A chessboard

Gins · 07-29-2006, 03:54 AM

# Making a chessboard
for (( i = 1; i <= 20; i++ )) ### Outer for loop ###
do
for (( j = 1 ; j <= 110; j++ )) ### Inner for loop ###
do
tot=`expr $i + $j`
tmp=`expr $tot % 2`
if [ $tmp -eq 0 ]; then
echo -e -n "\033[41m "
else
echo -e -n "\033[44m "
fi
done
echo -e -n "\033[40m" #### set back background colour to black
echo "" #### print the new line ###
done
-------------------------------------------------

The above works fine. However, I have a few questions.

1. What is the necessity of two paranthesis ' (( '
in the for statement?

2. There are two variables, namely 'tot' and 'tmp'. Why is ' tot%2 ' ? Why can't it be ' tot%5 ' or some other figure?

3. I can't see an outer loop and an inner loop when I look at the final outcome. It is just a chessboard.

4. Usually we initialize variables. Here we have two variables, namely 'i' and 'j'. Is it unnecessary to initialize in this program?

5. What is the use of 'else' statement here? We write the 'else' statement if something is not true. In this programme, the untrue aspect doesn't arise.

I hope our friends shed some light on my questions.

konsolebox · 07-29-2006, 04:41 AM

I don't know if you really mean to ask these questions. But I'll answer them anyway.

1. What is the necessity of two paranthesis ' (( '
in the for statement?
It's only part of the for syntax.

2. There are two variables, namely 'tot' and 'tmp'. Why is ' tot%2 ' ? Why can't it be ' tot%5 ' or some other figure?
Because there are only two colors in the board not 5.

3. I can't see an outer loop and an inner loop when I look at the final outcome. It is just a chessboard.
I guess the output of this program is only to create a board.

4. Usually we initialize variables. Here we have two variables, namely 'i' and 'j'. Is it unnecessary to initialize in this program?
Most of the time it's no longer needed in bash.

5. What is the use of 'else' statement here? We write the 'else' statement if something is not true. In this programme, the untrue aspect doesn't arise.
It's true if tmp is an even number and false if not.

Gins · 07-29-2006, 05:57 AM

Thanks konsolebox for the reply.

Usually we write only one set of paranthesis. Here we have two set of them.

I removed one of them. No, it didn't work.

tot%2
I wrote ' tot%5 '. It worked; but the output was a different picture.

Gins · 07-29-2006, 10:57 AM

I still need your help to understand those double parenthesis.

The explantion is as follows:

for (( i = 1; i <= 20; i++ ))
do
Begin the outer loop which runs 20 times., and the outer loop terminets when the value of i exceeds 20

for (( j = 1 ; j <= 110; j++ ))
do

Begins the inner loop, for each value of i the inner loop is cycled through 110 times, with the varible j taking values from 1 to 110. The inner for loop terminates when the value of j exceeds 110.

-----------------------------------------------------------

I can't fathom out the need of double parnthesis. Because we usually need a pair of parenthesis to write the 'for' statement.

Are you able to tell me more about it or show me a webpage to learn about it?

konsolebox · 07-29-2006, 11:14 AM

you can read the following docs:

Bash Reference Manual
http://www.gnu.org/software/bash/manual/bashref.html

Advanced Bash-Scripting HOWTO
http://docsrv.caldera.com:8457/en/AdvBashHowto/

Bash Beginners Guide
http://www.tldp.org/LDP/Bash-Beginne...tml/index.html

you can also try the following commands:

Code:

man bash
help
help for
help <command>

Gins · 07-29-2006, 11:45 AM

Thanks konsolebox for taking time to help me.

Now look at the following:

---------------------------------------------------
# vi nestedfor.sh
for (( i = 1; i <= 5; i++ )) ### Outer for loop ###
do

for (( j = 1 ; j <= 5; j++ )) ### Inner for loop ###
do
echo -n "$i "
done

echo "" #### print the new line ###

done
-----------------------------------------------------
I named the above as ' nested1 '.
The above works fine and the output is as follows:

[nissanka@c83-250-110-112 ~]$ chmod 755 nested1
[nissanka@c83-250-110-112 ~]$ ./nested1
1 1 1 1 1
2 2 2 2 2
3 3 3 3 3
4 4 4 4 4
5 5 5 5 5

The question is which is the outer loop and which is the inner loop.

Do you have any idea?

Nylex · 07-29-2006, 12:21 PM

The outer loop is the one controlling which number is being printed (and starting new lines) and the inner one is actually printing that number 5 times. I haven't done shell scripting before, but it's pretty simple. Maybe a good (?) and simple way to check you understand how the nested loops are working is to go through the code and kind of "execute" it yourself. What I mean is to get some paper and just write down the output and keep track of the loop variables (i and j) in your head. It helps me when I need to use nested loops, anyway.

Randux · 07-29-2006, 02:48 PM

cough *homework assignment* cough

homey · 07-29-2006, 03:31 PM

I don't think so as the example pops up pretty quick when googling for "linux script chess board"

At any rate, I changed it just abit so it will appear more like a chess board. Four squares each .... j = 2

Code:

#!/bin/bash

clear
echo ""

for (( i = 1; i <= 8; i++ )); do
  for (( j = 2 ; j <= 9; j++ )); do
    tot=$(expr $i + $j)
    tmp=$(expr $tot % 2)
    if [ $tmp -eq 0 ]; then
      echo -e -n "\033[47m  "
    else
      echo -e -n "\033[40m  "
    fi
  done
 echo ""
done
echo -e "\033[0m\n"

whk · 07-29-2006, 05:24 PM

actually a chessboard diagram is dark square at lower left at a1

Code:

#!/bin/bash

clear
echo ""
for (( i = 1; i <= 8; i++ )); do
  for (( j = 2 ; j <= 9; j++ )); do
    tot=$(expr $i + $j)
    tmp=$(expr $tot % 2)
    if [ $tmp -eq 0 ]; then
      echo -e -n "\033[40m   "
    else
      echo -e -n "\033[47m   "
    fi
  done
 echo -e "\033[0m\n"
 ### echo "" ### option ###
done