Predefined log function in C++?

[Math Is Hard]

Is there a predefined function in C++ that will allow me to take LOG(base 10) of a number? If not, is there a reasonably simple work-around?

thanks,

MIH

 

[Hurkyl]

The change of base formula.

log_a x = (log_b x) / (log_b a)


Now, it would surprise me if there isn't a built in base 10 log... maybe it's log10? I don't have the documentation installed on this computer, and the other one isn't hooked up.

 

[Math Is Hard]

even if I use C.O.B., is there a predefined log function in C++ at all?

(just saw your edit)

The thing is - I can't find this in my textbook and I thought the teacher had mentioned it in class.

 

[Hurkyl]

Certainly. It's log. It's declared in the cmath header. (math.h if you're using C-style headers)

 

[Math Is Hard]

ooh! that's it! I fergot the dern include! THANKS!

 

[plover]

There is a log10 function also.

 

[Math Is Hard]

is there? I went with c.o.b. using natural logs cause that's all I could find.

 

[Math Is Hard]

There's one little problem I'm still trying to solve. There's a place in the program where I want to set my int variable "num" to the remainder I get after dividing num by a power of 10, the exponent of which being determined by another int variable I have called "expo".

num = num % (pow(10, expo));

but I get this error: "'%' : illegal, right operand has type 'double'"

I was wondering if there was some way to use static_cast here to fix the problem?

 

[gerben]

yes there is
pow() apparently returns a double, but you can change the type of the returned variable by type-casting it to an int like this:

num = num % static_cast<int>( pow(10, expo) );

 

[Math Is Hard]

yes there is
pow() apparently returns a double, but you can change the type of the returned variable by type-casting it to an int like this:

num = num % static_cast<int>( pow(10, expo) );

sweet! thanks, gerben.

I was trying to use that but I must have been getting the syntax wrong.

works just peachy now.

 

[franznietzsche]

Certainly. It's log. It's declared in the cmath header. (math.h if you're using C-style headers)

this is why i should read first, because i just spent 15 minutes trying to figure out which header it was in (i don't use C++ much anymore, between F90 and Perl i don't need it.)

 

[Math Is Hard]

ah well...who knows, franz. maybe you'll need that info one day.

 

[franznietzsche]

ah well...who knows, franz. maybe you'll need that info one day.

Well i'd used it before, so i knew it existed, its just been a while because i find F90 easier for calculation work, which is 75% of all the programming i do. But its perfectly possible that'll use it again at some point.