Find the smallest number that is greater than a given number in a sorted list

Question

Given a sorted list of numbers, I need to find the smallest number that is greater than a given number. Consider this list:

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arr=[1,2,3,5,7,11,101,131,151,181,191,313,353,373,383]

Say the specified number is 320. Then, my method should return 353 as 353 is the smallest number greater than 320.

I am trying to use a slightly modified form of binary search; however on execution the program goes into infinite loop.

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def modBinarySearch(arr,x):
    l=len(arr)
    mid=l/2
    if arr[mid]>=x and arr[mid-1]<x:
        return arr[mid]
    elif arr[mid]>x and arr[mid-1]>x:
        modBinarySearch(arr[mid:l],x)
    else: 
        modBinarySearch(arr[0:mid],x)

N=int(raw_input())
arr=[1,2,3,5,7,11,101,131,151,181,191,313,353,373,383]
print modBinarySearch(arr,N)

Can someone point out what I am doing wrong ?

Answer 1

There is a standard module, bisect, that does this already:

In [49]: arr[bisect.bisect(arr, 320)]
Out[49]: 353

I think this should be the go-to method for searching sorted lists. There are a few examples in the manual.

As to your implementation, there is a number of problems:

1. Your recursion doesn't handle small arrays correctly.
2. The slicing done in the second branch is incorrect.
3. Your function doesn't return anything.
4. Because arr is in ascending order, arr[mid]>x and arr[mid-1]>x is equivalent to arr[mid-1]>x, suggesting you didn't write what you meant

Last but not least, recursion and all that slicing are completely unnecessary for this problem.

Answer 2

If the size of your lists is going to be 15, ditch the binary search altogether and use a sequential search.

You'll find the code much easier to write and, unless you need to do it many millions of times per second, the sequential solution will be more than fast enough.

If you do need to stick with the binary search, your first step should be to actually return the results of your recursive calls.

Answer 3

If arr[mid] and arr[mid-1], both are greater than your number, you should search in arr[0:mid], don't you think?

 elif arr[mid]>x and arr[mid-1]>x:
    modBinarySearch(arr[0:mid],x)
else: 
    modBinarySearch(arr[mid:1],x)

Answer 4

def modBinarySearch(arr, n):
    m = len(arr) / 2

    if arr[m] >= n and arr[m - 1] < n:
        return arr[m]
    elif arr[m] > n and arr[m - 1] > n:
        return modBinarySearch(arr[:m], n)
    else:
        return modBinarySearch(arr[m:], n)


arr = [1, 2, 3, 5, 7, 11, 101, 131, 151, 181, 191, 313, 353, 373, 383]
n = 320
print(modBinarySearch(arr, n))