CODEWITHSUNDEEP
bash
Breelin
Breelin

Reputation: 1

For loop syntax?

I'm just learning to code with bash, and I'm running into some problems doing some exercises. I feel like I might be doing something wrong with the syntax, but no matter how I look at it it doesn't look wrong.

The code I've got here is supposed to find the 'lucky numbers' between 1000 and 10000. In this case, a lucky number is a number who's digits sum to a number that in turns sums to 7. The part that does the calculation of the lucky number works on it's own, but once I put it into a for loop, where there is no single input, the output is infinite and wrong. I can only assume the for loop is the problem: 

digit=0
sum=0
for (( i=1000; i<=10000; i++ ))
do
     while [ $i -gt 0 ]
     do
          digit=$(( $i % 10 ))
          i=$(( $i / 10 ))
          sum=$(( $sum + $digit ))
    done
    while [ $sum -gt 0 ]
    do
          digit=$(( $sum % 10 ))
          sum$(( $sum / 10 ))
          sum2=$(( $sum2 + $digit ))
    done
    if [[ $sum2 == 7 ]];
    then
          echo $sum2
    fi
done

If anyone has any idea what I'm doing wrong, or even a way to make it better, I would really appreciate it!

Upvotes: 0

Views: 146

Answers (2)

James K. Lowden
James K. Lowden

Reputation: 7837

for (( i=1000; i<=10000; i++ ))

If you want to learn bash for bash's sake, every last nook and cranny, that's fine, but I suggest you focus your attention on portable Bourne shell syntax.

  • It works in bash, and it has worked for millions of programmers for decades. Very often, the Old Ways are good ways.

  • You will encounter non-bash code in a variety of circumstances.

Most Linux systems include seq(1). So these work:

$ for i in $(seq 1000 10000); do echo $i; done | wc -l
9001
$ seq 1000 10000 | while read i; do echo $i; done | wc -l
9001

Shell scripts usually iterate over some input (a list of filenames, say) not for N times. I guess seq was invented to support counted loops before bash developed the counted-loop syntax.

As a by-product of using seq, I think your loop body will work unaltered. Because the loop (while or for) is assigning i from its input from seq, whatever you do to i in the body is lost at the next iteration.

Upvotes: 1

John Kugelman
John Kugelman

Reputation: 361565

i=$(( $i / 10 ))

You're modifying the for loop's variable. Don't do that; make a copy.

Upvotes: 1

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