UNIX for Dummies Questions & Answers

Hi,
I was hoping someone could explain this please

I'm using bash, scientific linux... and I don't know what else you need to know. With

Code:

awk '{ if( 0.3 == 0.1*3) print $1}' file.dat

nothing will be printed since apparently the two numbers do not equate. (Using 0.3 != 0.1*3 is seen as true however)

0.2==0.1*2 is seen as true
0.5==0.1*5 is true
0.7==0.1*7 is false...

Many thanks

One way would be to avoid decimal numbers. For me, this works

Code:

awk '{ if( 10*0.3 == 10*0.1*3) print $1}' file.dat
awk '{ if( 10*0.7 == 10*0.1*7) print $1}' file.dat

A different approach would be to avoid equal compares with decimal numbers. Instead you could write

Code:

awk '{ if( abs(0.3 - 0.1*3) < 0.00001) print $1}' file.dat

But since awk does not have a builtin abs function, you would have to provide one in your awk-script:

Code:

function abs(x) {
    return ((x < 0.0) ? -x : x)
}

Thanks -

avoiding using decimals works for the case I'm doing now but could you tell me WHY this is even a problem for awk?
And why are there certain decimal numbers (0.3, 0.7) that pose a problem while the vast majority do not?

Floating point numbers are a problem for all languages, because many non-integral numbers can not be represented precisely. The fact, that a certain number can be represented with a finite number of fractional digits in the decimal system does not mean, that they also have a finite number of fractional digits in a certain binary floating point representation. That's why you never should check for equality with floating point numbers, no matter what language you are using, because of the internal representation.

Commercial software uses fixed-point representations for this purpose or even plain integer variables.

FWIW I tried this in ksh93 and it seemed to work there:

Code:

$ printf "%.20f %.20f\n" 0.3 $((0.1*3))
0.30000000000000000000 0.30000000000000000000
$ printf "%.20f %.20f\n" 0.3 0.1*3 
0.30000000000000000000 0.30000000000000000000

Various awks:
gawk / mawk / bwk:

Code:

$ gawk 'BEGIN{printf "%.20f %.20f\n",0.3,0.1*3}'
0.29999999999999998890 0.30000000000000004441