Python | Counting sign change in list containing Positive and Negative Integers

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 :

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 :

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 :

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