Why does num2str not work for double digits

Question

Tomtom 2016-3-24

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编辑： Stephen23 2016-3-24

采纳的回答： Star Strider

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Hey everybody,

why does

num = [0 1 2 3 5];
for(i=1:5)
  X(i,:) = ['Hello Nr.',num2str(num(i))];
end
disp(X(:,:))

work fine but when I change num to have numbers with double digits like

num = [0 1 12 3 5];

it doestn't work anymore?

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Answer 1

Star Strider 2016-3-24

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编辑：Star Strider 2016-3-24

在 MATLAB Online 中打开

You can use a cell array, but probably the easiest solution is to use the sprintf function:

num = [0 1 12 3 5];
for(i=1:5)
  X(i,:) = sprintf('Hello Nr.%3.0f',num(i));
end
disp(X(:,:))
Hello Nr.  0
Hello Nr.  1
Hello Nr. 12
Hello Nr.  3
Hello Nr.  5

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Star Strider 2016-3-24

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You need to add a few esoteric tweaks (such as a regexp call) to make that work:

Angle_Vector = [0 pi 19 exp(1) 20000];
legend_text = regexp(sprintf('Angle %6.3f\n',Angle_Vector), '\n', 'split');
figure(1)
plot(1:10, randi(9, 5, 10))
legend(legend_text(1:end-1))

Stephen23 2016-3-24

编辑：Stephen23 2016-3-24

"a good site for information on the formatting operator."

Why not start by reading the very helpful written MATLAB documentation:

http://www.mathworks.com/help/matlab/ref/sprintf.html

Everything about the format string is explained there, with examples

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Answer 2

Guillaume 2016-3-24

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编辑：Guillaume 2016-3-24

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Your original problem is that you're trying to stuff strings of different lengths into a matrix.

On the first round of the loop, X does not exist. So the line,

X(i, :) = ...

instructs matlab to create a X matrix with 1 row (i = 1) and as many columns as necessary to fit the right hand side, that is 10 columns when num(i) is a single digit.

On subsequent rounds of the loop, X already exists so the line

X(i, :) = ...

tells matlab to fill row i and column 1:10 with the right hand side. However, if num(i) is two digits, then the right hand side ends up being 11 characters. At this point matlab tells you it can't put 11 characters in 10 columns.

Using sprintf with a field with forces all the strings to be the same length (as long as no number has more digits than the field width.

In general it's not a good idea to use a matrix to store multiple strings unless they're more or less the same lengths. All rows of a matrix have to have the same length, so shorter strings must be padded with spaces. Typically you store multiple strings in cell arrays, where each cell can be a different length. In your case:

num = [0, 10, pi, Inf];
X = cell(1, numel(num));
for i = 1 : numel(num)
   X{i} = ['Hello Nr.',num2str(num(i))]; %I personally prefer sprintf to num2str
end
celldisp(X)

Note that it's then trivial to transform the cell array into a matrix of character with char. Matlab automatically pads the shorter strings:

char(X)

but to be honest there's no advantage to storing strings in matrices.

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Tomtom 2016-3-24

Thank you for the answer. It seems sprintf is the better option. But to get it working I need these % formatting operator. For me the most confusing think in Matlab are these formatting operator.

Guillaume 2016-3-24

Note that matlab inherits these % formatting operator from C. It's well worth learning them. Granted they may look complex at first but they're also incredibly powerful.

You should be able to find plenty of tutorials on the web about sprintf (in matlab, or C, the format string is more or less the same)

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