How to write to a file using awk to find a string

问题:

I have some files in a folder that has several sqrt(x+y); functions. I want to find out what is in x+y. So I want to read the value inside sqrt(x+y ); and add a printf(x+y ); after the semicolon. I was trying this for about a month. But still cannot find a way.

Please help. If you know where I can find any good tutorial or book about AWK please let me know.. Thanks. Following is the algorithm that I figured. (I am totally new to AWK)

loop thro all files{ if one line starts with sqrt( start put the strings in to variable a /or array a until find ); write Print ( , concatenate, then the value , concatenate,); in the next line.

回答1:

You can try the following bash script:

#! /bin/bash

files=(*.txt)

for ((i=0; i<${#files[@]}; i++)) ; do
    file="${files[$i]}"
    awk -f f.awk "$file" > "${file}.mod"
done

where f.awk is:

{
    gsub(/\([^)]*\)/,"&; printf&")
    print
}

Note that this will not work for nested parenthesis, like sqrt(4+2*(x+y)).. (If that is needed I can try to update my answer.).

Given example file input.txt:

x=7;
sqrt(x+5*4); sqrt(x*x); 
i=3;
a=2+sqrt(8);

Running: awk -f f.awk input.txt gives:

x=7;
sqrt(x+5*4); printf(x+5*4); sqrt(x*x); printf(x*x); 
i=3;
a=2+sqrt(8); printf(8);

For Gnu Awk, I recommend the following Awk resource: http://www.gnu.org/software/gawk/manual/gawk.html

回答2:

awk is certainly a fine tool, but why not stretch a little? If you are willing to discard the whitespace inside the parentheses, try:

cat << 'EOF' - input-file | m4
divert(-1)
define(`sqrt',`divert(1)'$1
`divert(-1)')
EOF

That should print out every string that appears within a call to sqrt in the file named input-file. This will not evaluate the strings, but it's not clear to me if you want that.

The primary advantage of this solution is that it will work well with nested parentheses. That is, lines of the form sqrt( foo( bar( x + y ))) will correctly print foo( bar( x + y )), and that will be difficult to parse correctly with awk. The primary disadvantage is that it requires correct syntax on the input (unbalanced parentheses will cause problems).