PadLeft
 Usage
• PadLeft[list, n] 通过在左边对list填充零，构成一个长度为 n的列表。
• PadLeft[list, n, x] 通过重复元素 x 填充。
• PadLeft[list, n,   ,  , ...  ] 通过循环重复元素  填充。 • PadLeft[list, n, padding, m]留下一个放在右边的m个元素的边距。
• PadLeft[list,   ,  , ...  ] 构成在第 i 层的一个长度为  的嵌套列表。
Notes
• PadLeft[list, n, ... ] 总是返回一个长度为 n 的列表。 • PadLeft[list, n, padding, -m]截断list的最后m个元素。 • Round[(n-Length[list])/2] 的边距有效的以 list为中心。 • PadLeft[list, n, list] 有效的把list处理为循环。 • PadLeft[list, n,  xlist ] 可以用于本身是一个列表的个体元素。 • PadLeft[  , n,   ,  , ...  ] repeats the sequence of  as many times as fits in a list of length n. • PadLeft[list,   ,  , ...  ] 创建一个维数为   ,  , ...  的完全数组，即使 list被分开。 • 负  指定放在右边。 • PadLeft[list,   ,  , ...  , list] 在每一维上有效的把 list处理为循环。 • 对象list不必有头List。 • 参见 Mathematica 全书 : 节 1.8.6.
 Further Examples
Padding a list This makes a list of length  by padding to the left with x's.
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Using  gives padding on the right.
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Using an empty list you can see how a cyclic padding ends on the last element of the padding.
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Now the nonempty list is superimposed on the padding.
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Here the list is padded on the right by  elements.
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And here the last  elements of the list are dropped.
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This treats the list as cyclic.
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This has the same effect.
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Padding a matrix This pads a  x  matrix to give  rows and  columns.
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This does the same thing, but uses a  x  block to provide the padding.
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This shifts the matrix up  row and to the left  columns.
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Padding graphics Here is a nested list.
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This represents the list using Raster. (Raster starts from the lower-left corner so the picture seems upside-down.) Evaluate the cell to see the graphic.
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Here the list is padded.
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Evaluate the cell to see the graphic.
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Here are two different paddings.
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Evaluate the cell to see the graphic.
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Evaluate the cell to see the graphic.
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• 通过
，有效的规定循环重复 
， 然后在它们顶部强加列表，列表的最后一个元素放在 
的位置。
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