Comparing two integer variables

Question

I want to print two iterators named i and j of nested for loops whenever they are not equal. The error message:

syntax error near unexpected token `then'

#!/bin/bash
read n
array=()
for i in `seq 1 $n`
do
    read num
    array+=$num
done

count=0
count2=0
countmax=0
for i in `seq 1 $n`; do
    for j in `seq 1 $n`; do
        if [ $i -ne $j ]; then
            echo $i $j
        fi
    done
done

What have I done wrong?

Answer 1

What have I done wrong?

In no particular order,

• You didn't have #!/bin/bash or equivalent as the first line.
• You don't seem to have posted the actual script that's failing. (What you have here is syntactically correct.)
• You haven't double-quoted your variables, "$n" instead of $n.
• You're using obsolete backticks for evaluation instead of $( ...).
• You haven't tested your code in https://shellcheck.net/
• You haven't indented your code blocks to make them easier to read.
• If you are adding elements to an array, use array+=($num). What you have in your code is string concatenation to the last element.
• In the (new) first block you assign to elements (0 .. n-1) but you later use elements (1 .. n).
• In the (new) second block you are comparing the indices for inequality rather than the array elements. Did you mean to compare "${array[$i]}" -ne "${array[$j]}" (or, for the (( ... )) construct, array[i] != array[j])?

(I should probably point out that some of these are good practice rather then essential coding rules.)

Here is your code with my suggestions applied:

#!/bin/bash
read -p 'Number of elements: ' n

array=(0)    # We don't use element zero
for i in $(seq "$n")
do
    read -p 'Element: ' num
    array+=($num)
done

for i in $(seq "$n")
do
    for j in $(seq "$n")
    do
        # Compare the array elements rather than the indices
        # Is this what you intended?
        if (( array[i] != array[j] ))
        then
            echo "${array[$i]} ${array[$j]}"
        fi
    done
done

Answer 2

I would also eliminate the use of seq by using for loops:

#!/bin/bash

read -p 'Number of elements: ' n

array=(0)
for (( i = 1; i <= n; i++ ))
do
    read -p 'Element: ' num
    array+=("$num")
done

for (( i = 1; i <= n; i++ ))
do
    for (( j = 1; j <= n; j++ ))
    do
        (( array[i] != array[j] )) && echo "${array[i]} ${array[j]}"
    done
done