip(prefix, sequence))

Related

Return immediately for recursive calls

Problem Statement:
You are given an integer array nums. You are initially positioned at the array's first index, and each element in the array represents your maximum jump length at that position. Return true if you can reach the last index, or false otherwise.
How can I change my code so that it returns immediately when I have found a path that works for this problem instead of going through all the recursive calls that I have made previously
def canJump(self, nums: List[int]) -> bool:
solve = [False]
def backtrack(i):
if solve[0] == True:
return
if i == len(nums)-1:
solve[0] = True
return
if i >= len(nums) or nums[i] == 0:
return
for x in range(1, nums[i]+1):
backtrack(i+x)
backtrack(0)
return solve[0]
General Form of a Recursive Function
The general form of a recursive function has two mutually exclusive types of conditions that can be met on each iteration of the recursion. These are either:
terminal conditions, or
non-terminal conditions.
Both types of condition contain a return statement.
Terminal Conditions
The return statement in terminal conditions typically takes the form return <value>.
The solution to the problem you are trying to solve requires two possible terminal conditions:
The case where you know you can reach the last index. return True
The case where you know you can NOT reach the last index. return False
Non-Terminal Conditions
The non-terminal condition will occur on iterations where neither of the terminal cases are met. In this situation, you will call the recursive function and return what it returns.
This answer covers terminal and non-terminal conditions in more detail.
Example
Consider a recursive function that sums the numbers of an array.
def sum(position, array, end):
if(position == end): # terminal condition
return 0
else: # non-terminal condition
return array[position] + sum(position+1, array, end)
Another Example
Depending on any constraints to your problem that I might have missed, a solution might be the following:
def jump(current_position, nums, finish_line):
"""
greedy algorithm:
choose the next position with the largest sum of (jump_range + index)
"""
jump_range = nums[current_position]
choice = current_position + jump_range
if(jump_range == 0): # terminal condition
return False
if(choice >= finish_line): # terminal condition
return True
else: # non-terminal condition
utility = 0
for next_position in range(current_position+1, jump_range+1):
next_jump_range = nums[next_position]
if(utility <= next_position + next_jump_range):
utility = next_position + next_jump_range
choice = next_position
return jump(choice, nums, finish_line)
input1 = [2,0,0,10,3]
input2 = [2,3,0,10,3]
current_position = 0
finish_line = len(input1)
print(jump(0, input1, finish_line)) # False
finish_line = len(input2)
print(jump(0, input2, finish_line)) # True
The most noteworthy difference from your solution is that return statements always return a value.
How can I change my code so that it returns immediately when I have found a path that works for this problem instead of going through all the recursive calls that I have made previously
One particularly straightforward way is to throw an exception, which will immediately unwind the stack.
def can_jump(nums: list[int]) -> bool:
if not nums:
return False
class _Success(Exception):
pass
def backtrack(i):
if i >= len(nums):
return
if i == len(nums) - 1:
raise _Success()
for x in range(1, nums[i] + 1):
backtrack(i + x)
try:
backtrack(0)
return False
except _Success:
return True
We create a local exception type called _Success that the backtracking search can throw to indicate that it found a solution.
If it never finds a solution, the backtrack function will simply return and the return False line will run. Otherwise, it will raise _Success() and then the return True line will run.

how to check if a list is a consecutive number in python?

How to write python code that let the computer know if the list is a right sequence and the position doesn't matter, it will return true, otherwise it return false.
below are some of my example, I really don't know how to start
b=[1,2,3,4,5] #return true
b=[1,2,2,1,3] # return false
b=[2,3,1,5,4] #return true
b=[2,4,6,4,3] # return false
sort function is O(nlogn), we can use for loop which is O(n):
def check_seq(in_list):
now_ele = set()
min_ele = max_ele = in_list[0]
for i in in_list:
if i in now_ele:
return False
min_ele = min(i, min_ele)
max_ele = max(i, max_ele)
now_ele.add(i)
if max_ele-min_ele+1 == len(in_list):
return True
return False
Create a set and one to compare with -- based on minimum and maximum:
isRightSequence = set(range(min(b), max(b)+1)) == set(b)
This question is quite simple and can be solved a few ways.
The conditional approach - if there is a number that is bigger than the length of the list, it automatically cannot be a sequence because there can only be numbers from 1-n where n is the size of the list. Also, you have to check if there are any duplicates in the list as this cannot be possible either. If none of these conditions occur, it should return true
Using dictionary - go through the entire list and add it as a key to a dictionary. Afterwards, simply loop through numbers 1-n where n is the length of the list and check if they are keys in the dictionary, if one of them isn't, return false. If all of them are, return true.
Both of these are quite simply approaches and you should be able to implement them yourselves. However, this is one implementation for both.
1.
def solve(list1):
seen = {}
for i in list1:
if i > len(list1):
return False
if i in seen:
return False
seen[i] = True
return False
def solve(list1):
seen = {}
for i in list1:
seen[i] = True
for i in range (1, len(list1)+1):
if i not in seen:
return False
return True
This solution needs O(n) runtime and O(n) space
def is_consecutive(l: list[int]):
if not l:
return False
low = min(l)
high = max(l)
# Bounds Check
if high - low != len(l) - 1:
return False
# Test all indices exist
test_vec = [False] * len(l) # O(n)
for i in range(len(l)):
test_vec[l[i] - low] = True
return all(test_vec)
assert is_consecutive(range(10))
assert is_consecutive([-1, 1,0])
assert not is_consecutive([1,1])
assert not is_consecutive([1,2,4,6,5])

How to write this iterative function to be recursive?

I need to write this iterative function to do the same thing but it must be recursive.
def task1(string: str):
for i in range(len(string)):
if string[i] != string[len(string) - i - 1]:
return False
return True
This is what i tried but it does not work.
def task1_recursion(string: str):
print(string)
if len(string) > 1:
if string[0] == task1_recursion(string[1::1]):
return True
else:
return False
else:
return string
My code seems to one the last recursion return string "" and that makes it to return False.
Just check the tip and the tail, continue with the string without them:
def task1_recursion(string: str):
# recursion base condition (exit condition)
if len(string) <= 1:
return True
# unpack values
first, *_, last = string
# check if they are different
if first != last:
return False
# if not continue checking the remaining string
return task1_recursion(string[1:-1])
If I understand correctly you want to check if a string is symmetric with the code in task1. My solution is below:
def fct(s: str, i: int):
if len(s) <= 1 or i == len(s):
return True
return s[i] == s[len(s) - 1 - i] and fct(s, i + 1)
I tested and fct produces the same result as task1. It needs an additional parameter for the index though. But you can wrap it inside another function if you want the parameter to include only the input string. i is always set to 0 when you call the function, e.g. fct("ABCCBA", 0).

Loop through int array, return True if following int is equal to current

Given a list of ints, return True if the array contains a 3 next to a 3 somewhere.
has_33([1, 3, 3]) → True
has_33([1, 3, 1, 3]) → False
has_33([3, 1, 3]) → False
First Approch:
def has_33(nums):
for i in range(0,len(nums)):
return nums[i] == nums[i+1] ==3
Could someone explain me what's wrong with this approach, I see that this code is returning True only if all the elements in a list are true.
Second Approach:
def has_33(nums):
for i in range(0,len(nums)):
if(nums[i] == nums[i+1] ==3):
return True
The second approach satisfies my question.
What is the difference between these two approaches?
Well, the difference is rather obvious. In the first case, you inconditionnaly return the result of expression nums[i] == nums[i+1] ==3, whatever the value of this expression is. This actually means that you always return on the very first iteration, so your code could as well be written as
def has_33(nums):
if len(nums):
return nums[0] == nums[1] ==3
In the second case, you only return if the expression is true, so the iteration goes on until either you explicitely return (found a match) or the iteration naturally terminates and you've found nothing (in which case the function will implicitely return None).
Unrelated, but your code (second version) can be improved in quite a few ways. First point: Python "for" loop are of the "foreach" kind - you iterate on the sequence elements, not indices. If you don't need the indice, the proper way is
for item in iterable:
do_something_with(item)
no need for range(len(xxx)) and indexed access here.
If you do need both the item and the index, then enumerate() is your friend - it yields (index, item) tuples:
for index, item in enumerate(sequence):
print("item at {} is {}".format(index, item))
Now for your current need - geting (item, nextitem) pairs -, there's still another solution: zip(seq1, seq2) + slicing:
for item, nextitem in zip(sequence, sequence[1:]):
print("item: {} - nextitem : {}".format(item, nextitem))
and finally, if what you want is to check if at least one item in a sequence satisfies a condition, you can use any() with a predicate:
def has_33(nums):
return any((item == nextitem == 3) for item, nextitem in zip(nums, nums[1:]))
Another solution could be to turn nums into a string and look for the literal string "33" in it:
def has_33(nums):
return "33" in "".join(str(x) for x in nums)
but I'm not sure this will be more efficient (you can use timeit to find out by yourself).
In your first approach, you will return the value of
return nums[i] == nums[i+1] == 3 #Where i = 0 since it returns
first iteration.
return nums[0]==nums[1] == 3 #If nums = [0,3,3]
return false # would be your result. But it would never check the next pair of values.
In your second approach, you will return the value
return true #If the if-statement is satisfied
The return function, will end the function call when called. Therefore, if being called in a for-loop without an if-statement, it will be called for the first iteration. If there is an if-statement and the iteration passes through the if-statement, it will return and end the loop at that iteration. Basically, the return function ends the function call and returns the value given.

Keeping count in recursive function

I'm trying to figure out how to write a recursive function (with only one parameter) that returns the number of times the substring “ou” appears in the string. Where I'm confused at is that I'm not allowed to use any built-in string functions other than len, or the string operators [] and [:] for indexing and splicing. So I can't use the find built-in find function
I remember seeing something like this, but it uses two parameters and it also uses the find() method
def count_it(target, key):
index = target.find(key)
if index >= 0:
return 1 + count_it(target[index+len(key):], key)
else:
return 0
Very inefficient, but should work:
def count_it(target):
if len(target) < 2:
return 0
else:
return (target[:2] == 'ou') + count_it(target[1:])
See it working online: ideone
It's basically the same idea as the code you posted, except that it moves only one character at a time through the string instead of using find to jump ahead to the next match.
Try this, it works for the general case (any value of key, not only 'ou'):
def count_it(target, key):
if len(target) < len(key):
return 0
found = True
for i in xrange(len(key)):
if target[i] != key[i]:
found = False
break
if found:
return 1 + count_it(target[len(key):], key)
else:
return count_it(target[1:], key)

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