Python math.log1p() Method

1. Syntax
2. Example 1: Natural Logarithm of a number + 1
3. Example 2: If Input Less Than or Equal to -1
4. Example 3: If Input Is Not a Number
5. Example 4: If Input Is Infinity
6. Example 5: If Input Is NaN

The Python programming language offers a library, math, that contains implementation for various mathematical operations such as trigonometric functions and logarithmic functions.

Logarithm refers to the inverse function of exponentiation. In simple words, the logarithm of a number a is the exponent or number (x) to which another number, the base b, should be raised to make that number a.

In this article, we will discuss a method, log1p(), available in the math module that computes the logarithm of number + 1.

Syntax

math.log1p(x)

Parameters

Return

The log() method returns the natural logarithm of x + 1 or logarithm to the base of the mathematical constant e.

By default, it adds 1 to the number, which means that 0 turns to 1, and -0.5 turns to 0.5. Simply put, it calculates math.log(x + 1).

Example 1: Natural Logarithm of a number + 1

import math

print(math.log1p(0))
print(math.log1p(1))
print(math.log1p(-0.99999999999999))
print(math.log1p(99999))
print(math.log1p(2352.579))

Output:

0.0
0.6931471805599453
-32.236990899346836
11.512925464970229
7.763692427305668

The Python code above computes the natural logarithm value of x + 1 for 0, 1, -0.99999999999999, 99999, and 2352.579. 0 and -0.99999999999999 are valid inputs here because 1 will be added to it.

Hence, they will translate to 1 and 9.992007221626409e-15, respectively.

Example 2: If Input Less Than or Equal to -1

import math

print(math.log1p(-1))

Output:

Traceback (most recent call last):
  File "main.py", line 3, in <module>
    print(math.log1p(-1))
ValueError: math domain error

The Python code above takes -1 as input, and since -1 lies outside the domain, the log1p() method raises a ValueError.

import math

print(math.log1p(-35.54252))

Output:

Traceback (most recent call last):
  File "main.py", line 3, in <module>
    print(math.log1p(-35.54252))
ValueError: math domain error

The Python code above takes a value less than -1 as an input, and such values are outside the log1p() method’s domain. Hence, it raises a ValueError.

Example 3: If Input Is Not a Number

import math

print(math.log1p("this is a string"))

Output:

Traceback (most recent call last):
  File "main.py", line 3, in <module>
    print(math.log1p("this is a string"))
TypeError: must be real number, not str

The Python code above takes a string as an input. Since, the log1p() method expects a number, it raises a TypeError.

Example 4: If Input Is Infinity

import math

print(math.log1p(math.inf))

Output:

inf

Example 5: If Input Is NaN

import math

print(math.log1p(math.nan))

Output:

nan