while loop, need some explanation

manya · 10-14-2009, 07:39 AM

Hey Guys,

I am bit confused about while loop and not sure what it does. I need to know what while : does and how can you use that effectively.

Thnks,
Manya

rizhun · 10-14-2009, 07:46 AM

Hi manya,

You didn't specify what language you are talking about, so I'll have to be general...

A while loop is a flow control statement that allows code to be executed repeatedly based on a given condition.

I'll give you an example in Perl...

Say you wanted to cat a file and pipe it into a Perl program you've made, you'd process each line of the file by using a while loop:

Code:

#!/bin/perl

while ( <STDIN> ) {
  do something with the text line...
}

This code says "while data is coming from standard-input (STDIN) then do something with the text line"

The code will repeat this 'something' on every line from the file.

Hope this helps!

pixellany · 10-14-2009, 08:19 AM

http://tldp.org ..Get a copy of the Bash Guide for Beginners

Did you mean to ask literally what this does?:
"while :"
If so, whre did you see that?

sundialsvcs · 10-14-2009, 08:24 AM

How about classic BASIC?

Code:

10 IF condition is false THEN GOTO 40
20 do something
30 GOTO 10
40 ... the while-loop is now finished ...

Key points:

The condition is tested at the start of the loop. (It is not "continually tested during the loop.")
The body of the loop executes only so long as the condiion is true.
If the condition is initially false, the body of the loop is not executed at all.

manya · 10-14-2009, 09:12 AM

Oops !! sorry about providing half info. I was talking about bash scripting and here are some codes but not sure what they do or rather I need to know for what and how that while loop can be used

while : ; do
echo -n "Enter path to compressed image on cdrom [$cdpath or 'exit' to exit]: "
read P
if [ "$P" = "exit" ] ; then
cleanup 1
fi

IMGPATH=${P:=$cdpath}
if [ ! -z "$IMGPATH" ] ; then
break;
fi
done

manya · 10-14-2009, 09:48 AM

Or here is the simple script

X=0
while :
do
echo "X=$X"
X=`expr ${X} + 1`
sleep 1
done

Guttorm · 10-14-2009, 10:04 AM

Hi

while :
do
...
done

This means do what's between the "do" and the "done" forever. Well actually until someone kills the program, or there is a "break" or an "exit" inside the loop.

manya · 10-14-2009, 10:08 AM

hmmm..so is it mandatory to break the loop explicitly when you add ":" in while loop?

Guttorm · 10-14-2009, 10:20 AM

Yes : means do nothing, evaluate to true and set return code to 0.

"while :" is the same as "while true" but just a bit shorter and more difficult to read IMO.

pixellany · 10-14-2009, 01:42 PM

Quote:

Originally Posted by sundialsvcs

How about classic BASIC?

Code:

10 IF condition is false THEN GOTO 40
20 do something
30 GOTO 10
40 ... the while-loop is now finished ...

Yuck!!!---I thought GOTO went out with Fortran.....