长度指明了接下来包里面的值的长度。
值的长度越短，就越精确，键入错误或其他错误就越少。
The shorter the length of the value, the more precise it will be with less opportunity for keystroke or other errors.
如果初始值的长度比0大且和索引值相匹配，这两者都会以绿色显示；否则显示红色。
If the length of the original value is greater than zero and matches the index value, both are shown in green; otherwise the values are shown in red.
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