How to increment a string by .i in Java?

Question

My goal is to add value pairs to a HashMap and if a value is already taken it will increment the previous value by .i. What I mean by this is it would start as 0.1 ... 0.9 then 0.10, 0.11, 0.12 and so on...

I have started:

for (int i=0; i < 50; i++){ Double test = Double.parseDouble( 0 + "." + i);

But I cannot find a suitable test to add a decimal place onto the double once it has reached .9 (0.9, 0.10) Everything that I've tried doesn't work reliably. I was wondering if anyone could help.

floating point arithmetic is notoriously inaccurate due to the nature of computing decimals in binary. I would suggest scaling everything up x100 and storing your data as int — Woohoojin
– Woohoojin, Commented Aug 6, 2018 at 16:28
The problem with your logic is: what is i? 0.9 -> 0.10 suggests i = 0.1 but 0.10 -> 0.11 suggests i = 0.01. If your values actually are integers, i.e. 1 ... 9, 10, 11 etc., then please state so in your question - as well as why you prepend those with 0.. — Thomas
– Thomas, Commented Aug 6, 2018 at 16:29
If you want to count, you should use counting numbers: Integers. Counting with floating point numbers in a custom order is a bad idea. — lucasvw
– lucasvw, Commented Aug 6, 2018 at 16:30
Just use integers and a scaling factor, basically as you are doing now. What's wrong with your current approach? — Mad Physicist
– Mad Physicist, Commented Aug 6, 2018 at 17:46

Peter Lawrey · Accepted Answer · 2018-08-06 16:51:50Z

What I mean by this is it would start as 0.1...0.9 then 0.10, 0.11,0.12 and so on...

To print this pattern, there simplest solution is to print the value as it really a String operation rather than a mathematical one.

for (int i = 1; i <= 50; i++) System.out.println("0." + i);

NOTE: For double the value 0.1 == 0.10 and there is no way to tell them apart.

Ryuzaki L · Accepted Answer · 2018-08-06 16:45:35Z

You can use DecimalFormat to get two decimal digits. The ".00" in the parameter tells the formatter to use two decimal places while the "#" means to display the whole number as it is

DecimalFormat df = new DecimalFormat("#.00"); for(int i=0;i < 50 ;i++){ Double test = Double.parseDouble( 0 + "." + i); System.out.println(df.format(test));

Output:

.00 .10 .20 .30 .40 .50 .60 .70 .80 .90 .10 .11 .12 .13 .14 .15 .16 .17 .18 .19 .20

Magdrop · Accepted Answer · 2018-08-06 16:33:15Z

How about this

double division = 1.0; for(int i=0;i < 50 ;i++){ if (i%10 == 0) division *= 10; Double test = i/division; System.out.print(test + " "); }

It will have duplicate each time it goes down one decimal, but I hope for testing it should be ok.

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