execute a command during every n-th iteration

Question

I would like to print "is OK" during every odd loop iteration(first, third, fifth, etc) and print "is not OK" during every even loop iteration(second, fourth, sixth, etc). I'm able to accomplish this with:

$ for (( i=0,j=24,k=48; i<=200; i=i+48,j=j+48,k=k+48 )); do printf '%s\n%s\n' ""$i" to "$j" is OK" ""$j" to "$k" is not OK"; done
0 to 24 is OK
24 to 48 is not OK
48 to 72 is OK
72 to 96 is not OK
96 to 120 is OK
120 to 144 is not OK
144 to 168 is OK
168 to 192 is not OK
192 to 216 is OK
216 to 240 is not OK
$ 

However, is there a more elegant solution to accomplish the same?

Answer 1

You can use the modulo operator % for that:

for (( i=0; i<200; i+=24 )); do
  echo -n "$i to $((i+24)) is "
  ((i % 48 != 0)) && echo -n "not "
  echo OK
done

Answer 2

This code will output the same results as yours:

for (( i=0; i<=216; i+=24 )); do
    let j=i+24
    let v=(i/24)%2                           #calculate the iteration number
    if [ "$v" -eq 0 ]; then                  #if [ the iteration number is even ]
            printf ""$i" to "$j" is OK\n"
    else
            printf ""$i" to "$j" is not OK\n"
    fi
done