Calculate logarithm of a number in C++

This post will discuss how to calculate the logarithm of a number in C++.

The header <cmath> provides a several common logarithmic functions in C++. This post provides an overview of some of the commonly used functions.

1. Using `log10()` function

The log10() function returns the base-10 logarithm of the specified number. It takes a floating-point value whose logarithm is calculated and calculates the common logarithm of it. C++11 offers additional overloads for integral types, which effectively cast the argument to a double. It can be used as follows:

#include <iostream>

#include <cmath>

int main()

{

int x = 1000;

double lg = log10(x);

std::cout << lg << std::endl; // 3

return 0;

}

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2. Using `log()` function

The log() function returns the natural (base-e) logarithm of the specified number, which is the inverse of the natural exponential function. Its usage is demonstrated below:

#include <iostream>

#include <cmath>

int main()

{

int x = 1000;

double lg = log(x);

std::cout << lg << std::endl; // 6.90776

return 0;

}

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3. Using `log2()` function

If you need the binary (base-2) logarithm of the specified number, consider using the log2() function. You can invoke it like this:

#include <iostream>

#include <cmath>

int main()

{

int x = 1000;

double lg = log2(x);

std::cout << lg << std::endl; // 9.96578

return 0;

}

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Alternatively, you can use the logarithmic identity to calculate the base-2 logarithm of a value. The logarithmic identity is log_ba = log_y(a)/log_y(b), therefore, log₂n = log_y(n)/log_y(2), where y can be anything (10 or 2). Here’s the working example:

#include <iostream>

#include <cmath>

int main()

{

int x = 1000;

double lg = log10(x) / log10(2);

std::cout << lg << std::endl; // 9.96578

lg = log2(x) / log2(2);

std::cout << lg << std::endl; // 9.96578

return 0;

}

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That’s all about calculating the logarithm of a number in C++.

Calculate logarithm of a number in C++

1. Using log10() function

2. Using log() function

3. Using log2() function

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1. Using `log10()` function

2. Using `log()` function

3. Using `log2()` function