Combining cell array and double vector into sprintf

I would like to create a string for a plot annotation. My needs: Create annotation of the following format:

a1 = number

b1 = number

c1 = number

a2 = number

etc.

Where a1,b1, etc. come from a cell array and the numbers come from a vector. I would do it all manually but I am creating a large number of graphs and the number of coefficients changes between each plot.

What i would like to do is something like the following:

    coeffnote = []
        for l = 1:length(coeffVals)
           coeffnote = [coeffnote, coeffNames(l) ' = ' num2str(coeffVals(l)) '\n'];
           coeffnote = string(coeffnote);
        end
        sprintf(coeffnote(:))

When I run this, I keep getting an error. I am at my wits' end with this. Can anyone help.

Answer 1

[Stephen23]

You don't need a loop at all, it is simpler without one:

str = {'aa','bb','cc'};
vec = 1:3;
tmp = [str;num2cell(vec)];
out = sprintf('%s = %d\n',tmp{:});

[David Mueller]

First, thanks for answering. I think this is close, but I am getting weird results:

str =

      'a1 = 98
       1 = 99
       1 = 97
       2 = 98
       2 = 99
       2 = 97
       3 = 98
       3 = 99
       3 = 97
       4 = 98
       4 = 99
       4 = 1.101091e+00
       1.883974e+00 = -6.098070e-01
       7.002899e-01 = 2.033078e+00
       2.452218e+00 = 3.222503e-03
       5.724037e+00 = -1.616887e+00
       6.268332e-01 = 1.437858e+00
       2.806340e+00 = '

This is happening both with my variables and simply copying/pasting your exact code.

[Stephen23]

@David Mueller: the code I gave you assumes that both the cell array and numeric vector are row vectors. If they are not then you need to convert them to row, e.g.:

tmp = [str(:).';num2cell(vec(:).')];

The aim is to get a cell matrix shaped like this:

tmp = {'aa','bb','cc',...;
          1,   2,   3,...};

This can then be used in sprintf in the way that I showed in my answer. See also:

https://www.mathworks.com/help/matlab/matlab_prog/comma-separated-lists.html

Note that you might want to change the sprintf format string for the numeric: are the values integer, or decimal fractions? How do you want them formatted?

[David Mueller]

Oh okay, I was transposing the wrong one I guess. Thanks! Also - and this is a small matter - is there a simple way to remove the return on the final line?

[Stephen23]

"is there a simple way to remove the return on the final line?"

In my experience the simplest way is to put such character/s at the start and remove the first one/s:

out = sprintf('\n%s = %d',tmp{:});
out = out(2:end);

You could do something similar with the last newline, but I find this way easier to follow.

Answer 2

[Aditya Adhikary]

Another answer based on your code (not caring about efficiency):

 coeffVals = [1,2,3];
 coeffNames = {'a1','b1','c1'};
 coeffnote = [];
 for i = 1:length(coeffVals)
     val = coeffVals(i);
     name = coeffNames(i);
     x = [name,' = ',num2str(val),'\n'];
     x = strjoin(x);
     coeffnote = [coeffnote, x];
 end
 sprintf(coeffnote);

[David Mueller]

Perfect! Thanks! Just out of curiosity, what would a more efficient way be? I feel like Stephen Cobeldick's answer was very close, but giving a weird output.

[Jan]

Letting the output grow iteratively is a bad programming practice in general, because the costs grow exponentially.

Answer 3

[KSSV]

x = rand(10,1) ;
y = rand(10,1) ;

plot(x,y,'.r') ;

hold on

str = cell(10,1) ;
for i = 1:10
    str{i} = sprintf(['c',num2str(i),'=',num2str(i)]) ;
end

text(x,y,str)

[Stephen23]

@KSSV: what is the point of using both num2str and sprintf ? Your use of sprintf here does absolutely nothing. This does the same thing:

['c',num2str(i),'=',num2str(i)]

Note that it would be more efficient to use sprintf rather than num2str:

sprintf('c%d = %d',i,i)

Answer 4

[Guillaume]

I'm surprised nobody's mentioned compose which is the modern, more powerful, easier to work with, version of sprintf. Going back to Stephen's example:

str = {'aa','bb','cc'};
vec = 1:3;

compose('%s = %d', string(str(:)), vec(:))

Note that the (:) are to ensure the inputs are column vectors. If they already are, you don't need to bother.