How to coerce AWK to evaluate string as math expre

问题:

Is there a way to evaluate a string as a math expression in awk?

balter@spectre3:~$ echo "sin(0.3) 0.3" | awk '{print $1,sin($2)}'
sin(0.3) 0.29552

I would like to know a way to also have the first input evaluated to 0.29552.

回答1:

Here's a simple one liner!

math(){ awk "BEGIN{printf $1}"; }

Examples of use:

math 1+1  

Yields "2"

math 'sqrt(25)'

Yeilds "5"

x=100; y=5; math "sqrt($x) + $y"

Yeilds "15"

回答2:

awk lacks an eval(...) function. This means that you cannot do string to code translation based on input after the awk program initializes. Ok, perhaps it could be done, but not without writing your own parsing and evaluation engine in awk.

I would recommend using bc for this effort, like

[edwbuck@phoenix ~]$ echo "s(0.3)" | bc -l
.29552020666133957510

Note that this would require sin to be shortened to s as that's the bc sine operation.

回答3:

You can just create your own eval function which calls awk again to execute whatever command you want it to:

$ cat tst.awk
{ print eval($1), sin($2) }

function eval(str,      cmd,line,ret) {
    cmd = "awk \047BEGIN{print " str "; exit}\047"
    if ( (cmd | getline line) > 0 ) {
        ret = line
    }
    close(cmd)
    return ret
}

$ echo 'sin(0.3) 0.3' | awk -f tst.awk
0.29552 0.29552

$ echo '4*7 0.3' | awk -f tst.awk
28 0.29552

$ echo 'tolower("FOO") 0.3' | awk -f tst.awk
foo 0.29552

回答4:

With gawk version 4.1.2 :

echo "sin(0.3) 0.3" | awk '{split($1,a,/[()]/);f=a[1];print @f(a[2]),sin($2)}'

It's ok with tolower(FOO) too.

回答5:

You can try Perl as it has eval() function.

$  echo "sin(0.3)" | perl -ne ' print eval '
0.29552020666134
$

For the given input,

$ echo "sin(0.3) 0.3" | perl -ne ' /(\S+)\s+(\S+)/ and print eval($1), " ", $2 '
0.29552020666134 0.3
$