Writematrix loses precision with string present in data

I am saving a matrix to file and need full precision in the data. I am using the writematrix function, and my problem can be shown in this minimal example:

writematrix([1;0.123456789],"./testA.txt")
writematrix(["Test";0.123456789],"./testB.txt")

Then testA.txt:

1
0.123456789

And testB.txt

Test
0.12346

Is there a way to have strings (such as a header row) in my matrix but retain the full precision in the data?

Answer 1

[Walter Roberson]

writematrix(["Test";compose("%.999g", 0.123456789)],"./testB.txt")

dbtype testB.txt

1     Test
2     0.12345678899999999733605449137030518613755702972412109375

Retains full precision.

Answer 2

[dpb]

["Test";0.123456789]

ans = 2x1 string array
    "Test"
    "0.12346"

The problem, if there is one, is in combining the string and the numeric into one array; MATLAB can have only one data type in an array other than a cell array and so the numeric value was coerced to a string before it was written; ergo, it isn't writematrix that is the issue here, it's trying put a square peg in a round hole; mixing data types inappropriately.

Probably the easiest workaround with the minimal example would to use a table instead...

tT=table(0.123456789,'VariableNames',"Test")

tT = table
     Test  
    _______

    0.12346

writetable(tT,"testB.txt",'WriteVariableNames',1)
type testB.txt

Test
0.123456789