Python: Longest Plateau Problem: to find the length and location of the longest contiguous sequence of equal values

Question

The question is to solve the following question in Sedgewick Wayne's Python book:

Given an array of integers, compose a program that finds the length and location of the longest contiguous sequence of equal values where the values of the elements just before and just after this sequence are smaller.

I tried on this problem, and encountered some problems.

Here are my codes:

import sys
import stdio

# Ask the user input for the list of integers
numList = list(sys.argv[1])

maxcount = 0
value = None
location = None

i = 1
while i < len(numList) - 1:
    resList = []
    count = 0
    # If i > i-1, then we start taking i into the resList
    if numList[i] > numList[i - 1]:
        # start counting
        resList += [numList[i]]
        # Iterating through the rest of the numbers
        j = i + 1
        while j < len(numList):
            # If the j element equals the i, then append it to resList
            if numList[i] == numList[j]:
                resList += [numList[j]]
                j += 1
            elif numList[i] < numList[j]:
                # if j element is greater than i, break out the loop
                i = j
                break
            else:
                # if j element is smaller than i, count equals length of resList
                count = len(resList)
                if count > maxcount:
                    maxcount = count
                    value = resList[1]
                    location = i
                i = j
    else:
        # if not greater than the previous one, increment by 1
        i += 1

stdio.writeln("The longest continuous plateau is at location: " + str(location))
stdio.writeln("Length is: " + str(maxcount))
stdio.writeln("Number is: " + str(value))

The result shows:

python exercise1_4_21.py 553223334445554
The longest continuous plateau is at location: 11
Length is: 3
Number is: 5


python exercise1_4_21.py 1234567
The longest continuous plateau is at location: None
Length is: 0
Number is: None

But somehow, if the list given is in the format of having a group of continuous integers that is greater than the previous one, but then this group ends the list with no integer following it, my program simply doesn't end....

exercise1_4_21.py 11112222111444
Traceback (most recent call last):
  File "exercise1_4_21.py", line 32, in <module>
    if numList[i] == numList[j]:
KeyboardInterrupt


exercise1_4_21.py 111222211112223333
Traceback (most recent call last):
  File "exercise1_4_21.py", line 25, in <module>
    if numList[i] > numList[i - 1]:
KeyboardInterrupt

Not quite sure where the logical error is...thank you very much for your help and kindness!

Answer 1

Seems you overcomplicated the solution (while correctly selected key cases).

It requires only single run through the list.

def maxplat(l):
    if (len(l)==0):
        return 0, 0
    start, leng = 0, 1
    maxlen, maxstart = 0, 1
    for i in range(1, len(l) + 1):
        if (i == len(l)) or (l[i] < l[i-1]):
            if (leng > maxlen):
                maxlen, maxstart = leng, start
        elif (l[i] == l[i-1]):
            leng += 1
        else:
            start, leng = i, 1
    return maxlen, maxstart

#test cases
print(maxplat([]))  #empty case
print(maxplat([3]))  #single element
print(maxplat([3,2,4,4,2,5,5,5,3]))   #simple case
print(maxplat([3,2,4,4,2,5,5,5,6]))  #up after long run
print(maxplat([3,2,4,4,2,5,5,5]))    #run at the end
print(maxplat([3,3,3,3,2,4,4,2]))   #run at the start

>>> 
(0, 0)
(1, 0)
(3, 5)
(2, 2)
(3, 5)
(4, 0)

Answer 2

You need to add an extra check in your code to exit.

if j == len(numList):
   maxcount = len(resList)
   value = resList[1]
   location = i
   break

In your code it'll look like this:

import sys
import stdio

# Ask the user input for the list of integers
numList = list(sys.argv[1])

maxcount = 0
value = None
location = None

i = 1
while i < len(numList) - 1:
    resList = []
    count = 0
    # If i > i-1, then we start taking i into the resList
    if numList[i] > numList[i - 1]:
        # start counting
        resList += [numList[i]]
        # Iterating through the rest of the numbers
        j = i + 1
        while j < len(numList):
            # If the j element equals the i, then append it to resList
            if numList[i] == numList[j]:
                resList += [numList[j]]
                j += 1
            elif numList[i] < numList[j]:
                # if j element is greater than i, break out the loop
                i = j
                break
            else:
                # if j element is smaller than i, count equals length of resList
                count = len(resList)
                if count > maxcount:
                    maxcount = count
                    value = resList[1]
                    location = i
                i = j
        #EXTRA CHECK HERE
        if j == len(numList):
            maxcount = len(resList)
            value = resList[1]
            location = i
            break
    else:
        # if not greater than the previous one, increment by 1
        i += 1

stdio.writeln("The longest continuous plateau is at location: " + str(location))
stdio.writeln("Length is: " + str(maxcount))
stdio.writeln("Number is: " + str(value))