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In the Programming mode of gcalctool, you have a log button, which calculates the logarithm, but how do I know what base it's using and how do I change it?

For example, how would I calculate the base 2 logarithm for 16?

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up vote 12 down vote accepted

By default the log button calculates base-10 logarithms (while the "ln" button does, of course, base-e logarithms).

To specify a different base, you use a "subscript":

  • Type "log" in the input field.
  • Input your desired base with a subscript. This can be done either by clicking on the "downarrow-n" button (top left) and then entering the number, or by pressing Alt while typing the number.
  • Finally type the number whose logarithm you want to calculate.

By the way, I found this pretty easily by accessing gcalctool's help (press F1) and looking for logarithms in the index.

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To use a decimal base (e.g. 1.01), use the answer from user247983. E.g. to determine the monthly interest from an APR of 12%, I used ln 1.12 / ln 1.01. If I use the method from this answer (log₁.₀₁ 1.12), it says "Malformed expression". – Tyler Collier Apr 14 '15 at 1:07

You can also use the base change formula:

ln(1024)/ln(2) = log (base 2) (1024)
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You may use this frequently and like a button instead of writing out the base change formula again and again. Add within gcalculator (kde) by selecting:

edit preferences functions Name: log2 Variable: x Expression: log(x)/log(2) add update close view buttons functions

now to get log2(1024): type 1024 into the calculator and PRESS ENTER hit the 'fun' button select log2 the answer is 10

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