awk: Statements(1)

1. Conditional Statements:

text2:

 1 Hello world!  
 2 Hello Chicago!  
 3 Hello Boston!  

script_1:

 #! /bin/bash  
   
 awk '{  
   if($1 == 1)  
     print $0;  
   else if($1 == 2)  
     print $2;  
   else   
     print $3;  
   }' text2;  

terminal:

 aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ./script_1  
 1 Hello world!  
 Hello  
 Boston!  

2. Iterative Statements: for(1)
text2:

 Hello world!  
 Hello Chicago!  

script_1:
"i=0" is executed at the very beginning.
"i<=NF" is executed before each iteration, if true, continue to run
"i++" is executed after each iteration.

 #! /bin/bash  
   
 awk '{  
   for (i=0; i<=NF; i++)  
   {  
     print $i;  
   }  
   }' text2;  

terminal:

 aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ./script_1  
 Hello world!  
 Hello  
 world!  
 Hello Chicago!  
 Hello  
 Chicago!  

Don't use float number in condition:
script_1:

 #! /bin/bash  
   
 awk 'BEGIN {  
   for (i=1; i>=0; i-=0.05)  
   {  
     print i;  
   }  
   }'  

terminal:

 aubinxia@aubinxia-fastdev:~/Desktop/xxdev$ ./script_1  
 1  
 0.95  
 0.9  
 0.85  
 0.8  
 0.75  
 0.7  
 0.65  
 0.6  
 0.55  
 0.5  
 0.45  
 0.4  
 0.35  
 0.3  
 0.25  
 0.2  
 0.15  
 0.1  
 0.05  

It doesn't output the final 0. Because of the implementation of float number, every time i is decreased by more than 0.05 a little bit.