bash: use [ test in a for(( loop

Howard_L · #1 03-Oct-2009, 10:51

Is there a way to get an arithmetic for(( loop
to use a [ or [[ test?
Example:

Code:

#!/bin/sh

echo "Enter some strings , terminate with a single dot:";
REPLY="";

I am trying to replace this with a arithmetic for statement:
# j=0;
# while [ "${REPLY:0:1}" != "." ];  do
#   ((j++));

# This works:

# for((j=1; 1 ; j++ )); do
# Shouldn't the test statement in here evaluate to a numeric too?

# for((j=1; [ "${REPLY:0:1}" != "." ] ; j++ )); do

# That gets this error line:
# syntax error: operand expected (error token is "[  != . ] ")

# I tried variants of the following and still no luck.
# Can it be done at all?

# for((j=1; [ [ "${REPLY:0:1}" != "." ] ] ; j++ )); do
# for((j=1; ([ "${REPLY:0:1}" != "." ]) ; j++ )); do
# for((j=1; (( [ "${REPLY:0:1}" != "." ] )); j++ )); do
   read
 echo "${REPLY}";
done;
echo "goodbye";

Thanks

TurboPT · #2 04-Oct-2009, 21:43

Hello Howard,

I tried Googling around for some 'for loop' examples having a string test in the condition, but could not find any...

However, I did have the idea to use a function in the condition to do the same? Maybe this could help with the goal?

Code:

 #!/bin/sh

 echo "Enter some strings, terminate with a single dot:";
 REPLY="";

 j=0;

 function check_reply
 {
         retVal=1;

         if [ "${REPLY}" == "." ]
         then
                 let retVal=0;
         fi

         echo ${retVal};
 }

 for((j=0; $(check_reply)==1 ; j++)); do
         read
         echo "${REPLY}";
 done;

 echo "goodbye";

Note, though, that there is a 'side affect' of 'j' having +1 more than the input strings given because of the first check of REPLY's value.

Howard_L · #3 05-Oct-2009, 11:53

Ha, so it will take the result of your command there:

CPP / C++ / C Code:

for((j=0; $(check_reply)==1 ; j++)); do

interesting.
But I don't think that would build more efficient code , right?

From what I've heard so far it looks like it can't be done...
dave would give us the old "FULL STOP!" and all that

So I'll have to learn to accept this "while" form which is really that bad:

Code:

REPLY=;
j=0; while [ "${REPLY:0:1}" != "." ]; do ((j++));  # <-- make believe it's a for()  loop...
  read; 
  echo $j; 
done;  (Of course the increment can be moved to the bottom of the block the)

But still "personal problem" flares when I see stuff like this. There's got to be a way!:

Code:

~>  i=1
~>  [ $i -eq 1 ];  echo $?;
0
~>  [ $i -eq 2 ];  echo $?;
1
~>  [ $i -eq 1 ];  printf "%x \n" $?;
0
~>  [ $i -eq 2 ];  printf "%x \n" $?;
1
~>  $([ $i -eq 1 ]);  printf "%x \n" $?;
0
~>  $([ $i -eq 2 ]);  printf "%x \n" $?;
1
~>  x=
~>  x=$([ $i -eq 1 ]);  printf "%x \n" $x;
0
~>  x=$([ $i -eq 2 ]);  printf "%x \n" $x;
0
~>  #hmmm
~>  x=1; x=$([ $i -eq 2 ]);  printf "%x \n" $x;
0
~>  #ouch! , They win , I quit...  let's move on  (yeah right)

Thanks for taking the time turbo.
Howrd();

TurboPT · #4 05-Oct-2009, 12:25

Quote:

Originally Posted by Howard_L

But I don't think that would build more efficient code , right?

Since there is a side-affect (though minor) no.

Quote:

Originally Posted by Howard_L

Code:

REPLY=;
# make believe it's a for()  loop...
j=0; while [ "${REPLY:0:1}" != "." ]; do ((j++));  
  read; 
  echo $j; 
done;

but darn it, when I see stuff like this I think there's got to be a way

I agree, but I wasn't able to find a way either. While investigating, though, I did find the 'until' syntax to be interesting too. (very similar to the while)

Quote:

Originally Posted by Howard_L

Thanks for taking the time turbo.
Howrd();

No problem. (Things like this make a great refresher -- especially when one gets away from the 'tinkering' for awhile)

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bash: use [ test in a for(( loop

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