WolframLang: Get Parts of List

Get Parts by Index

the Part function is the most versatile and most frequently used function to get part of a expression.

Part[expr, i]

(Short syntax: expr[[i]])

ith part of expr. If i is negative, count from right. Part

x = {a, b, c, {d1, d2}, e};
Part[x, 2] === b

(* short syntax *)
x[[2]] === b

(* count from right *)
x[[-2]] === {d1, d2}

Part[expr, {a, b, c}]

(Short syntax: expr[[{a, b, c}]])

Same as {Part[expr, a], Part[expr, b], Part[expr, c]}

x = {a, b, c, {d1, d2}, e};
x[[{2,-1}]] === {b, e}

Part[expr, a, b, c]

(Short syntax: expr[[a, b, c]])

same as Part[Part[Part[expr, a], b], c]

x = {a, b, c, {d1, d2}, e};
x[[4,1]] === d1

Part[expr, m;;n]

(Short syntax: expr[[m;;n]])

(note: a;;b is short for Span[a,b])

parts m through n. If m is omitted, default to beginning. If n is omitted, default to end.

x = {a, b, c, {d1, d2}, e};
x[[2;;4]] === {b, c, {d1, d2}}

• expr[[m;;]] → part m to end
• expr[[;;n]] → beginning to n
• expr[[;;,j]] → column j
• expr[[m1;;n1,m2;;n2]] → submatrix

(* column 1 *)
{{a,b}, {c,d}}[[;;,1]] === {a, c}

Get Parts by Filter Function

Select

Get elements by a filter function Select

Select[ {3,4,5,6}, EvenQ ] === {4, 6}

Select[ Range[20], PrimeQ ] === {2, 3, 5, 7, 11, 13, 17, 19}

Select[
 {{3}, {0, 23}, {9}, {4,5,6}},
 Function[ Length[#] === 2 ]
] === {{0, 23}}

Get Parts by Pattern

Cases

Get elements by Pattern Cases

Cases[ {a, f[3], f[9]}, f[_] ] === {f[3], f[9]}

Cases[ {a, f[3], g[4], f[5]}, _[_] ] === {f[3], g[4], f[5]}

Cases[ {{2}, {4, 8}, {c}}, {_ , _} ] === {{4, 8}}

Other convenient functions by index

First

get first element. First

Last

get last element. Last

Most

get all elements except last. Most

Rest

get all elements except first. Rest

Take

take nth to mth item Take

Drop

Return a list with nth to mth dropped Drop