how to use setvartype on a table that is already existing

i am struggling with tables very badly.

I can not find a single shred of information on how to use setvartype function on an existing table

All the examples do it when reading in a table from a file.

I created a table in my code using

cell2table(cell(2,80))

and it gave me a 2X 80 table

However it wont let me put anything in it because the variales are not the correct type

Can someone tell me how to set all the variable types to one uniform type using setvartype function?

I can only find example that do it when importing a new table from a file not on an existing new file

the main problem im having is.... im trying to put data in the table

and im getting the following error

"conversion to cell from char is not possible"

The data im trying to put in the table location is of the type char

What am i doing wrong.... Can someone please lend a hand

Answer 1

[Jeremy Hughes]

This will do what you're literally asking for.

T = array2table(strings(200,0)) % 200 variables, 0 rows, all string type

T =

  200×0 empty table

But I have a feeling it would be easier to give you the best answer with a bit more context.

From the small amount of code you submitted, I tried this:

T = cell2table(cell(2,80))

T = 2×80 table
        Var1            Var2            Var3            Var4            Var5            Var6            Var7            Var8            Var9           Var10           Var11           Var12           Var13           Var14           Var15           Var16           Var17           Var18           Var19           Var20           Var21           Var22           Var23           Var24           Var25           Var26           Var27           Var28           Var29           Var30           Var31           Var32           Var33           Var34           Var35           Var36           Var37           Var38           Var39           Var40           Var41           Var42           Var43           Var44           Var45           Var46           Var47           Var48           Var49           Var50           Var51           Var52           Var53           Var54           Var55           Var56           Var57           Var58           Var59           Var60           Var61           Var62           Var63           Var64           Var65           Var66           Var67           Var68           Var69           Var70           Var71           Var72           Var73           Var74           Var75           Var76           Var77           Var78           Var79           Var80    
    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________

    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}
    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}

T(1,1) = {'SomeData'} % Need to insert a cell.

T = 2×80 table
        Var1            Var2            Var3            Var4            Var5            Var6            Var7            Var8            Var9           Var10           Var11           Var12           Var13           Var14           Var15           Var16           Var17           Var18           Var19           Var20           Var21           Var22           Var23           Var24           Var25           Var26           Var27           Var28           Var29           Var30           Var31           Var32           Var33           Var34           Var35           Var36           Var37           Var38           Var39           Var40           Var41           Var42           Var43           Var44           Var45           Var46           Var47           Var48           Var49           Var50           Var51           Var52           Var53           Var54           Var55           Var56           Var57           Var58           Var59           Var60           Var61           Var62           Var63           Var64           Var65           Var66           Var67           Var68           Var69           Var70           Var71           Var72           Var73           Var74           Var75           Var76           Var77           Var78           Var79           Var80    
    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________    ____________

    {'SomeData'}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}
    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}    {0×0 double}

Which works.

"However it wont let me put anything in it because the variales are not the correct type"

Are you trying to do this:

T(1,1) = 'SomeData'

The number of table variables in an assignment must match.

If not, can you show the example code that you tried to use?

(edited to run the code!)

[Walter Roberson]

T(1,1) = {'SomeData'} % Need to insert a cell.

T = 1×1 cell array
    {'SomeData'}

That results in variable #1 of T being a cell array of character vectors. If I understand correctly, the difficulty is that Robert would like variable #1 to instead become a character array -- to emphasize, fixed width entries

A = {'abc'; 'pqr'}

A = 2×1 cell array
    {'abc'}
    {'pqr'}

B = ['abc'; 'pqr']

B = 2×3 char array
    'abc'
    'pqr'

T = table(A,B)

T = 2×2 table
       A        B 
    _______    ___

    {'abc'}    abc
    {'pqr'}    pqr

When I tested, I did not seem to find a way to replace a table variable that was a cell with a table variable that is a char array... not without deleting the original variable and adding a new one. Attempting

%T{:,1} = ['stu'; 'xyz'];

gives an error about the new values needing to be width 1. If you try

%T{:,1} = string(['stu';'xyz'])

then you get an error because value of type 'string' is not convertible to 'cell'. And trying to replace the entire variable with a new variable gives

T(:,1) = table(['stu';'xyz'], 'VariableNames', {'C'})

Conversion to cell from char is not possible.

[Jeremy Hughes]

I didn't see anywhere that char arrays were mentioned. /shrug

There are a couple of ways to add/replace a variable. The easy way (assuming your data is the correct hight) is just to use the table.VarName syntax.

T = array2table(rand(2))

T = 2×2 table
     Var1        Var2   
    _______    _________

    0.33128      0.91374
    0.71777    0.0005792

T.C = ['stu';'xyz'] % ought to work if the table is height=2

T = 2×3 table
     Var1        Var2        C 
    _______    _________    ___

    0.33128      0.91374    stu
    0.71777    0.0005792    xyz

You can even overwrite that with another datatype.

T.C = [1 2 3; 7 8 9] % as long as the RHS is the same height as the table.

T = 2×3 table
     Var1        Var2            C     
    _______    _________    ___________

    0.33128      0.91374    1    2    3
    0.71777    0.0005792    7    8    9

If it's not the right height, then it will fill the rest with the default value if you use the proper indexing (whether or not the variable was already there.)

T = array2table(rand(5,2));
T.C(1:2,1:3) = ['stu';'xyz'] 

T = 5×3 table
     Var1       Var2       C 
    _______    _______    ___

    0.46489     0.9575    stu
     0.7387     0.5462    xyz
    0.64852     0.1423       
    0.52138    0.39645       
     0.4863    0.53726       

The works the same as array assignment.

A = char(randi(uint8('az'),1,3))

A = 'hla'

A(4:5,1:3) = ['stu';'xyz']

A = 5×3 char array
    'hla'
    '   '
    '   '
    'stu'
    'xyz'

Notice the filled values are char(0)

double(A)

ans = 5×3

104 108 97 0 0 0 0 0 0 115 116 117 120 121 122

[Walter Roberson]

With regards to char array: note the question asked

"conversion to cell from char is not possible"

The data im trying to put in the table location is of the type char

[Walter Roberson]

It is interesting and important for this purpose that you can change the data type of a variable to char when you use dot syntax, but not with () or {} indexing. I don't think it is obvious that there should be difference in functionality for this purpose; I was under the impression that dot indexing was syntactic sugar for {} indexing.

[Jeremy Hughes]

For most things, () returns a subset of the data of the same array type, while {} returns the contents of container types like cell or table. On assignment, () preserves the orginal type, which is why you need a table (or something that can be converted into a table) on the RHS of T(...) = rhs.

For cell arrays, C{k} = rhs, you're assigning into a cell, so the type is always an overwrite, but for table, T{:,k} = rhs is the same as T.(k)(:) = rhs. The original type is preserved. Then T.(k) = rhs (or T.VarName) is assiment just like C = newdata, but into the table variable instead of the workspace.

[Peter Perkins]

Walter, dot is not exactly syntactic sugar for braces (and I am always puzzled by the number of people who use braces for accessing one variable -- I find dot far more readable). Consider:

>> t = table([1;2;3],["AAA";"BB";"C"])
t =
  3×2 table
    Var1    Var2 
    ____    _____
     1      "AAA"
     2      "BB" 
     3      "C"  

Surely you would not want this assignment

>> t{1:2,1} = single([11;12])
t =
  3×2 table
    Var1    Var2 
    ____    _____
     11     "AAA"
     12     "BB" 
      3     "C"  

to turn Var1 into a single, and it does not -- it converts the RHS to the LHS, just as it would if these were arrays directly in the workspace. So neither should

>> t{:,1} = single([21;22;23])
t =
  3×2 table
    Var1    Var2 
    ____    _____
     21     "AAA"
     22     "BB" 
     23     "C"  

convert Var1, despite the :. This is where dot comes in: you can dot assign into the variable using parens

>> t.Var1(:) = single([21;22;23])
t =
  3×2 table
    Var1    Var2 
    ____    _____
     21     "AAA"
     22     "BB" 
     23     "C"  

but if you leave off any paren subscripting

>> t.Var1 = single([21;22;23])
t =
  3×2 table
    Var1    Var2 
    ____    _____
     21     "AAA"
     22     "BB" 
     23     "C"

then you are overwriting the whole variable. Pretty sure this is in the doc but maybe it's easy to miss. I'll make a note about that.

[Walter Roberson]

Thanks, @Peter Perkins

Answer 2

[Simon Chan]

Please refer to the following example without using setvartype.

(1) When you create the table, you can create the preset variable names and variable types like the following, noticed that type char may not be supported but you may use type cell. On the other hand, you can assign the size of the table and my example shows a table with size 50 rows and 3 column.

(2) Suppose your data is stored in variable and the following code put the first 15 rows of data from variable into table T.

T.Properties.VariableNames = {'Name','Class','Value'};
T = table('Size',[50 3],'VariableTypes',{'cell','cell','double'})
T(1:15,:) = variable(1:15,:);

[Robert]

This doesn’t help me I am trying to create blank table that has hundreds of columns and what them all to be the same type

How can I do that

[Robert]

and by the way, your code does not run, it errors out

Why post code on here that doesnt work

[Walter Roberson]

@Robert

Reminder: In the great majority of cases, people who respond on MATLAB Answers are doing so as volunteers. That includes the great majority of cases where Mathworks Staff respond: nearly the only people who respond here as part of their job are the people who run MATLAB Central itself, such as staff responding about bugs in the MATLAB Answers facility.

And volunteers contribute as much or as little as they feel like, and do whatever testing (or non-testing) they feel like.

You have been around for years, Robert, and by now the regular volunteers have largely formed the opinion that you are competent enough that you only need guideance on approaches and can work from there, rather than you needing fully-tested code due to you being inexperienced. Perhaps the volunteers are mistaken about that point and have greatly over-estimated your skills. If so, then it would help the volunteers if you were mention if you are not an experienced worker who is able to make projections from incomplete information; doing that would help alleviate perceptions that are perhaps being formed that you might be demanding more than is reasonable from volunteers.

Answer 3

[Robert]

Does anyone know how to do this? I am at a complete standstill

All i want to do is create a table of one variable type! Why is everything so hard in matlab. Its rediculous

create a table of x,y size of a single variable type without having to address every single type individualy, im not going to type the word 'string' 200 times for a table that is 200 wide with all strings.

This is utterly rediculous. This is why kids today are learning python not matlab

Answer 4

[Walter Roberson]

Can someone tell me how to set all the variable types to one uniform type using setvartype function?

That is not possible. setvartype() applies only to options structures such as are created by detectImportOptions() and related direct constructors.

The data im trying to put in the table location is of the type char

cell2table(cell(2,80))

Those would be empty cells. If you were to suceed in converting them to char, then the result would be a table full of 2 x 0 char arrays, which you would not be able to store into.

All i want to do is create a table of one variable type!

Are you changing your question? Your original question is about converting existing table variables to datatype char. MATLAB discourages using char variables in tables, recommending that you instead use cell array of character vector or string() objects -- when you use char variables then the fields are fixed width (just like char arrays). MATLAB deliberately makes it difficult to convert variables to char array.

If you want to create a table of all char variables, that is a quite different matter than converting existing variables.

im not going to type the word 'string' 200 times for a table that is 200 wide with all strings.

Please clarify whether you are needing the entires to be char (fixed-width fields) or string (variable width, additional methods).

I see several possibilities here:

1. You want to create a 2 x 200 table in which every variable is character array of width 0
2. You want to create a 2 x 200 table in which every variable is a character array of a particular non-zero width (e.g., 5 characters wide for all of them)
3. You want to create a 2 x 200 table in which every variable is a string
4. You might want to convert existing table variables into char array -- this one is difficult
5. You might want to convert existing table variables into string objects

I have code solutions for all of these (but #4 is substantially more complicated, and I would rather not post that unless you have a specific need to use char arrays as table objects).

Normally I would have gone ahead and posted some solutions, but considering your remarks to Simon about not posting code that does not work, I need to know exactly what your needs are so that I can post code that will work for your needs.

[Robert]

Lets take the attitude down a notch and come down off your high horse ok

Your justification for posting none working code while answering a question makes no logical sense nore does your attack on me. So cool it, i dont need your personal assessment and that is not the intended purpose of this forum. If you dont like my questions simply dont answer them.

If mathworks did not deisgn this rediculous data type and then have terrible documentation on it these questions would not exist.

[Walter Roberson]

https://www.mathworks.com/matlabcentral/answers/885269-how-to-use-setvartype-on-a-table-that-is-already-existing#comment_1655764

You asked why post code that doesn't work. In order to be sure that any code I might post would work for your situation, I need clarification from you as to what your situation is. Why would I post code that doesn't work for your purpose?

Note by the way that I am the only person who answered the question you originally asked.

[Robert]

@Walter Roberson and i appreciate your effort and skills however your delivery could use some work.

Your right i have been around a while and im a big fan of matlab. However, in my line of work i am subjected to using it extensivley for 6 months then not touching it for 4 years and then coming back. I am jealous of your everyday exposure to my favorite program.

Answer 5

[Peter Perkins]

setvartype is an import feature. let's come back to that.

cell creates a cell array containing nothing but 0x0 doubles:

>> c = cell(3,2)
c =
  3×2 cell array
    {0×0 double}    {0×0 double}
    {0×0 double}    {0×0 double}
    {0×0 double}    {0×0 double}

cell2table tries to create a table the same size as c, by vertcat'ing the contents of each cell in each column of the cell array. But those contents are all 0x0, so it can't do that. So it falls back to leaving every column of c as a cell array in t (this is all in the doc):

>> t = cell2table(c)
t =
  3×2 table
         c1              c2     
    ____________    ____________
    {0×0 double}    {0×0 double}
    {0×0 double}    {0×0 double}
    {0×0 double}    {0×0 double}

And that's probably not what you want, because you won't be able to do anything useful, unless all you have is text data. Even if all you have is text data, you are not doing yourself a favor by using cell arrays instead of string arrays, but let's come back to that. The reason why you can't assign into those variables in the table is (I guess? I have not read through the entire thread) because they are cell arrays and you are (I guess) assigning as if it were something else. If you really do have only text data, and you are tyrig to assign a char row into the table, you need to remember that you are assigning into cell vars in the table, and the RHS should be a cell:

>> t(1,1) = {'abc'}
t =
  3×2 table
         c1              c2     
    ____________    ____________
    {'abc'     }    {0×0 double}
    {0×0 double}    {0×0 double}
    {0×0 double}    {0×0 double}

This is not all that convenient. So use dot to assign to one element of one var in the table. The c1 var is a cell array, how would you assign into it if it were directly in the workspace? You say c1{2} = 'def'. Do that, in the table:

>> t.c1{2} = 'def'
t =
  3×2 table
         c1              c2     
    ____________    ____________
    {'abc'     }    {0×0 double}
    {'def'     }    {0×0 double}
    {0×0 double}    {0×0 double}

Still, it's easier by far to use strings, because no braces:

>> s = strings(3,2)
s = 
  3×2 string array
    ""    ""
    ""    ""
    ""    ""
>> t = array2table(s)
t =
  3×2 table
    s1    s2
    __    __
    ""    ""
    ""    ""
    ""    ""
>> t.s1(2) = "def"
t =
  3×2 table
     s1      s2
    _____    __
    ""       ""
    "def"    ""
    ""       ""

OK, back to the original question. You've asked about setvartype, which made me think you were really asking about import. But I think that's a red herring. To create a table all of whose variables are one type, use the syntax specifically for that:

>> t = table('Size',[3 2],'VariableTypes',repmat("double",1,2))
t =
  3×2 table
    Var1    Var2
    ____    ____
     0       0  
     0       0  
     0       0  

Maybe in your case, you want t = table('Size',[3 2],'VariableTypes',repmat("string",m,n)). Perhaps using repmat with VariableTypes ought to be a doc example, I'll make a note of that.

The other question, about changing the type of a variable in a table: any assignment syntax that involves parens or brace subscripting on the table is by definition assigning INTO the variables in the table. Only a dot assignment, with no subscripts, can change the type. It's equivalent to deleting and replacing. The other alternative is the convertvars function. But I'm not even sure what type you want to change from or to. If you are tyring to make a raw char variable in a table, don't do that, it is far more awkward that either cell arrays or (the best choice) string arrays.

[Walter Roberson]

convertvars() has a specific built-in limitation that prevents@char from being used as the datatype, substituting in @cellstr instead. One warning is given for each variable for which @cellstr is substituted for @char .

table() also has a specific built-in limitation that prevents 'char' from being used as the VariableTypes. One warning is given for each variable for which cell is substituted for char.

(I do wonder if it is necessary to issue one warning for each variable, as opposed to one warning for it happening at all.)

[Peter Perkins]

"If you are trying to make a raw char variable in a table, don't do that."