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howto use a for(( exp1; [ test exp2 ]; exp3 )); construct

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# 1  
Old 10-04-2009
howto use a for(( exp1; [ test exp2 ]; exp3 )); construct
Hi,
I am fairly novice at bash but not bad at C and so wondered if a for loop could be done as shown below:
Code:
#!/bin/sh

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

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

# My logic:  This works (1 is true):
# for((j=1; 1 ; j++ )); do  

# So shouldn't the test statement in here evaluate to 1 or 0 and work too?   

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

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

# I tried variants of the following and still no luck.
 
# for((j=1; [[ "${REPLY:0:1}" != "x" ]] ; j++ )); do
# for((j=1; ([ "${REPLY:0:1}" != "x" ]) ; j++ )); do
# for((j=1; (( [[ "${REPLY:0:1}" != "x" ]] )); j++ )); do

# Can it be done at all?

  read 
  echo "${REPLY}";
done;
echo "goodbye";

Am I nuts?
Thanks, Howard
# 2  
Old 10-04-2009
The only way I could make it work (so far ...):

bash code:
  1. #! /bin/bash
  2.  
  3. echo -n "Input: " ; read INPUT
  4.  
  5. for (( INDEX=0; $INDEX<${#INPUT}; INDEX++ ))
  6. do
  7.   if &#91; "${INPUT:$INDEX:1}" == "x" ]
  8.   then
  9.     break
  10.   else
  11.     echo "${INPUT:$INDEX:1}"
  12.   fi
  13. done
  14.  
  15. exit 0
Last edited by Neo; 10-04-2009 at 05:11 PM.. Reason: Debugging (sort of ...) .. .neo: added highlight=bash bbcode
# 3  
Old 10-04-2009
Quote:
Originally Posted by HowardL
Hi,
I am fairly novice at bash but not bad at C and so wondered if a for loop could be done as shown below:
Code:
#!/bin/sh

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

#I am trying to replace this with a arithmetic for statement:


Why? That for syntax is non-standard.
Quote:
Code:
j=0;
while [ "${REPLY:0:1}" != "x" ];  do
  ((j++));

# My logic:  This works (1 is true):
# for((j=1; 1 ; j++ )); do  

# So shouldn't the test statement in here evaluate to 1 or 0 and work too?   

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


The second expression must be an arithmetic expression, not a command.
Quote:
Code:
# That gets this error line:
# syntax error: operand expected (error token is "[  != . ] ")

# I tried variants of the following and still no luck.
 
# for((j=1; [[ "${REPLY:0:1}" != "x" ]] ; j++ )); do
# for((j=1; ([ "${REPLY:0:1}" != "x" ]) ; j++ )); do
# for((j=1; (( [[ "${REPLY:0:1}" != "x" ]] )); j++ )); do

# Can it be done at all?

  read 
  echo "${REPLY}";
done;
echo "goodbye";

Am I nuts?

Use the standard syntax:

POSIX code:
  1. j=0
  2. while [ "${REPLY:0:1}" != "x" ]
  3. do
  4.   read
  5.   j=$(( $j + 1 ))
  6. done
Last edited by cfajohnson; 10-04-2009 at 09:56 PM.. Reason: added highlight=bash bbcode tags; it's not bash, so changed to POSIX code
# 4  
Old 10-04-2009
The "standard syntax' is fine.
I had a feeling what I was trying to do was just plain wrong , but
there was a grain of uncertainty and I'm stubborn...
...like Dr. House's there! Nice effort.
I guess it's that challenge thing.
Thanks to both of you. Nice forum youhave here.
Howard();
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