Partitioning and Padding Lists
| Partition[list,n] | partition list into sublists of length n |
| Partition[list,n,d] | partition into sublists with offset d |
| Split[list] | split list into runs of identical elements |
| Split[list,test] | split into runs with adjacent elements satisfying test |
Partitioning elements in a list.
This partitions in blocks of 3.
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This partitions in blocks of 3 with offset 1.
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The offset can be larger than the block size.
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This splits into runs of identical elements.
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This splits into runs where adjacent elements are unequal.
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Partition in effect goes through a list, grouping successive elements into sublists. By default it does not include any sublists that would "overhang" the original list.
This stops before any overhang occurs.
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You can tell
Partition to include sublists that overhang the ends of the original list. By default, it fills in additional elements by treating the original list as cyclic. It can also treat it as being padded with elements that you specify.
This includes additional sublists, treating the original list as cyclic.
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Now the original list is treated as being padded with the element
.
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This pads cyclically with elements
and
.
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This introduces no padding, yielding sublists of differing lengths.
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You can think of
Partition as extracting sublists by sliding a template along and picking out elements from the original list. You can tell
Partition where to start and stop this process.
This gives all sublists that overlap the original list.
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This allows overlaps only at the beginning.
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| Partition[list,n,d] or Partition[list,n,d,{1,-1}] | keep only sublists with no overhangs |
| Partition[list,n,d,{1,1}] | allow an overhang at the end |
| Partition[list,n,d,{-1,-1}] | allow an overhang at the beginning |
| Partition[list,n,d,{-1,1}] | allow overhangs at both the beginning and end |
| Partition[list,n,d,{kL,kR}] | specify alignments of first and last sublists |
| Partition[list,n,d,spec] | pad by cyclically repeating elements in list |
| Partition[list,n,d,spec,x] | pad by repeating the element x |
| Partition[list,n,d,spec,{x1,x2,...}] |
| pad by cyclically repeating the  |
| Partition[list,n,d,spec,{}] | use no padding |
Specifying alignment and padding.
An alignment specification
tells
Partition to give the sequence of sublists in which the first element of the original list appears at position
in the first sublist, and the last element of the original list appears at position
in the last sublist.
This makes
appear at position 1 in the first sublist.
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This makes
appear at position 2 in the first sublist.
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Here
is in effect made to appear first at position 4.
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This fills in padding cyclically from the list given.
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Functions like
ListConvolve use the same alignment and padding specifications as
Partition.
In some cases it may be convenient to insert explicit padding into a list. You can do this using
PadLeft and
PadRight.
| PadLeft[list,n] | pad to length n by inserting zeros on the left |
| PadLeft[list,n,x] | pad by repeating the element x |
| PadLeft[list,n,{x1,x2,...}] | pad by cyclically repeating the  |
| PadLeft[list,n,list] | pad by cyclically repeating list |
| PadLeft[list,n,padding,m] | leave a margin of m elements on the right |
| PadRight[list,n] | pad by inserting zeros on the right |
Padding a list.
This pads the list to make it length 6.
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This cyclically inserts
as the padding.
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This also leaves a margin of 3 on the right.
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PadLeft,
PadRight, and
Partition can all be used on nested lists.
This creates a 3×3 array.
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This partitions the array into 2×2 blocks with offset 1.
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If you give a nested list as a padding specification, its elements are picked up cyclically at each level.
This cyclically fills in copies of the padding list.
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Here is a list containing only padding.
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