Interpolation Search producing wrong output

Question

Here I am trying to implement interpolation search in Python(3.6.1). My logic seems correct but still the output is always giving me False unless I give the first index of the list as the search key. The code follows :

def NSearch(l,n,s):
    beg = 0
    end = (n-1)

    while (beg<=end) and (s>=l[beg]) and (s<=l[end]):
       p  = beg + int(((float(end - beg) / ( l[end] - l[beg])) * ( s - l[beg])))
       if (l[p]==s):
         return True
       if (l[p]<=s):
         beg = p + 1
       else:
         end = p - 1

    return False    

liost = [23, 76, 17, -87, 56]
#x = int(input())
x = 76
print(NSearch(liost,len(liost),x))

Answer 1

Read: How-to-debug-small-programs/

I set a breakpoint on your while and looked hard at the values for your example.

# while (beg<=end) and (s>=l[beg]) and (s<=l[end]):

while (0<=4) and (76>=23) and (76<=56):     # this is false, function does not enter while.

    # all code in while block never executed == irrelevant 

return False

Then I looked up interpolation search .

Interpolation search is an algorithm for searching for a given key in an indexed array that has been ordered by numerical values

Then I looked at your inputs:

liost = [23, 76, 17, -87, 56]  # not ordered

And fixed them to:

x = 76
print(NSearch(sorted(liost),len(liost),x))

Now if finds it - added couple of other values to check and voila: works.

---

Sidenote:

print (x in liost) # prints True as well...