Variable for a "for loop"

Question

I've got two variables and I'm trying to run a for loop from one of them:

a=5  
z="i=0;i<=$a;i++"
x="i=$a;i>=1;i--"
read -p "choose loop:" loop
  case "$loop" in
    plus ) l="$z" ;;
    minus ) l="$x" ;;
  esac

for (($l)) do
#also tried (('$l')) & (("$l"))
  ...
done

When I'm doing this I've got:

syntax error: arithmetic expression requiered
syntax error:'(($l))'

So I tried:

a=5
x="\(\(i=1;i<=$a;i++\)\)"
#also tried "((...))"

for $x do
  ...
done
#also tried "$x" & '$x'

But when I'm doing this i've got:

bash: '"$l"': not a valid identifier.

But I want only one "loop construction" I don't want to copy my code twice.
So what should i do for run my loop and don't have to write my code twice inside the loop?

SLePort · Accepted Answer · 2017-02-04 08:56:27Z

3

You can store the list in a variable using seq:

loop_max=5
read -p "choose loop:" loop
case "$loop" in
  plus) myseq=$(seq 0 "$loop_max");;
  minus) myseq=$(seq "$loop_max" -1 0);;
esac

for i in $myseq; do
  echo "$i"
done

Edit:

You can also set the seq arguments in the case statement and call seq in you for loop:

loop_max=5
read -p "choose loop:" loop
case "$loop" in
  plus) start="0";;
  minus) end="-1 0";;
esac

for i in $(seq $start "$loop_max" $end); do
  echo "$i"
done

edited Feb 4, 2017 at 8:56

answered Feb 3, 2017 at 7:08

SLePort

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4 Comments

codeforester Over a year ago

This is very elegant!

SLePort Over a year ago

Thanks @codeforester. I was thinking of using just one seq command but I found it would have been less readable.

user7427983 Over a year ago

@SLePort Just for knowledge you can post how you do with only seq command ?

user7427983 Over a year ago

@SLePort , first one is good for understand but second one is shortest and even more what i was really excepting for . Thanks you .

codeforester · Accepted Answer · 2017-02-03 05:02:34Z

I don't think for syntax allows what you are trying to do. Instead, do this to keep it more readable:

loop_max=5
read -p "choose loop:" loop
case "$loop" in
   plus) loop_type="ascending"
    ;;
  minus) loop_type="descending"
    ;;
esac

[[ $loop_type == "ascending" ]] && for ((i = 0; i <= $loop_max; i++)); do
  : your code goes here
done

[[ $loop_type == "descending" ]] && for ((i = $loop_max; i > 0; i--)); do
  : your code goes here
done

The case statement is not adding much value. So, you could rewrite the whole thing this way:

loop_max=5
read -p "choose loop:" loop_type
if [[ $loop_type == "plus" ]]; then
  for ((i = 0; i <= $loop_max; i++)); do
    : your code goes here
  done
elif [[ $loop_type == "minus" ]]; then
  for ((i = $loop_max; i > 0; i--)); do
    : your code goes here
  done
else
  printf "Invalid choice.\n"
fi

Rishi · Accepted Answer · 2017-02-03 04:53:33Z

0

You can modify your for loop to look like this:

for j in $l 
do
    echo $j; # whatever you need to do here 
done

Hope this will solve your purpose.

answered Feb 3, 2017 at 4:53

Rishi

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2 Comments

codeforester Over a year ago

Did you test it? It won't iterate the way OP wants it - it will run the loop only once with j set to the value of $l.

Rishi Over a year ago

@codeforester, I see that you have posted the correct answer. I tried to address his issues with "not a valid identifier" error that he was facing. As is evident from your answer, there are multiple issues with his script.

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