Python program to print negative numbers in a list

Given a list of numbers, write a Python program to print all negative numbers in the given list. 

Example:

Input: list1 = [12, -7, 5, 64, -14]
Output: -7, -14

Input: list2 = [12, 14, -95, 3]
Output: -95

Example #1: Print all negative numbers from the given list using for loop Iterate each element in the list using for loop and check if the number is less than 0. If the condition satisfies, then only print the number. 

Python3

# Python program to print negative Numbers in a List

# list of numbers
list1 = [11, -21, 0, 45, 66, -93]

# iterating each number in list
for num in list1:

    # checking condition
    if num < 0:
    print(num, end=" ")

Output:

-21 -93 

Time complexity: O(n)
Auxiliary space: O(n), where n is total number of values in list1.

Example #2: Using while loop 

Python3

# Python program to print negative Numbers in a List

# list of numbers
list1 = [-10, 21, -4, -45, -66, 93]
num = 0

# using while loop
while(num < len(list1)):

    # checking condition
    if list1[num] < 0:
        print(list1[num], end=" ")

    # increment num
    num += 1

Output

-10 -4 -45 -66 

Time complexity: O(n), where n is the length of the input list. 
Auxiliary space: O(1).

Example #3: Using list comprehension

Python3

# Python program to print negative Numbers in a List

# list of numbers
list1 = [-10, -21, -4, 45, -66, 93]

# using list comprehension
neg_nos = [num for num in list1 if num < 0]

print("Negative numbers in the list: ", *neg_nos)

Output

Negative numbers in the list:  -10 -21 -4 -66

Time complexity: O(n), where n is the number of elements in the input list.
Auxiliary space: O(n), as it creates a new list to store the negative numbers in the input list. 

Example #4: Using lambda expressions

Python3

# Python program to print negative Numbers in a List

# list of numbers
list1 = [-10, 21, 4, -45, -66, 93, -11]


# we can also print negative no's using lambda exp.
neg_nos = list(filter(lambda x: (x < 0), list1))

print("Negative numbers in the list: ", *neg_nos)

Output

Negative numbers in the list:  -10 -45 -66 -11

Time complexity: O(n), where n is the number of elements in the list.
Auxiliary space: O(m), where m is the number of negative numbers in the list.

Method: Using enumerate function 

Python3

l=[12, -7, 5, 64, -14]
print([a for j,a in enumerate(l) if a<0])

Output

[-7, -14]

Method: Using startswith() method

Python3

# Python program to print negative Numbers in a List

# list of numbers
list1 = [11, -21, 0, 45, 66, -93]
res = []
list2 = list(map(str, list1))
for i in range(0, len(list2)):
    if(list2[i].startswith("-")):
        res.append(str(list1[i]))
res = " ".join(res)
print(res)

Output

-21 -93

The time complexity of the given program is O(n), where n is the length of the input list.

The auxiliary space complexity of the program is also O(n), where n is the length of the input list.

Method:  Using numpy.array

Python3

# Python program to print negative Numbers in a List
import numpy as np
# list of numbers
list1 = [-10, 21, 4, -45, -66, 93, -11]


# Using numpy to print the negative number
temp = np.array(list1)
neg_nos = temp[temp <= 0]

print("Negative numbers in the list: ", *neg_nos)

Output:

Negative numbers in the list:  -10 -45 -66 -11

Time Complexity: O(n)
Auxiliary Space: O(n)

Method : Using Recursion

Python3

#Recursive function to check current element negative or not
def PrintNegative(itr,list1):
  if itr == len(list1): #base condition
    return
  if list1[itr] < 0: #check negative number or not
    print( list1[itr],end = " ")
  PrintNegative(itr+1,list1) #recursive function call

list1 = [-1,8,9,-5,7]
PrintNegative(0,list1)

Output

-1 -5 

Method: Using numpy.array() and numpy.where()

note: install numpy module using command “pip install numpy”

Algorithm:

Convert the input list to a numpy array using numpy.array() method.
Use numpy.where() method to find all the negative numbers in the array.
Print the negative numbers.

Python3

import numpy as np

# list of numbers
list1 = [12, -7, 5, 64, -14]

# converting list to numpy array
arr1 = np.array(list1)

# finding negative numbers in the array
neg_nums = arr1[np.where(arr1 < 0)]

# printing negative numbers
print("Negative numbers in the list: ", *neg_nums)

Output:

Negative numbers in the list:  -7 -14

Time complexity: O(n), where n is the length of the input list.

Auxiliary space: O(n), as we are creating a numpy array to store the input list and another numpy array to store the negative numbers.

Method : Using functools.reduce

Algorithm: 

• Initialize the test list. 
• Use reduce function with the lambda function which adds a negative element to the list. 
• Print list. 

Python3

# Python program to print negative Numbers in a List
from functools import reduce

# list of numbers
list1 = [-10, -21, -4, 45, -66, 93]

# using reduce method
neg_nos = reduce(lambda a, b : a + [ b ] if b < 0 else a ,list1, [])

print("Negative numbers in the list: ", *neg_nos)

Output

Negative numbers in the list:  -10 -21 -4 -66

Time complexity: O(N), where N is the length of the test list.

Auxiliary space: O(M), where M is length of the negative number list.