奇怪的双到字符串的转换 [英] Strange double-to-string conversion

问题描述

我用下面的code在我的活动之一来填充微调...

I am using the following code to populate a Spinner in one of my Activities...

    for( double i = 0; i < 10 ; i+=0.1 ) {
        rVoltsList.add( Double.toString( i ) );
    }
    Spinner rVoltsSpinner = (Spinner) findViewById( R.id.recloseVoltsSpinner );
    ArrayAdapter<String> rVoltsAdapter = new ArrayAdapter<String>( this, android.R.layout.simple_spinner_item, rVoltsList );
    rVoltsSpinner.setAdapter( rVoltsAdapter );

我是假设这会给我一个名单如下：0.0，0.1，0.2，0.3，0.4，依此类推。
然而，这是该列表看起来什么，当我运行我的程序，如：

I was assuming this would give me a list as follows : 0.0, 0.1, 0.2, 0.3, 0.4, and so on. However, this is what the list looks like when I run my program:

0.0
0.1
0.2
0.30000000000000000000000004
0.4
0.5
0.6
0.7
0.79999999999999999999
0.89999999999999999999
0.99999999999999999999
1.09999999999999999999
1.2
1.3
and this goes on until 9.99999999999999999998

什么想法？

推荐答案

使用这样的：

rVoltsList.add( String.format("%.1f", i) );

问题是， Double.toString（我）将不是圆的。

但奇怪的值是由于一个事实，即0.1（基数为10）没有一个确切的再presentation为双击在Java中，所以每次时间它添加到循环变量，要添加的东西比你想的是什么（请原谅我的双关语）有一点不同。

The strange values are due to the fact that 0.1 (base 10) does not have an exact representation as a double in Java, so every time you add it to the loop variable, you are adding something a bit different than what you think (if you'll pardon the pun).

舍入的同一个问​​题建议您不要使用被双作为循环变量。你是不太可能恰好准确地打你的循环限制。我会改写你的循环如下：

The same issue of rounding suggests that you should not be using a double as a loop variable. You are very unlikely to exactly hit your loop limit exactly. I would rewrite your loop as follows:

for( int i = 0; i < 100 ; ++i ) {
    rVoltsList.add( String.format("%.1f", i / 10.0) );
}

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