How can you use Ln function in C programing?

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Header file math.h is for mathematical functions like cos,sin, tan.. But how to write the ln function and not log?

Wein answered 10/12, 2015 at 15:3 Comment(8)

Why? Ln is just log base e. – Carraway 10/12, 2015 at 15:5

log base e is log. What you're thinking of as log (log base 10) is log10 in the library. – Notebook 10/12, 2015 at 15:6

You could use the taylor series or paulo's formula – Sandler 10/12, 2015 at 15:6

Don't you mean log base e is Ln? Or am I confused? – Carraway 10/12, 2015 at 15:6

no, I mean that the standard library function called log computes log base e. – Notebook 10/12, 2015 at 15:7

OK, thanks. :) I'm more of a math/physics guy than a CS guy, so I didn't actually know that. :D – Carraway 10/12, 2015 at 15:7

Thankyou guys. Very helpful – Wein 10/12, 2015 at 15:12

The documentation is "helpful" too! ;-) – Inviolate 1/10, 2024 at 13:2

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The C function log is the natural logarithm, which mathematicians usually write as "ln".

The C function log10 is the logarithm base 10, which is sometimes written "log".

Did answered 10/12, 2015 at 15:7 Comment(0)

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Remember math at school: logA_B = logA_C/logB_C

Dit answered 10/12, 2015 at 15:5 Comment(0)

C

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double ln(double x) { return log(x); }

Or in a pinch

#define ln(x) log(x)

Coss answered 10/12, 2015 at 15:3 Comment(0)

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double ln(double x) {
    // ln(1 - y) = sum of -(y ^ k) / k from k = 1 to n but only if 0 < y < 1
    // taylor expansion formula of ln(x): https://furthermathematicst.blogspot.com/2011/06/71-taylor-polynomial-part-1.html
    int exponent = 0; // map the value x between 0 and 1
    while (x > 1.0) {x /= 2.0; exponent++;}
    while (x < 0.0) {x *= 2.0; exponent--;}
    int k, n = 1000; // iteration
    double y = 1 - x; // ln(1 - y) = ln(x) >> ln(y) = ln(1 - x)
    double initial_y = y;
    double result = 0;
    for (k = 1; k <= n; k++) { // sum of -(y ^ k) / k from k = 1 to n
        result += y / k;
        y *= initial_y; // y ^ k
    }
    return -1 * result + exponent * LN2;
}

Sweetmeat answered 1/10, 2024 at 12:48 Comment(1)

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