How can I get the expected output in Dart? - dart

Question

I am trying to get the output as below:
The problem is from URI-1098 Sequence IJ 4.
My Code is:
void main() {
double x = 0;
double y = 1;
for(double i = x; i<=2; i+= .2){
for(double j=y; j <= y + 2; j++){
if(i==0 || i==1 || i==2){
print('I=${i.toStringAsFixed(0)} J=${j.toStringAsFixed(0)}');
}else{
print('I=${i.toStringAsFixed(1)} J=${j.toStringAsFixed(1)}');
}
}
y += 0.2;
}
}
And my output is:
I=0 J=1
I=0 J=2
I=0 J=3
I=0.2 J=1.2
I=0.2 J=2.2
I=0.2 J=3.2
I=0.4 J=1.4
I=0.4 J=2.4
I=0.4 J=3.4
I=0.6 J=1.6
I=0.6 J=2.6
I=0.6 J=3.6
I=0.8 J=1.8
I=0.8 J=2.8
I=0.8 J=3.8
I=1 J=2
I=1 J=3
I=1 J=4
I=1.2 J=2.2
I=1.2 J=3.2
I=1.2 J=4.2
I=1.4 J=2.4
I=1.4 J=3.4
I=1.4 J=4.4
I=1.6 J=2.6
I=1.6 J=3.6
I=1.6 J=4.6
I=1.8 J=2.8
I=1.8 J=3.8
I=1.8 J=4.8
I=2.0 J=3.0
I=2.0 J=4.0
I=2.0 J=5.0
I am not getting desired outputs for the last 3 lines of the output. Can anyone kindly show the the error in my logic.
Thanking you all in advance.

Answer 1

You're using doubles.
That's the issue here. You are adding 0.2 repeatedly to a value starting at 0, but 0.2 is not precisely representable as a double. The 0.2 literal really represents the double value 0.200000000000000011102230246251565404236316680908203125 which is close to 0.2, but not exactly there.
The values you get by adding 0.2 to itself ten times loses some precision along the way. It hits 1.0 precisely, but then the next addition needs to drop some bits so the next value is actually below 1.2. The final result is 1.9999999999999997779553950749686919152736663818359375 which is not == 2.0, so your check for == 2.0 doesn't trigger. When you ask for the value with one decimal, it does round to "2.0", so you can't tell unless you print the actual value.
There are multiple ways to get around the problem.
One is to not use doubles. If you keep the numbers as integers by multiplying them by ten, then you just need a way to create the string to print. That could be
String toPrint(int value) {
var string = value.toString();
var lead = string.substring(0, string.length - 1);
if (string.endsWith('0')) return lead;
return "$lead.${string.substring(string.length - 1)}";
}
Alternatively, you can keep using doubles, but do .toStringAsFixed(1) and then check whether the last digit is zero. If it is, cut off the last two characters before printing.