awk初學(xué)之常見問題

yuloveban

回復(fù) 1# yinyuemi


    非常感謝共享！

yuloveban

回復(fù) 1# yinyuemi


    樓主具體解析下 awk -v RS="#" '{$1=$1;print $0}'  $1=$1應(yīng)該如何理解啊？ 剛剛看了你給發(fā)的那個(gè)帖子，沒有看明白！

yinyuemi

回復(fù)  yinyuemi


    樓主具體解析下 awk -v RS="#" '{$1=$1;print $0}'  $1=$1應(yīng)該如何理解�。� 剛剛 ...
yuloveban 發(fā)表于 2011-11-07 22:37

http://www.gnu.org/s/gawk/manual/gawk.html#Fields
Advanced Notes: Understanding $0

It is important to remember that $0 is the full record, exactly as it was read from the input. This includes any leading or trailing whitespace, and the exact whitespace (or other characters) that separate the fields.

It is a not-uncommon error to try to change the field separators in a record simply by setting FS and OFS, and then expecting a plain ‘print’ or ‘print $0’ to print the modified record.

But this does not work, since nothing was done to change the record itself. Instead, you must force the record to be rebuilt, typically with a statement such as ‘$1 = $1’, as described earlier.

1. $1=$1可以使OFS生效
   
2. 
   
3. 
   
4. echo '1#1#1
   
5. 2#2#2'  |awk -vRS="#" '{$1=$1;print $0}'
   
6. 1
   
7. 1
   
8. 1 2 # OFS默認(rèn)值為空格，生效！或者說，任意一個(gè)對(duì)域進(jìn)行操作的action，都會(huì)使得OFS生效，比如$2=$2,NF+=0;
   
9. 2
   
10. 2
    
11. 
    
12. 再舉兩個(gè)例子：
    
13.     echo '1 1 1
    
14.     2 2 2' |awk 'NR==1{OFS=":";$1=$1;print}NR==2{OFS="#";$1=$1;print}'  
    
15.     1:1:1
    
16.     2#2#2
    
17. 
    
18.     echo '1 1 1
    
19.     2 2 2' |awk 'NR==1{OFS=":";$0=$0;print}NR==2{OFS="#";$0=$0;print}'   # $0=$0的指令并不會(huì)引起OFS生效。
    
20.     1 1 1
    
21.     2 2 2
    
22. 
    

復(fù)制代碼

yuloveban

回復(fù) 43# yinyuemi


    3qs

rayzhang11

總結(jié)的真好，感謝

xiaopan3322

關(guān)于第四點(diǎn)，今天看manual的時(shí)候找到了，貼下：

Finally, there are times when it is convenient to force awk to rebuild the entire record, using the current value of the fields and OFS. To do this, use the seemingly innocuous assignment:

     $1 = $1   # force record to be reconstituted
     print $0  # or whatever else with $0

This forces awk rebuild the record. It does help to add a comment, as we've shown here.

There is a flip side to the relationship between $0 and the fields. Any assignment to $0 causes the record to be reparsed into fields using the current value of FS. This also applies to any built-in function that updates $0, such as sub() and gsub() (see String Functions).

Advanced Notes: Understanding $0
It is important to remember that $0 is the full record, exactly as it was read from the input. This includes any leading or trailing whitespace, and the exact whitespace (or other characters) that separate the fields.

It is a not-uncommon error to try to change the field separators in a record simply by setting FS and OFS, and then expecting a plain ‘print’ or ‘print $0’ to print the modified record.

But this does not work, since nothing was done to change the record itself. Instead, you must force the record to be rebuilt, typically with a statement such as ‘$1 = $1’, as described earlier.

yinyuemi

回復(fù) 46# xiaopan3322


    多謝Bob

manganese_zh

不錯(cuò)，給力的貼子

freeterman

10. awk ‘! a[$0]++’ 怎么理解？

這是一個(gè)非常經(jīng)典的去重復(fù)項(xiàng)的awk語句，雖然短小，不過涉及到了不少知識(shí)點(diǎn)，下面一一解讀：

<1> ：”!” 即非。

<2>：a[$0]，以$0為數(shù)據(jù)下標(biāo)，建立數(shù)組a

<3>：a[$0]++，即給數(shù)組a賦值，a[$0]+=1

<4> ：那么組合起來，awk是怎么執(zhí)行!a[$0]++的呢？我用一個(gè)實(shí)際例子來解釋：


01.cat file

02.111

03.222

04.111

05.222

06.333

07.

08.awk '{print a[$0],!a[$0]++,a[$0],!a[$0],$0}' file

09.  1 1 0 111

10.  1 1 0 222

11.1 0 2 0 111

12.1 0 2 0 222

13.  1 1 0 333
復(fù)制代碼


原來，第一個(gè)a[$0]的值為空，所以!a[$0]++是先作判斷，結(jié)果為1（非空為真，即為1），再作數(shù)組賦值a[$0]++。這也就是為什么前面的!a[$0]++并不一定等于后面的!a[$0]。

awk ‘++a[$0]==1’ 和上面的代碼作用一樣，你理解了么?


不懂

ulovko

總結(jié)的真好 ，感謝分享！