###Binary Search###
def search(list1,n):
l=0
u=len(list1)
print(u)
while l<=u:
mid = (l+u)//2
if list1[mid]==n:
global index1
index = mid
return True
else:
if list1[mid]<n:
l = list1[mid]
else:
u = list1[mid]
list1 = [4,7,8,12,45,99.102,702,10987,56666]
n = 12
list1.sort()
if search(list1, n):
print("Found at ",index)
else:
print("Not Found")
The error I am getting is:
line 26, in <module> if search(list1, n): line 11, in search if list1[mid]==n: IndexError: list index out of range
I am assuming your the u in your code is supposed to be the high pointer. In that case it should be initialized to len(list1) - 1 and in the end the left and right pointers should be set to l = list1[mid+1] and u = list1[mid-1]. For binary search its always easier to implement the recursive method. Find out more info here
Related
I have this code in python and I am trying to make a counter for the iteration of the binary search (yeah I know it is incomplete...), but I am stuck with the variable inside the function, when i try to print the variable count I get this error
name 'count' is not defined in python
can someone explain why i get this error?
import csv
def binarySearch(arr, l, r, x):
count=0
while l <= r:
mid = int(l + (r - l) / 2)
# Check if x is present at mid
if arr[mid] == x:
return mid
# If x is greater, ignore left half
elif arr[mid] < x:
l = mid + 1
# If x is smaller, ignore right half
else:
r = mid - 1
# If we reach here, then the element
# was not present
return -1
with open('bl_printed_music_500.csv', newline='', encoding="utf-8-sig") as csvfile:
reader = csv.DictReader(csvfile)
arr=[]
for row in reader:
if row ["Publication date (standardised)"] != "":
arr.append(int(row["Publication date (standardised)"])) #create list for searching
list.sort(arr) #list must be sorted to work
#print (arr)
x = 1850 #year to search
# Function call
result = binarySearch(arr, 0, len(arr) - 1, x)
found = False
if result != -1:
found = True
print(found)
print(count)
I think it's because you defined count in binarySearch but try to use it outside of the method. Try using a global variable (define it outside of binarySearch), it should work.
You can return count as well.
For example:
def myFunc():
x = 5
y = 10
return x,y
a, b = myFunc()
print(a)
print(b)
This will be:
5
10
Note that, I could have written x, y = myFunc(). These x and y are not the same as the ones inside myFunc(). The latter are local to the function.
In your code, you can return your local count variable:
return mid, count #(A)
return -1, count #(A)
And get its value by:
result, count = binarySearch(arr, 0, len(arr)-1, x) #(B)
Again, these two count variables, (A) and (B) are different variables with different scopes.
See, for instance:
https://docs.python.org/3/faq/programming.html#why-am-i-getting-an-unboundlocalerror-when-the-variable-has-a-value
Alternatively, if a global variable, as suggested in the other answer, suits you best, you can see an example of its usage in the link.
I'm writing this piece of code, in which I've used bisect_left function from the bisect module which is a first-party module of Python. I'm using it with two parameters only i.e. sorted_list and target(the one for which I have to find the suitable index value).
The issue is: If my target is greater than the sum of lowest value and highest value, the function is returning the index = len(sorted_li), due to which I'm getting index error. I can use try and except but more than that I'm curious to know why it is behaving like so.
Following is my code:
from bisect import bisect_left
li = [10,15,3,6,10]
k = 19
def binary_search(sorted_list,target):
index = bisect_left(sorted_list,target)
print(index)
if sorted_list[index] == target:
return index
else:
return False
def function(sorted_li,k):
"""
Given a list of numbers and a number k, return whether any two numbers from the list add up to k.
For example, given [10, 15, 3, 7] and k of 17, return true since 10 + 7 is 17.
"""
print(sorted_li)
for i in range(len(sorted_li)):
print('Next iteration')
print(sorted_li[i])
target = k - sorted_li[i]
j = binary_search(sorted_li,target)
if j:
if j != i:
print(sorted_li[i])
print(sorted_li[j])
return True
else:
if j + 1 < len(sorted_li):
if sorted_li[j+1] == target:
print(sorted_li[i])
print(sorted_li[j+1])
return True
if j - 1 > 0:
if sorted_li[j-1] == target:
print(sorted_li[i])
print(sorted_li[j-1])
return True
return False
if __name__ == "__main__":
li.sort()
a = function(li,k)
print(a)
It's output is as follows:
but when I'm changing k to 18, the code is working fine, the output is as follows:
I've tried with various sets of numbers for the same. The output remains the same.
You're using bisect_left which has next purpose: it looking for the insertion point for x (which is target in your case) in a to maintain sorted order.
So for your case when you call first binary_search first time for 16 (19 - 3), it compare your number with items in li list using binary algorithm and then it returns position for insert 5, because in your list [3, 6, 10, 10, 15] insertion point should be after 15 which is correct.
If you open documentation you can find next method in searching sorted list
which does exactly you need, it searching for the exact item in list and return position of it if it exists either it raises ValueError because item not found.
def index(a, x):
'Locate the leftmost value exactly equal to x'
i = bisect_left(a, x)
if i != len(a) and a[i] == x:
return i
raise ValueError
I have this line of code that i used for one assignment, but i can't figure out how to add the median and mode into the code to let it run without error.
def main():
filename = input('File name: ')
num=0
try:
infile = open(filename, 'r')
count = 0
total = 0.0
average = 0.0
maximum = 0
minimum = 0
range1 = 0
for line in infile:
num = int(line)
count = count + 1
total = total + num
if count == 1:
maximum = num
minimum = num
else:
if num > maximum:
maximum = num
if num < minimum:
minimum = num
if count > 0:
average = total / count
range1 = maximum - minimum
I'll jump right in and show you the code. It's a very simple and quite pythonic solution.
Solution
import statistics
def open_file(filename):
try:
return open(filename, 'r')
except OSError as e:
print(e)
return None
def main():
# Read file. Note that we are trusting the user input here without sanitizing.
fd = open_file(input('File name: '))
if fd is None: # Ensure we have a file descriptor
return
data = fd.read() # Read whole file
if data == '':
print("No data in file")
return
lines = data.split('\n') # Split the data into a list of strings
# We need to convert the list of strings to a list of integers
# I don't know a pythonic way of doing this.
for number, item in enumerate(lines):
lines[number] = int(item)
total_lines = len(lines)
total_sum = sum(lines)
maximum = max(lines)
minimum = min(lines)
# Here is the python magic, no need to reinvent the wheel!
mean = statistics.mean(lines) # mean == average
median = statistics.median(lines)
mode = "No mode!"
try:
mode = statistics.mode(lines)
except statistics.StatisticsError as ec:
pass # No mode, due to having the same quantity of 2 or more different values
print("Total lines: " + str(total_lines))
print("Sum: " + str(total_sum))
print("Max: " + str(maximum))
print("Min: " + str(minimum))
print("Mean: " + str(mean))
print("Median: " + str(median))
print("Mode: " + str(mode))
if __name__ == '__main__':
main()
Explanation
Generally, in python, it's safe to assume that if you want to calculate any mundane value using a well known algorithm, there will already be a function written for you to do just that. No need to reinvent the wheel!
These functions aren't usually hard to find online either. For instance, you can find suggestions regarding the statistics library by googling python calculate the median
Although you have the solution, I strongly advise looking through the source code of the statistics library (posted below), and working out how these functions work for yourself. It will help your grow as a developer and mathematician.
statistics.py
mean
def mean(data):
"""Return the sample arithmetic mean of data.
>>> mean([1, 2, 3, 4, 4])
2.8
>>> from fractions import Fraction as F
>>> mean([F(3, 7), F(1, 21), F(5, 3), F(1, 3)])
Fraction(13, 21)
>>> from decimal import Decimal as D
>>> mean([D("0.5"), D("0.75"), D("0.625"), D("0.375")])
Decimal('0.5625')
If ``data`` is empty, StatisticsError will be raised.
"""
if iter(data) is data:
data = list(data)
n = len(data)
if n < 1:
raise StatisticsError('mean requires at least one data point')
T, total, count = _sum(data)
assert count == n
return _convert(total/n, T)
median
def median(data):
"""Return the median (middle value) of numeric data.
When the number of data points is odd, return the middle data point.
When the number of data points is even, the median is interpolated by
taking the average of the two middle values:
>>> median([1, 3, 5])
3
>>> median([1, 3, 5, 7])
4.0
"""
data = sorted(data)
n = len(data)
if n == 0:
raise StatisticsError("no median for empty data")
if n%2 == 1:
return data[n//2]
else:
i = n//2
return (data[i - 1] + data[i])/2
mode
def mode(data):
"""Return the most common data point from discrete or nominal data.
``mode`` assumes discrete data, and returns a single value. This is the
standard treatment of the mode as commonly taught in schools:
>>> mode([1, 1, 2, 3, 3, 3, 3, 4])
3
This also works with nominal (non-numeric) data:
>>> mode(["red", "blue", "blue", "red", "green", "red", "red"])
'red'
If there is not exactly one most common value, ``mode`` will raise
StatisticsError.
"""
# Generate a table of sorted (value, frequency) pairs.
table = _counts(data)
if len(table) == 1:
return table[0][0]
elif table:
raise StatisticsError(
'no unique mode; found %d equally common values' % len(table)
)
else:
raise StatisticsError('no mode for empty data')
Line 11 produces the error. Stepping through the code doesn't reveal a problem?
The code just points at from left and right ends of list, moving pointers toward per iteration until a target sum is found or not! Doesn't look like the loops can step on itself but seems to anyway.
def twoSum(num_array, sum):
'''1.twoSum
Given an array of integers, return indices of the two numbers that
add up to a specific target.
'''
array = sorted(num_array)
l = array[0]
r = array[len(array)-1]
indx_Dict = dict(enumerate(array))
while (l < r) :
if (array[l] + array[r]) == sum:
return [indx_Dict[l], indx_Dict[r]]
elif array[l] + array[r] < sum:
l += 1
else:
r -= 1
num_array1 = [2, 7, 11, 15,1,0]
target1 = 9
twoSum(num_array1, target1)
that is what i changed:
array[len(array)-1] -> len(array)-1 (that's what caused your IndexError)
indx_Dict: i changed it such that indx_Dict[sorted_index] = original_index
sum -> sum_: sum is a built-in. it is never a good idea to use one of those as variable name! (yes, the new name could be better)
this is the final code:
def two_sum(num_array, sum_):
'''1.twoSum
Given an array of integers, return indices of the two numbers that
add up to a specific target.
'''
array = sorted(num_array)
l = 0
r = len(array)-1
indx_Dict = {array.index(val): index for index, val in enumerate(num_array)} ##
while (l < r) :
if (array[l] + array[r]) == sum_:
return [indx_Dict[l], indx_Dict[r]]
elif array[l] + array[r] < sum_:
l += 1
else:
r -= 1
here is a discussion about this problem:
Find 2 numbers in an unsorted array equal to a given sum (which you seem to be aware of - looks like what you are trying to do). this is a python version of just that:
def two_sum(lst, total):
sorted_lst = sorted(lst)
n = len(lst)
for i, val0 in enumerate(sorted_lst):
for j in range(n-1, i, -1):
val1 = sorted_lst[j]
s = val0 + val1
if s < total:
break
if s == total:
return sorted((lst.index(val0), lst.index(val1)))
return None
this version is based on looping over the indices i and j.
now here is a version that i feel is more pythonic (but maybe a little bit harder to understand; but it does the exact same as the one above). it ignores the index j completely as it is not really needed:
from itertools import islice
def two_sum(lst, total):
n = len(lst)
sorted_lst = sorted(lst)
for i, val0 in enumerate(sorted_lst):
for val1 in islice(reversed(sorted_lst), n-i):
s = val0 + val1
if s < total:
break
if s == total:
return sorted((lst.index(val0), lst.index(val1)))
return None
aaaaand just for the fun of it: whenever there is a sorted list in play i feel the need to use the bisect module. (a very rudimentary benchmark showed that this may perform better for n > 10'000'000; n being the length of the list. so maybe not worth it for all practical purposes...)
def two_sum_binary(lst, total):
n = len(lst)
sorted_lst = sorted(lst)
for i, val0 in enumerate(sorted_lst):
# binary search in sorted_lst[i:]
j = bisect_left(sorted_lst, total-val0, lo=i)
if j >= n:
continue
val1 = sorted_lst[j]
if val0 + val1 == total:
return sorted((lst.index(val0), lst.index(val1)))
else:
continue
return None
for (a bit more) completeness: there is a dictionary based approach:
def two_sum_dict(lst, total):
dct = {val: index for index, val in enumerate(lst)}
for i, val in enumerate(lst):
try:
return sorted((i, dct[total-val]))
except KeyError:
pass
return None
i hope the code serves as its own explanation...
l and r are not your indices, but values from your array.
Say you have an array: [21,22,23,23]. l is 21, r is 23; therefore, calling array[21] is out of bounds.
Additionally, you would have a problem with your indx_Dict. You call enumerate on it, which returns [(0,21),...(3,23)]. Calling dict gives you {0:21,1:22,2:23,3:23}. There is no key equivalent to 21 or 23, which will also give you an error.
What you could try is:
def twoSum(num_array, asum):
'''1.twoSum
Given an array of integers, return indices of the two numbers that
add up to a specific target.
'''
array = sorted(num_array)
l = 0
r = len(array)-1
while (l < len(array)-1) :
while (r > l):
if (array[l] + array[r]) == asum:
return [num_array.index(array[l]),\
num_array.index(array[r])]
r -= 1
r = len(array)-1
l += 1
num_array1 = [2, 7, 11, 15,1,0]
target1 = 9
twoSum(num_array1, target1)
This way, your l and r are both indices of the sorted array. It goes through every possible combination of values from the array, and returns when it either has found the sum or gone through everything. It then returns the index of the original num_array that contains the correct values.
Also, as #hiro-protagonist said, sum is a built-in function in Python already, so it should be changed to something else (asum in my example).
So the code below:
Takes a given square, and if it is an X does nothing. If the square has an O in it, it changes the O to an X and autofills the square above, below, to the left, and to the right.
#openFile(filename): This function opens and prints the users txt file for paint
#input: none
#output: The file that needs to be painted
def openFile(filename):
file = open(filename, 'r')
for line in file:
print(line)
file.close()
#convertFile(filename): This function is used to convert the .txt file into a 2D arrary
#input: none
#output: none
def convertFile(filename):
empty = []
filename = open(filename, 'r')
for line in filename:
line = line.rstrip("\n")
empty.append(list(line))
return empty
#getCoordinates(x,y): This function is used to get the coordinates the user wants to pain from
#input: user coordinates.
#output: none
def getCoordinates(x, y):
coordinates = []
userInt = 0
user = []
try:
user = input("Please enter a square to fill , or q to exit: ")
user.split()
coordinates.append(int(user[0]))
coordinates.append(int(user[2]))
except ValueError:
print("Enter a valid input!")
user = input("Please enter a square to fill, or q to exit: ")
user.split()
coordinates.append(int(user[0]))
coordinates.append(int(user[2]))
return coordinates
def printGrid(grid):
for innerList in grid:
for item in innerList:
print(item, end = "")
print()
#autoFill(board, row, col): This is the heart of the program and the recursive program
# use to fill the board with x's
#input: none
#output: none
def autoFill(grid, rows, cols):
if grid[cols][rows] == "X":
return 0
else:
grid[cols][rows] = "X"
if rows > 0:
autoFill(grid, rows - 1, cols)
if rows < len(grid[cols]) - 1:
autoFill(grid, rows + 1, cols)
if cols > 0:
autoFill(grid, rows, cols - 1)
if cols < len(grid) - 1:
autoFill(grid, rows, cols + 1)
def main():
coordinates = []
empty = []
while True:
filename = input("Please enter a filename: ")
openFile(filename)
empty = convertFile(filename)
coordinates = getCoordinates(len(empty), len(empty[0]))
empty = autoFill(empty(coordinates[0], coordinates[1]))
for item in empty:
s = ""
s.join(item)
for x in item:
s += str(x)
print(s)
if user == "q":
return 0
main()
output should look like:
Please enter a filename: input.txt
OOOOOOXOOOO
OOOOOXOOOOO
OOOOXOOOOOO
XXOOXOOOOOO
XXXXOOOOOOO
OOOOOOOOOOO
Please enter a square to fill, or q to exit: 1, 1
XXXXXXXOOOO
XXXXXXOOOOO
XXXXXOOOOOO
XXXXXOOOOOO
XXXXOOOOOOO
OOOOOOOOOOO
But when i type in the coordinate points i get:
empty = autoFill(empty(coordinates[0], coordinates[1]))
TypeError: 'list' object is not callable
Any guidance in fixing this issue will be much appreciated
The particular error you're asking about is happening because you're trying to call empty (which is a list, as returned by convertFile) as if it were a function.