IndexError: list index out of range

[rajeev1729]

please help me. i am unable to understand IndexError.

def binarySearch (arr,first,last,element):
    mid = first+last//2
    print("Value of index ",mid)
    print("Value of element ",arr[mid])
    if last>=first:
        if(element==arr[mid]):
            print("element",element,"found at location ",mid)
        else:
            if(element>arr[mid]):
                return binarySearch(arr,mid+1,last,element)
            else:
                return binarySearch(arr,first,mid-1,element)
    else:
        print("Element is not present in array")
    
arr = [int(x) for x in input("Enter list of element:").split(',')]
element=int(input("Enter search element:"))
binarySearch(arr,0,len(arr)-1,element)

==================== RESTART: C:/python/program/binary.py ====================
Enter list of element:2,3,4,5,6
Enter search element:2
Value of index 2
Value of element 4
Value of index 0
Value of element 2
element 2 found at location 0
>>>
==================== RESTART: C:/python/program/binary.py ====================
Enter list of element:2,3,4,5,6
Enter search element:3
Value of index 2
Value of element 4
Value of index 0
Value of element 2
Value of index 1
Value of element 3
element 3 found at location 1
>>>
==================== RESTART: C:/python/program/binary.py ====================
Enter list of element:2,3,4,5,6
Enter search element:3
Value of index 2
Value of element 4
Value of index 0
Value of element 2
Value of index 1
Value of element 3
element 3 found at location 1
>>>
==================== RESTART: C:/python/program/binary.py ====================
Enter list of element:2,3,4,5,6
Enter search element:5
Value of index 2
Value of element 4
Value of index 5
Traceback (most recent call last):
File "C:/python/program/binary.py", line 18, in <module>
binarySearch(arr,0,len(arr)-1,element)
File "C:/python/program/binary.py", line 10, in binarySearch
return binarySearch(arr,mid+1,last,element)
File "C:/python/program/binary.py", line 4, in binarySearch
print("Value of element ",arr[mid])
IndexError: list index out of range
>>>
==================== RESTART: C:/python/program/binary.py ====================
Enter list of element:2,3,4,5,6
Enter search element:6
Value of index 2
Value of element 4
Value of index 5
Traceback (most recent call last):
File "C:/python/program/binary.py", line 18, in <module>
binarySearch(arr,0,len(arr)-1,element)
File "C:/python/program/binary.py", line 10, in binarySearch
return binarySearch(arr,mid+1,last,element)
File "C:/python/program/binary.py", line 4, in binarySearch
print("Value of element ",arr[mid])
IndexError: list index out of range

[hbknjr]

please help me. i am unable to understand IndexError.

mid = first+last//2

You are getting that error due to logical a mistake. Read about the order of operation and operator precedence here.

Here's how this expression will be evaluated in python:
1- last is divided by 2 and only integer(//) is stored.
2- result from above step is added to first and stored in mid.

what you want is:

mid = (first+last)//2

[Larz60+]

what hbknjr says is correct:

If you add one more statement to for routine and slight modification to print statements, the error becomes clear:

def binarySearch(arr, first, last, element):
    print('value of arr: {}'.format(arr))
    mid = first + last // 2
    print("Value of mid: {}".format(mid))
    print("Value of arr[mid]: {}".format(arr[mid]))
    if last >= first:
        if (element == arr[mid]):
            print("element", element, "found at location ", mid)
        else:
            if (element > arr[mid]):
                return binarySearch(arr, mid + 1, last, element)
            else:
                return binarySearch(arr, first, mid - 1, element)
    else:
        print("Element is not present in array")


arr = [int(x) for x in input("Enter list of element:").split(',')]
element = int(input("Enter search element:"))
binarySearch(arr, 0, len(arr) - 1, element)

first crash from your attempts:
inputs:

Output:
Enter list of element:2,3,4,5,6
Enter search element:5

results:
C:\Python36\python.exe M:/python/ProvingGround/newStuff/FindIndex1.py
Enter list of element:2,3,4,5,6
Enter search element:5

Output:
Traceback (most recent call last):
  File "M:/python/ProvingGround/newStuff/FindIndex1.py", line 20, in <module>
    binarySearch(arr, 0, len(arr) - 1, element)
  File "M:/python/ProvingGround/newStuff/FindIndex1.py", line 11, in binarySearch
    return binarySearch(arr, mid + 1, last, element)
  File "M:/python/ProvingGround/newStuff/FindIndex1.py", line 5, in binarySearch
    print("Value of arr[mid]: {}".format(arr[mid]))
IndexError: list index out of range
value of arr: [2, 3, 4, 5, 6]
Value of mid: 2
Value of arr[mid]: 4
value of arr: [2, 3, 4, 5, 6]
Value of mid: 5

You can see here that there is no element 5 ... remember index is 0 to 4