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Using a variable as a for loop expression

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# 1  
Old 09-21-2012
Using a variable as a for loop expression
I'm writing a script to merge the xkcd webcomic tiles for comic 1110. So far, I have written about 100 lines, and instead of doing each quadrant of the image separately, I've decided to use functions to do this, repeating for every quadrant and using variables for each quadrant to make the function act differently each time. My problem is, I'm having trouble using variables for the for loop expressions, it goes "line 8: ``eval "$a"`': not a valid identifier". This is what I have so far.

PS: I know I'm designated this as PHP, but that's only to get syntax highlighting, since php appears to highlight it the same way.

PHP Code:
#!/bin/bash
# Date Created: September 20th, 2012
# Created by Jonathan Bondhus - https://github.com/jbondhus
# Credit to Andreas Reichinger for analysis of image tile placement, xkcd.com for comic. Used ImageMagick code from Antonio Frascarelli.
#
#  This script will merge the files from xkcd comic number 1110, "Click and Drag".

function function_join() {
    echo "#";
    if [[ quadrant -eq 1 ]]; then
    echo "# First Quadrant:"; # Join together the first quadrant
    quadrantName="first"
    function_quadrant
    else
        if [[ quadrant -eq 2 ]]; then
        echo "# Second Quadrant:"; # Join together the second quadrant
        quadrantName="second"
        function_quadrant
        else
            if [[ quadrant -eq 3 ]]; then
            echo "# Third Quadrant:"; # Join together the third quadrant
            quadrantName="third"
            function_quadrant
            else
                if [[ quadrant -eq 4 ]]; then
                echo "# Fourth Quadrant:"; # Join together the fourth quadrant
                quadrantName="fourth"
                function_quadrant
                fi
            fi
        fi
    fi
    echo "#";
    for `eval "$a"`;
    do
        echo "# row number "$n" ...";
        convert $b;
        for `eval "$c"`;
        do
            if [ $d ];      # If the tile file doesn't exist, an empty square will be used.
                then convert $e;
                else convert $f;
            fi;
        done;
        convert $g;    # Append the image onto the current quadrant
        rm -f $h;
        echo "# ... OK";
        echo "#";
    done;
}

function function_quadrant() {
    if [[ quadrant -eq 1 ]]; then # Set the variables for the first quadrant
    a='((n=$north; n>=1)); n--'
    b='-size 0x1 xc:white resultn$n''w.png'
    c='((w=$west; w>=1; w--))'
    d='-e $n''n$w''w.png'
    e='+append resultn$n''w.png $n''n$w''w.png resultn$n''w.png'
    f='+append resultn$n''w.png _blank.png resultn$n''w.png'
    g='-append $quadrantNameQuadrant.png resultn$n''w.png $quadrantNameQuadrant.png'
    h='resultn$n''w.png'
    else
        if [[ quadrant -eq 2 ]]; then
        a='n=$north; n>=1; n--'
        b='-size 0x1 xc:white resultn$n''e.png'
        c='e=$east; e<=20; e++'
        d='-e $n''n$e''e.png'
        e='+append resultn$n''e.png $n''n$e''e.png resultn$n''e.png'
        f='+append resultn$n''e.png _blank.png resultn$n''e.png'
        g='-append secondQuadrant.png resultn$n''e.png secondQuadrant.png'
        h='resultn$n''e.png'
        else
            if [[ quadrant -eq 3 ]]; then
            a='s=1; s<=$south; s++'
            b='-size 0x1 xc:black results$s''w.png'
            c='w=$west; w>=1; w--'
            d='-e $s''s$w''w.png'
            e='+append results$s''w.png $s''s$w''w.png results$s''w.png'
            f='+append results$s''w.png _black.png results$s''w.png'
            g='-append thirdQuadrant.png results$s''w.png thirdQuadrant.png'
            h='results$s''w.png'
            else
                if [[ quadrant -eq 4 ]]; then
                a='s=1; s<=$south; s++'
                b='-size 0x1 xc:black results$s''e.png'
                c='e=1; e<=20; e++'
                d='-e $s''s$e''e.png'
                e='+append results$s''e.png $s''s$e''e.png results$s''e.png'
                f='+append results$s''e.png _black.png results$s''e.png'
                g='-append fourthQuadrant.png results$s''e.png fourthQuadrant.png'
                h='results$s''e1.png'
                else # If more than 4 quadrants, just to be safe, exit
                    exit 1
                fi
            fi
        fi
    fi
}

for (( quadrant = 0; quadrant < 4; quadrant++ )); do
    function_join
done

echo "#################################"
echo "Joining completed!"
echo "#################################"

exit 0 
# 2  
Old 09-21-2012
Why it's not working really depends on what $a is.

I'm not sure assembling a big giant command then feeding it into eval is a good idea.
This User Gave Thanks to Corona688 For This Post:
jbondhus
# 3  
Old 09-21-2012
Code:
((n=$north; n>=1; n--))

is what $a is supposed to be. That way,
Code:
for ${a}

actually should be
Code:
for ((n=$north; n>=1; n--))

This function has to run multiple times in slightly different ways. Normal commands and command options can easily be written as variables, but I'm not sure whether it's possible to do that for a for loop's expression.
# 4  
Old 09-21-2012
You can't put part of a command inside an eval. It has to be a whole one.

I can't help thinking there must be a simpler way to do this.
# 5  
Old 09-21-2012
Quote:
Originally Posted by Corona688
You can't put part of a command inside an eval. It has to be a whole one.

I can't help thinking there must be a simpler way to do this.
How am I supposed to do that, other than evaluating a variable for a for loop expression? Any ideas?
# 6  
Old 09-21-2012
A normal for-loop? I don't see why you've taken this approach at all.

I'm trying to work on an expanded version of this:

Code:
for Y in 19n 18n 17n ... 17s 18s 19s
do
        ARGS=""
        for X in 19w 18w 17w ... 17e 18e 19e
        do
                ARGS="$ARGS +append $Y$X.png"
        done

        convert $ARGS $Y.png
done

...but running into memory limitations. I may have to shrink all the images before I begin. Which makes sense since I'd have to shrink them severely to view the entire image at once anyway.
# 7  
Old 09-21-2012
That is some comic Smilie

Managed to compose a quarter-scale image of it, I think, but it was the .png equivalent of a zip-bomb, a 300K file which would cause things to run out of memory when they opened it. Trying a sixteenth-scale...
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