how to find missing value in sorted array

# A binary search based program to find  
# the only missing number in a sorted  
# in a sorted array of distinct elements  
# within limited range 
def search(ar, size): 
    a = 0
    b = size - 1
    mid = 0
    while b > a + 1: 
        mid = (a + b) // 2
        if (ar[a] - a) != (ar[mid] - mid): 
            b = mid 
        elif (ar[b] - b) != (ar[mid] - mid): 
            a = mid 
    return ar[mid] + 1
  
# Driver Code 
a = [1, 2, 3, 4, 5, 6, 8] 
n = len(a) 
  
print("Missing number:", search(a, n)) 
  
# This code is contributed 
# by Mohit Kumar