Bash script - a puzzle

Question

The script takes only one argument (call it n). It then alternately adds and subtracts each digit of the numbers from 1 through n.

For example:

n = 12

1 - 2 + 3 - 4 + 5 - 6 + 7 - 8 + 9 - 1 + 0 - 1 + 1 - 1 + 2

output: 5

#!/bin/bash
number=$1
result=1
if (( number == 1 || number == 0 )); then
    echo
    echo number: $number
    echo
else
    for (( i = 1; i < number; i++ )); do
        if (( i < 10 )); then
            if (( i % 2 == 0 )); then
                let "result = result + i + 1"
            else
                let "result = result - i - 1"
            fi
        else
            for (( i = 10; i <= number; i++ )); do
                if (( i < 100 && i >= 10)); then
                    let "result = result - i/10 + i%10"
                else
                    let "result = result - i/100 + (i%100)/10 - (i/100)%10"
                fi
            done
        fi
    done

    echo
    echo result: $result
    echo number: $number
    echo
fi

It works fine for ints 0 to 9, but for n>9 it gives the wrong answer.

Please correct my script and explain to me why it's behaving like this.

PS I'm new to bash scripting, so I'd love to see another solution to this puzzle

PPS I'm familiar to java

EDIT

I fixed the code, and it works correct now

#!/bin/bash
number=$1
result=1
if (( number == 1 || number == 0 )); then
    echo
    echo number: $number
    echo
    else
    for (( i = 1; i < number; i++ )); do
        if (( i < 9 )); then
            if (( i % 2 == 0 )); then
                let "result = result + i + 1"
            else
                let "result = result - i - 1"
            fi
        fi
    done
    for (( i = 10; i <= number; i++ )); do
        if (( i < 100 )); then
            let "result = result - i/10 + i%10"
        else
            if (( i % 2 == 0 )); then
                let "result = result - i/100 + (i%100)/10 - (i%100)%10"
            else
                let "result = result + i/100 - (i%100)/10 + (i%100)%10"
            fi
        fi
    done

    echo
    echo result: $result
    echo number: $number
    echo
fi

Answer 1

Ok this should work apologies that I miss-read the question earlier

number=$1
MATSTR="";
for((i=1; i <= number; i++)); do
        MATSTR="$MATSTR$i";
done;
echo $MATSTR | sed -e 's/\(.\)\(.\)/\1-\2+/g' | sed 's/+$//' | bc 

Explanation:

• For loop to create string concatenating all numbers from 1 to the number.
• sed statement replaces the every two digits with 1st digit minus second digit plus
• sed removes any trailing plus
• bc calculates the statement

Answer 2

Here's your original solution, but with bug fixes (in comments). It still won't work for number > 99, of course.

#!/bin/bash
number=$1
result=0                                            # Don't start at 1!!
if (( number == 1 || number == 0 )); then
    echo
    echo number: $number
    echo
else
    for (( i = 1; i <= number; i++ )); do           # <=, not <
        if (( i < 10 )); then                       # < 10, not < 9
            if (( i % 2 == 0 )); then
                let "result = result - i"           # subtraction! Also, the +1 was unnecessary
            else
                let "result = result + i"           # addition!
            fi
        else
            if (( i < 100 && i >= 10)); then
                let result=result-i/10+i%10
            else
                let "result=result-i/100+(i%100)/10-(i/100)%10"
            fi
        fi
    done

    echo
    echo result: $result
    echo number: $number
    echo
fi 

Answer 3

After misreading the question, (or the question being unclear), the answer took only minor adjustments. The sting solution is probably the simplest approach using modulo to togget +/-:

#!/bin/bash

declare -i num=$1
declare -i res=0
str=""

[ "$num" -gt 1 ] || {
    printf "\n Error: invalid input. usage: %s int (greater than 1)\n\n" "${0//*\//}"
    exit 1
}

for ((i = 1; i <= $num; i++)); do    # create the string of digits
    str="${str}${i}"
done

printf "\n Calculations:\n\n"

for ((i = 0; i < ${#str}; i++)); do  # walk down the string adding/subtracting each digit
    if [ $((i % 2)) -eq 0 ]; then
        ((res+=${str:$i:1}))
        printf "  res + %3s = %3s\n" "${str:$i:1}" "$res"
    else
        ((res-=${str:$i:1}))
        printf "  res - %3s = %3s\n" "${str:$i:1}" "$res"
    fi
done

printf "\n Final Result: %s\n\n" "$res"

exit 0

output:

$ ./puzzle.sh 12

Calculations:

  res +   1 =   1
  res -   2 =  -1
  res +   3 =   2
  res -   4 =  -2
  res +   5 =   3
  res -   6 =  -3
  res +   7 =   4
  res -   8 =  -4
  res +   9 =   5
  res -   1 =   4
  res +   0 =   4
  res -   1 =   3
  res +   1 =   4
  res -   1 =   3
  res +   2 =   5

Final Result: 5