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problem with floating point number loops

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# 1  
Old 06-18-2010
problem with floating point number loops
Hey,

I guess I am just to stupid and am not seeing the "wood for the trees", but I am always getting strange errors.

I want to create a mesh with coordinates like:

x y z
3.1 3.0 0.75 0 0 1
3.1 2.9 0.75 0 0 1
3.1 2.8 0.75 0 0 1
3.1 2.7 0.75 0 0 1
3.0 3.0 0.75 0 0 1
3.0 2.9 0.75 0 0 1
3.0 2.8 0.75 0 0 1
3.0 2.7 0.75 0 0 1

Here is my skript:

Code:
echo "Abstand Berechnungspunkte in Meter="
read abstand

z=0.75
a=0
b=0
c=1

for (( i=$abstand; i<0; i=$i+$abstand )) 
do
			for (( h=$abstand; h<4.61; h=$h+$abstand )) 
			do
						x=`echo "-3.616+$i" |bc -l `
						y=`echo "-0.085+$h" |bc -l `
						print $x " " $y " " $z " " $a " " $b " " $c
			done
done

or I also tried this one:

Code:
echo "Abstand Berechnungspunkte in Meter="
read abstand 

    z=.75
    a=0
    b=0
    c=1
    ab=$abstand
    i=-3.616
    h=0
    maxx=0
    maxy=4.61
    
    while [ $i -le $maxx ]
    do 
            x=`echo "-3.616+$i" |bc -l`
            i=`echo "$i+$ab" |bc -l`
    done

But it is always creating the same problem regarding numbers with commata. Becasue my input is e.g., 0.02
line 11: ((: i=0.1: syntax error: invalid arithmetic operator (error token is ".1")

Does anyone has an idea? Thanks for any help and sorry for the stupid question,
Sam
# 2  
Old 06-18-2010
The shell will not allow you to loop using a floating point value. As a general rule floating point values should not be used in loop counters even when the language allows it.
# 3  
Old 06-18-2010
ok. thanks.

Do you then have an idea how to realize such a grid by a loop easily?

And thanks for your reply, sam
# 4  
Old 06-18-2010
A possible work-around: convert all meter into centimeter values, e.g. by means of sed or awk Smilie
# 5  
Old 06-18-2010
Quote:
Originally Posted by ergy1983
ok. thanks.

Do you then have an idea how to realize such a grid by a loop easily?

And thanks for your reply, sam
My Deutsch isn't all that great, so to make sure I'm clear on what you're doing here - you want to find the number of steps between between two constant values.

Isn't this essentially a division problem? For example if the abstand=2, your xmax is 6 and your ymax is 4 the output table would be:

Code:
6 4 0.75 0 0 1
6 2 0.75 0 0 1
6 0 0.75 0 0 1
4 4 0.75 0 0 1
4 2 0.75 0 0 1
4 0 0.75 0 0 1
2 4 0.75 0 0 1
2 2 0.75 0 0 1
2 0 0.75 0 0 1
0 4 0.75 0 0 1
0 2 0.75 0 0 1
0 0 0.75 0 0 1

Also, are you limited to using a shell script? The only way I know of to do floating point comparisons in bash is pretty kludgy. If you have perl available and my understanding is correct I could definitely create a script to handle this.

Quote:
Originally Posted by dr.house
A possible work-around: convert all meter into centimeter values, e.g. by means of sed or awk Smilie
This would only push the failure point to millimeters.
# 6  
Old 06-18-2010
Even floating point has a failure point. The question is how much accuracy is necessary.
# 7  
Old 06-18-2010
Quote:
Originally Posted by Corona688
Even floating point has a failure point. The question is how much accuracy is necessary.
Math::BigFloat could express atoms in meters, so I figured that wouldn't be an issue. Likewise, I assumed that more than a couple of decimal places were necessary. If this is just for a limited degree of accuracy an expr or bc kludge would be sufficient, but I wouldn't want to maintain it.
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