How can I convert a negative integer to a positive number using bc

eflint · 05-25-2016, 05:18 PM

I'm never use bc that much, but If I have an integer of -5 can I use bc to make it a positive number? much appreciated

astrogeek · 05-25-2016, 05:39 PM

Just multily by -1.

In bc...

Code:

-5 * -1

Shadow_7 · 05-25-2016, 05:48 PM

Why not use sed to remove the "-"?

A slow way would be to square then square root it. Or multiply by -1 and take the higher value of the original versus post bc comparison. And probably many other tricks. Although bc might have an abs() function for absolute value. Or bash if you're not doing floating point numbers. I'm not that up on either to know though.

eflint · 05-25-2016, 05:56 PM

Quote:

Originally Posted by astrogeek

Just multily by -1.

In bc...

Code:

-5 * -1

I found this on the web.

echo -"(-5)" | bc -l and it output the number 5

Quote:

Originally Posted by Shadow_7

Why not use sed to remove the "-"?

I need to use the code in a math situation in a bash script.

Much appreciate to both

dunne · 05-25-2016, 11:33 PM

More generally:

Code:

define test (n) {
if (n < 0) n *= -1
return n
}

So in a shell script you could do:

Code:

(echo 'define test (n) {' ; echo ' if (n < 0) n *= -1 ; return n}' ; echo "test($myvar)") | bc

Need two echoes for function defintion because newline expected after "{".

EDIT:
Thinking about it, bc can read a file of function defintions, so you could put
this and other functions into a file, and invoke:

Code:

bc myfunctionsfile

grail · 05-26-2016, 02:03 AM

All looks a bit complicated to me. If it is bash and we are simply using integers, something like:

Code:

x=-5

(( x = x < 0 ? x * -1 : x ))

echo $x

You can put it in a function or use where necessary.

dunne · 05-26-2016, 02:48 AM

Quote:

Originally Posted by grail

All looks a bit complicated to me

The question was how to do it in bc.
And why on Earth would anyone use a behemoth like Bash to do simple maths?

szboardstretcher · 05-26-2016, 03:05 AM

A quick absolute value script would be:

Code:

[sysop@ArchTerminal Downloads]$ bc -l

bc 1.06.95
Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'. 

define abs(x) {if (x<0) {return -x}; return x;}
abs(-2)
2

And of course in bash you can quickly do something like:

Code:

[sysop@ArchTerminal Downloads]$ asdf=-1
[sysop@ArchTerminal Downloads]$ echo $asdf
-1
[sysop@ArchTerminal Downloads]$ asdf=${asdf/#-/}
[sysop@ArchTerminal Downloads]$ echo $asdf
1

grail · 05-26-2016, 03:29 AM

Quote:

Originally Posted by dunne

The question was how to do it in bc.
And why on Earth would anyone use a behemoth like Bash to do simple maths?

Gee, maybe it had something to do with the OP saying:

Quote:

I need to use the code in a math situation in a bash script.

And as it is a newbie forum he/she may have thought that bc was their only option

szboardstretcher · 05-26-2016, 03:36 AM

I would say in this case,. bc is a behemoth and complete overkill,. You are already IN the bash shell, and bash is completely capable of 'simple maths' .. so why break out a calcuating program that requires an entirely new language to use?

JJJCR · 05-26-2016, 04:40 AM

Just use cut.

Quote:

echo "-5" | cut -c 2

Output: 5

Shadow_7 · 05-26-2016, 05:17 AM

Quote:

Originally Posted by szboardstretcher

I would say in this case,. bc is a behemoth and complete overkill,. You are already IN the bash shell, and bash is completely capable of 'simple maths' .. so why break out a calcuating program that requires an entirely new language to use?

Because bash doesn't do floating point math. Unless you shift the decimal and add it back into your number (visually) with string manipulation. Although it is disheartening that a kernel build now requires bc.

$ echo $(( -5 * -1 ))
5

$ echo $(( 5 / 10 ))
0

$ echo "."$(( 50 / 10 ))
.5

syg00 · 05-26-2016, 07:07 AM

The OP specifically mentioned integer.

szboardstretcher · 05-26-2016, 09:26 PM

Looks like OP has gotten a lot of good answers. It's really up to them to decide on the way that works for them.

@eflint, Let us know if you have further need or explanation!