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Python | Counting sign change in list containing Positive and Negative Integers
  • Last Updated : 21 Jun, 2019

Given a list containing Positive and Negative integers, We have to find number of times the sign(Positive or Negative) changes in the list.

Input: [-1, 2, 3, -4, 5, -6, 7, 8, -9, 10, -11, 12] 
Output:9
Explanation : 
Sign change from -1 to 2, ans = 1
Sign change from 3 to -4, ans = 2
Sign change from -4 to 5, ans = 3
Sign change from 5 to -6, ans = 4
Sign change from -6 to 7, ans = 5
Sign change from 8 to -9, ans = 6
Sign change from -9 to 10, ans = 7
Sign change from 10 to -11 ans = 8
Sign change from -11 to 12, ans = 9

Input: [-1, 2, 3, -4, 5, -11, 12]  
Output:5
Explanation :
Sign change from -1 to 2, ans = 1
Sign change from 3 to -4, ans = 2
Sign change from -4 to 5, ans = 3
Sign change from 5 to -11, ans = 4
Sign change from -11 to 12, ans = 5

Let’s discuss certain ways in which this task is performed.

Method #1: Using Iteration
Using Iteration to find number of time sign changes in the list.




# Python Code to find number of 
# time sign changes in the list.
  
# Input list Initialization
Input = [-1, 2, 3, -4, 5, -6, 7, 8, -9, 10, -11, 12
  
# Variable Initialization
prev = Input[0]
ans = 0
  
# Using Iteration
for elem in Input:
    if elem == 0:
        sign = -1
    else:
        sign = elem / abs(elem)
  
    if sign == -prev:
        ans = ans + 1
        prev = sign
  
# Printing answer
print(ans)

Output :

9

Method #2: Using Itertools and groupby
This is yet another way to perform this particular task using itertools.






# Python Code to find number of 
# time sign changes in the list.
  
# Input list Initialization
Input = [-1, 2, 3, -4, 5, -6, 7, 8, -9, 10, -11, 12
  
# Importing
import itertools
  
# Using groupby
Output = len(list(itertools.groupby(Input, lambda Input: Input > 0)))
  
Output = Output -1
  
# Printing output
print(Output)

Output :

9

Method #3: Using Zip
The most concise and readable way to find number of time sign changes in the list is using zip.




# Python Code to find number of 
# time sign changes in the list.
  
# Using zip to check
def check(Input):
    Input = [-1 if not x else x for x in Input]
    # zip with leading 1, so that opening negative value is 
    # treated as sign change
    return sum((x ^ y)<0 for x, y in zip([1]+Input, Input))
  
# Input list Initialization
Input = [-1, 2, 3, -4, 5, -6, 7, 8, -9, 10, -11, 12
Output = check(Input)
  
Output = Output -1
  
# Printing output
print(Output)

Output :

9

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