Manipulating Lists

Constructing Lists
Lists are widely used in the Wolfram Language, and there are many ways to construct them.
the list {1,2,3,,n}
the values of expr with i from 1 to n
the list {f[1],f[2],,f[n]}
{x,f[x],f[f[x]],} with up to n nestings
a length n list with element ik being vk
the list {e1,e2,}
Some explicit ways to construct lists.
This gives a table of the first five powers of 2:
Here is another way to get the same result:
This gives a similar list:
SparseArray lets you specify values at particular positions:
You can also use patterns to specify values:
Often you will know in advance how long a list is supposed to be, and how each of its elements should be generated. And often you may get one list from another.
the values of expr with i taking on values from list
apply f to each element of list
give f[elem,{i}] for the i th element
give elements of list that match form
select elements for which test[elem] is True
pick out elements of list for which the corresponding elements of sel match form
give elements ei from the beginning of list as long as test[ei] is True
give a list of the specified parts of list
Constructing lists from other lists.
This selects elements less than 5:
This takes elements up to the first element that is not less than 5:
This explicitly gives numbered parts:
This picks out elements indicated by a 1 in the second list:
Sometimes you may want to accumulate a list of results during the execution of a program. You can do this using Sow and Reap.
sow the value val for the nearest enclosing Reap
evaluate expr, returning also a list of values sown by Sow
Using Sow and Reap.
This program iteratively squares a number:
This does the same computation, but accumulating a list of intermediate results above 1000:
An alternative but less efficient approach involves introducing a temporary variable, then starting with t={}, and successively using AppendTo[t,elem].
Manipulating Lists by Their Indices
part or parts of a list
part or parts of a nested list
the n th part from the beginning
the n th part from the end
a list of parts
parts m through n
all parts
Getting parts of lists.
This gives a list of parts 1 and 3:
Here is a nested list:
This gives a list of its first and third parts:
This gives a list of the first part of each of these:
And this gives a list of the first two parts:
This gives the first two parts of m:
This gives the last part of each of these:
This gives the second part of all sublists:
This gives the last two parts of all sublists:
You can always reset one or more pieces of a list by doing an assignment like m[[]]=value.
This resets part 1,2 of m:
This is now the form of m:
This resets part 1 to x and part 3 to y:
This resets parts 1 and 3 both to p:
This restores the original form of m:
This now resets all parts specified by m[[{1,3},{1,2}]]:
You can use ;; to indicate all indices in a given range:
It is sometimes useful to think of a nested list as being laid out in space, with each element at a coordinate position given by its indices. There is then a direct geometrical interpretation for list[[spec1,spec2,]]. If a given speck is a single integer, then it represents extracting a single slice in the k th dimension, while if it is a list, it represents extracting a list of parallel slices. The final result for list[[spec1,spec2,]] is then the collection of elements obtained by slicing in each successive dimension.
Here is a nested list laid out as a twodimensional array:
This picks out rows 1 and 3, then columns 1 and 2:
Part is set up to make it easy to pick out structured slices of nested lists. Sometimes, however, you may want to pick out arbitrary collections of individual parts. You can do this conveniently with Extract.
the list {list[[i1]],list[[i2]],}
the element list[[i1,i2,]]
parts specified by successive slicing
the list of individual parts {list[[i1,i2,]],list[[j1,j2,]],}
Getting slices versus lists of individual parts.
This extracts the individual parts 1,3 and 1,2:
An important feature of Extract is that it takes lists of part positions in the same form as they are returned by functions like Position.
This sets up a nested list:
This gives a list of positions in m:
This extracts the elements at those positions:
take the specified parts of a list
drop the specified parts of a list
take or drop specified parts at each level in nested lists
the first n elements
the last n elements
element n only
elements m through n (inclusive)
elements m through n in steps of s
all parts
no parts
Taking and dropping sequences of elements in lists.
This takes every second element starting at position 2:
This drops every second element:
Much like Part, Take and Drop can be viewed as picking out sequences of slices at successive levels in a nested list, you can use Take and Drop to work with blocks of elements in arrays.
Here is a 3×3 array:
Here is the first 2×2 subarray:
This takes all elements in the first two columns:
This leaves no elements from the first two columns:
add element at the beginning of list
add element at the end of list
insert element at position i
insert at position {i,j,}
delete the element at position i
delete at position {i,j,}
Adding and deleting elements in lists.
This makes the 2,1 element of the list be x:
This deletes the element again:
replace the element at position i in list with new
replace list[[i,j,]] with new
replaces parts at positions in by newn
replace parts at positions {in,jn,} by newn
replace all parts list[[ik,jk,]] with new
Replacing parts of lists.
This replaces the third element in the list with x:
This replaces the first and fourth parts of the list. Notice the need for double lists in specifying multiple parts to replace:
Here is a 3×3 identity matrix:
This replaces the 2,2 component of the matrix by x:
It is important to understand that ReplacePart always creates a new list. It does not modify a list that has already been assigned to a symbol the way m[[]]=val does.
This assigns a list of values to alist:
This gives a copy of the list in which the third element has been replaced with x:
The value of alist has not changed:
Nested Lists
list of lists
m×n× table of values of expr
m×n× array of values f[i,j,]
m×n× array with element {is,js,} being vs
generalized outer product with elements combined using f
all possible m×n× arrays of elements from list
Ways to construct nested lists.
This generates a table corresponding to a 2×3 nested list:
This generates an array corresponding to the same nested list:
Elements not explicitly specified in the sparse array are taken to be 0:
Each element in the final list contains one element from each input list:
Functions like Array, SparseArray, and Outer always generate full arrays, in which all sublists at a particular level are the same length.
the dimensions of a full array
test whether all sublists at a given level are the same length
the depth to which all sublists are the same length
Functions for full arrays.
The Wolfram Language can handle arbitrary nested lists. There is no need for the lists to form a full array. You can easily generate ragged arrays using Table.
This generates a triangular array:
flatten out all levels of list
flatten out the top n levels
create a flattened array from an array of arrays
Flattening out sublists and subarrays.
This generates a 2×3 array:
Flatten in effect puts elements in lexicographic order of their indices:
This creates a matrix from a block matrix:
transpose the top two levels of list
put the k th level in list at level nk
Transposing levels in nested lists.
This generates a 2×2×2 array:
This permutes levels so that level 3 appears at level 1:
This restores the original array:
map f across elements at level n
apply f to the elements at level n
map f onto parts at level n and their indices
Applying functions in nested lists.
Here is a nested list:
This maps a function f at level 2:
This applies the function at level 2:
This applies f to both parts and their indices:
partition into n1×n1× blocks
pad on the left to make an n1×n1× array
pad on the right to make an n1×n1× array
rotate nk places to the left at level k
rotate nk places to the right at level k
Operations on nested lists.
Here is a nested list:
This rotates different amounts at each level:
This pads with zeros to make a 2×3×3 array:
Partitioning and Padding Lists
partition list into sublists of length n
partition into sublists with offset d
split list into runs of identical elements
split into runs with adjacent elements satisfying test
Partitioning elements in a list.
This partitions in blocks of 3:
This partitions in blocks of 3 with offset 1:
The offset can be larger than the block size:
This splits into runs of identical elements:
This splits into runs where adjacent elements are unequal:
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:
The same is true here:
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:
Now the original list is treated as being padded with the element x:
This pads cyclically with elements x and y:
This introduces no padding, yielding sublists of differing lengths:
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:
This allows overlaps only at the beginning:
keep only sublists with no overhangs
allow an overhang at the end
allow an overhang at the beginning
allow overhangs at both the beginning and end
specify alignments of first and last sublists
pad by cyclically repeating elements in list
pad by repeating the element x
pad by cyclically repeating the xi
use no padding
Specifying alignment and padding.
An alignment specification {kL,kR} 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 a appear at position 1 in the first sublist:
This makes a appear at position 2 in the first sublist:
Here a is in effect made to appear first at position 4:
This fills in padding cyclically from the list given:
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.
pad to length n by inserting zeros on the left
pad by repeating the element x
pad by cyclically repeating the xi
pad by cyclically repeating list
leave a margin of m elements on the right
pad by inserting zeros on the right
Padding a list.
This pads the list to make it length 6:
This cyclically inserts {x,y} as the padding:
This also leaves a margin of 3 on the right:
PadLeft, PadRight, and Partition can all be used on nested lists.
This creates a 3×3 array:
This partitions the array into 2×2 blocks with offset 1:
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:
Here is a list containing only padding:
Sparse Arrays: Manipulating Lists
Lists are normally specified in the Wolfram Language just by giving explicit lists of their elements. But particularly in working with large arrays, it is often useful instead to be able to say what the values of elements are only at certain positions, with all other elements taken to have a default value, usually zero. You can do this in the Wolfram System using SparseArray objects.
ordinary lists
sparse arrays
Ordinary lists and sparse arrays.
This specifies a sparse array:
Here it is as an ordinary list:
This specifies a two-dimensional sparse array:
Here it is an ordinary list of lists:
sparse array version of list
sparse array with values vali at positions posi
the same sparse array
banded sparse array with values val
d1×d2× sparse array
sparse array with default value val
ordinary list version of array
position-value rules for array
Creating and converting sparse arrays.
This generates a sparse array version of a list:
This converts back to an ordinary list:
This makes a length 7 sparse array with default value x:
Here is the corresponding ordinary list:
This shows the rules used in the sparse array:
This creates a banded matrix:
An important feature of SparseArray is that the positions you specify can be patterns.
This specifies a 4×4 sparse array with 1 at every position matching {i_,i_}:
The result is a 4×4 identity matrix:
Here is an identity matrix with an extra element:
This makes the whole third column be a:
You can think of SparseArray[rules] as taking all possible position specifications, then applying rules to determine values in each case. As usual, rules given earlier in the list will be tried first.
This generates a random diagonal matrix:
You can have rules where values depend on indices:
This fills in even-numbered positions with p:
You can use patterns involving alternatives:
You can also give conditions on patterns:
This makes a band-diagonal matrix:
Here is another way:
For many purposes, the Wolfram System treats SparseArray objects just like the ordinary lists to which they correspond. Thus, for example, if you ask for parts of a sparse array object, the Wolfram System will operate as if you had asked for parts in the corresponding ordinary list.
This generates a sparse array object:
Here is the corresponding ordinary list:
Parts of the sparse array are just like parts of the corresponding ordinary list:
This part has the default value 0:
Many operations treat SparseArray objects just like ordinary lists. When possible, they give sparse arrays as results.
This gives a sparse array:
Here is the corresponding ordinary list:
Dot works directly with sparse array objects:
You can mix sparse arrays and ordinary lists:
The Wolfram System represents sparse arrays as expressions with head SparseArray. Whenever a sparse array is evaluated, it is automatically converted to an optimized standard form with structure SparseArray[Automatic,dims,val,].
This structure is, however, rarely evident, since even operations like Length are set up to give results for the corresponding ordinary list, not for the raw SparseArray expression structure.
This generates a sparse array:
Here is the underlying optimized expression structure:
Length gives the length of the corresponding ordinary list:
Map also operates on individual values: