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Bash Calulation

Posted on 2011-09-07
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Last Modified: 2012-05-12
HI,

I have a bash script and need to do a calculation and return an integer.
The issue I have is expr and bc don't deal with decimal exponents.

This is the excel equation I am trying to get working in bash.
ANSWER=INT($START*$CPU^0.422)

Does anyone know how to do this in bash and just return the rounded down value?
All decimal answers should be rounded down.

Thanks
Fixitben
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Question by:fixitben
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5 Comments
 
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Expert Comment

by:woolmilkporc
ID: 36498629
echo $START $CPU | awk '{printf "%d", $1*$2^0.422}'

or, to use the variables inside awk and to avoid "echo"

1)
awk 'BEGIN {printf "%d", '$START'*'$CPU'^0.422}'

2)

awk -v S=$START -v C=$CPU 'BEGIN {printf "%d", S*C^0.42}'

wmp
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LVL 68

Accepted Solution

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woolmilkporc earned 2000 total points
ID: 36498637
The full statement

ANSWER=$(awk -v S=$START -v C=$CPU 'BEGIN {printf "%d", S*C^0.42}')
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Author Closing Comment

by:fixitben
ID: 36498689
That worked great. Thanks
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Expert Comment

by:edster9999
ID: 36498747
bc is a command that does just that.  If you do not specify the 'scale' to give it the number of decimal point places to use then it assumes no places and rounds down to the nearest integer.

Now the next step.  You can't do 'to the power of' in bc with a real number.  The value to the right has to be an integer.  Its just not going to handle that.

You'll need to convert it first.  Get your maths text book out and look up :
x^y = e^(y*log(x))
This is an alternative way of working out x^y and can  be handled by bc.
I've never done this part so you'll have to play round with it.
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LVL 38

Expert Comment

by:Gerwin Jansen, EE MVE
ID: 36498948
With bc your answer would have been:

START=1.5
CPU=2
echo "e(0.477*l(${START}*${CPU}))" | bc -l
1.68883351312073475842

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and rounded:

START=1.5
CPU=2
echo "e(0.477*l(${START}*${CPU}))" | bc -l | xargs printf "%1.0f\n"
2

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