log

log

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Calculates the natural logarithm of a number

 #include <math.h> double log ( double x  ); float logf ( float x  );         (C99) long double logl ( long double x  );         (C99) 

The log( ) functions calculate the natural logarithm of their argument. The natural logarithmcalled "log" for short in English as well as in Cis the logarithm to base e, where e is Euler's number, 2.718281....

The natural log of a number x is defined only for positive values of x. If x is negative, a domain error occurs; if x is zero, a range error may occur (or not, depending on the implementation).

Example

The following code prints some sample values for base 2, base e, and base 10 logarithms:

 double x[ ] = { 1E-100, 0.5, 2, exp(1), 10, 1E+100 }; puts("       x           log2(x)            log(x)           log10(x)\n"      " ---------------------------------------------------------------"); for ( int i = 0; i < sizeof(x) / sizeof(x[0]); ++i ) {   printf("%#10.3G %+17.10G %+17.10G %+17.10G\n",          x[i], log2(x[i]), log(x[i]), log10(x[i]) ); } 

This code produces the following output:

        x           log2(x)            log(x)           log10(x)  ---------------------------------------------------------------  1.00E-100      -332.1928095      -230.2585093              -100      0.500                -1     -0.6931471806     -0.3010299957       2.00                +1     +0.6931471806     +0.3010299957       2.72      +1.442695041                +1     +0.4342944819       10.0      +3.321928095      +2.302585093                +1  1.00E+100      +332.1928095      +230.2585093              +100 

See Also

log10( ), log1p( ), log2( ), exp( ), pow( )