merge some elements into one in a list

Question

I hava a list just like

list = {{a1},
        {a2},   
        {a31,a32,a33}, 
        {a41,a42}}

I want to save it to excel, but it will in different column like

Now I want they are all in the same column, maybe separate by comma or other mark symbol.

Answer 1

We can use the Row command to convert a list into comma separate values. However, we must convert the row into a string before we export. If we do this

Export["tmp1.xls",
  ToString[Row[#,","]]&  /@  list
]

then we get an Excel file with one column that contains only text data. Even if we set a1=3.14 in MMA, it will be text in Excel.

If our row contains only one value we do not need the Row command or the ToString command. So, we could do this

Export["tmp2.xls",
  If[Length[#] > 1,
    ToString[Row[#, ","]],
    First[#]] & /@ list
 ]

The second version will export rows that contain single numbers as numbers.

So far we have only considered a list of lists. What if we want to export this: list3 = { 3.14, {1, 4, 9} }; ? We would want the number 3.14 to appear in Excel, along the text "1, 4, 9". Here is how we could do it.

list3 = {3.14, {1, 4, 9}};
Export["tmp3.xls",
 Which[
   Head[#] === List,
    If[Length[#] > 1,
     ToString[Row[#, ","]],
     First[#]],
   True, #] & /@ list3
 ]

This code checks to see if the element is a list. If it is a list, and its length is greater than 1, we need to convert it to a row. Otherwise, we do not change the element.