How to use Accumulate on a matrix column

Question

For a matrix

{{1,a},{2,b},{3,c},{4,d}},

how do I use the command Accumulate to get the matrix

{{1,a},{2,a+b},{3,a+b+c},{4, a+b+c+d}}?

One can break apart the matrix

list1={1,2,3,4} and list2 = {a,b,c,d}

and let

list3=Accumulate[list2]

and then let

list4=Transpose[{list1,list3}] 

This gives what I want.

Is there a more direct way?

Answer 1

Using SubsetMap:

amat = {{1, a}, {2, b}, {3, c}, {4, d}}
SubsetMap[Accumulate, amat, {All, 2}]

Using FoldList:

FoldList[{First@#2, Last@#1 + Last@#2} &, amat]

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Result:

{{1, a}, {2, a + b}, {3, a + b + c}, {4, a + b + c + d}}

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Using Construct and MapThread:

To apply different functions to columns, let's start with with a 3-column example matrix called bmat.

bmat = {{1, a, w}, {2, b, x}, {3, c, y}, {4, d, z}}
funcs = {Identity, Accumulate, #^2 &};
Transpose@MapThread[Construct, {funcs, Transpose@bmat}] // TableForm

{{1, a, w^2}, {2, a + b, x^2}, {3, a + b + c, y^2}, {4, a + b + c + d, z^2}}

$$\left( \begin{array}{ccc} 1 & a & w^2 \\ 2 & a+b & x^2 \\ 3 & a+b+c & y^2 \\ 4 & a+b+c+d & z^2 \\ \end{array} \right)$$

Answer 2

Another possibility:

list = {{1,a},{2,b},{3,c},{4,d}};

list[[All,2]] = Accumulate[list[[All,2]]];

Then list has the desired output:

list

{{1, a}, {2, a + b}, {3, a + b + c}, {4, a + b + c + d}}

Answer 3

Using MapAt:

amat = {{1, a}, {2, b}, {3, c}, {4, d}}
Transpose@MapAt[Accumulate, Transpose@amat, {2}]
(*{{1, a}, {2, a + b}, {3, a + b + c}, {4, a + b + c + d}}*)

Or using Table and Partition:

Table[{amat[[i, 1]], Total@(First@Partition[amat[[All, 2]], amat[[i, 1]]])}, {i, 1, Length[amat]}]
(*{{1, a}, {2, a + b}, {3, a + b + c}, {4, a + b + c + d}}*)

Answer 4

Some other methods

Query

Module[{i=0},Query[All, {2 -> (i+=#&)}]]@mat

(* {{1, a}, {2, a + b}, {3, a + b + c}, {4, a + b + c + d}} *)

The 'compiled form' of Query here uses MapAt 'under the hood'

Query[All, {2 -> (i+=#&)}]//Normal

MapAt[i += #1 & , {All, 2}]

MapAt

Module[{i=0},MapAt[i+=#&,{All,2}]]@mat

(* {{1, a}, {2, a + b}, {3, a + b + c}, {4, a + b + c + d}} *)

ApplyTo and Accumulate

Using a modification of the neat method posted below by Carl Woll (without in-place modification):

Module[{temp=#},temp[[All,2]]//=Accumulate;temp]&@mat

{{1, a}, {2, a + b}, {3, a + b + c}, {4, a + b + c + d}}

Matrix

mat={{1,a},{2,b},{3,c},{4,d}};