I'm trying to assign the element of an array to a variable so I can use it later. For example:
constraints=("A" "B" "C" "E" "Q" "P" "S")
constraint=A
... a loop ...
rand=$[$RANDOM % ${#constraints[@]}]
let constraint="${constraints[${rand}]}"
echo $constraint
... end of loop ...
If the array is all numeric values, it works. But if they are strings the echo always outputs 0. Why?
let is only for arithmetic operations. Your strings have no numeric value (unless they name variables which themselves have numeric values), so they all evaluate to 0 in that context.
Make it:
constraint="${constraints[${rand}]}"
...with no let.
In context, this might look like:
constraints=(A B C E Q P S)
rand=$(( RANDOM % ${#constraints[@]} ))
constraint=${constraints[${rand}]}
echo "$constraint"
Note:
$[ ] and let. Neither is good form in modern scripting.echo $var if the values themselves, or the value of the IFS variable, are unknown or not controlled. See BashPitfalls #14.Get rid of the "let."
constraints=("A" "B" "C" "E" "Q" "P" "S")
constraint=A
rand=$[$RANDOM % ${#constraints[@]}]
constraint="${constraints[${rand}]}"
echo $constraint
$[ ] syntax, etc).
let.letmakes your operation arithmetic. All the stringsA,B,C,E,Q,PandSevaluate to arithmetic 0.$[ ... ]is 1970s-era math syntax; on modern (post-1991-POSIX-sh) shells, use$(( ))instead of either that orlet.$RANDOMisn't the typical choice if you're deciding a lottery or generating keys to encrypt military secrets, after all), but if${#constraints[@]}doesn't divide evenly into 32768, then the "random" choice being made here isn't quite fairly weighted. If in a case where this matters, you'll want to compare the randomly-chosen value to see if it's greater than$(( 32768 - (32768 % ${#constraints[@]}) ))and pick a different random number until that isn't true.