When execute the below code ? the error message appear 'Index exceeds the number of array elements'?

Question

Hussain 2022-9-12

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1803435-when-execute-the-below-code-the-error-message-appear-index-exceeds-the-number-of-array-elements

评论： Walter Roberson 2022-9-21

M_h="FFFFFFFFFFFFFFFF";
M="";
for i=1:16
    M(end+1:end+4)=dec2bin(hex2dec(M_h(i)), 4);
end
Warning: Hexadecimal numbers representing integers greater than or equal to flintmax might not be represented exactly as double-precision floating-point values.
Index exceeds the number of array elements. Index must not exceed 1.

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Answer 1

Jérôme 2022-9-12

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在 MATLAB Online 中打开

M_h and M are string arrays because you used "". Here, you want to work with char arrays using '' (See for example these links for details about them: https://www.mathworks.com/help/matlab/characters-and-strings.html, https://www.mathworks.com/help/matlab/ref/char.html, https://www.mathworks.com/help/matlab/ref/string.html).

If you use the following code, you will not have the error anymore:

M_h='FFFFFFFFFFFFFFFF';
M='';
for i=1:16
    M(end+1:end+4)=dec2bin(hex2dec(M_h(i)), 4);
end

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Jérôme 2022-9-19

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In Text in String and Character Arrays, you can see that MATLAB shows how to access individual characters of a char array, but not for a string. Because strings are made to work at a higher level, not to work characters per characters, which is the perfect job for char arrays.

In your case, you can simply convert the string to a char array before your operation, and convert it back to a string after that:

M_h="FFFFFFFFFFFFFFFF";
M_h2 = convertStringsToChars(M_h);
M2='';
for i=1:16
    M2(end+1:end+4)=dec2bin(hex2dec(M_h2(i)), 4);
end
M = convertCharsToStrings(M2);

Otherwise, if you really want to manipulate string directly, I imagine it's possible using the function extractBetween.

Hussain 2022-9-21

Thanks for tour reply, appreciate your support.

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Answer 2

Walter Roberson 2022-9-21

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在 MATLAB Online 中打开

Using extractBetween() as suggested by @Jérôme is the approved way to work with characters inside strings. But you can also go to a lower level... which will give you faster code.

Double-quotes like you had is used for a string array, so below I show looping over a string array and using {} to extract characters

M_h = ["FFFFFFFFFFFFFFFF"; "DEADBEAFDEADBEAF"];
M = strings(size(M_h));
for K = 1 : numel(M_h)
    this_M_in = M_h{K};
    this_M_out = blanks(64);
    for i=1:16
        this_M_out(i*4-3:i*4)=dec2bin(hex2dec(this_M_in(i)), 4);
    end
    M(K) = string(this_M_out);
end
M
M = 2×1 string array
    "1111111111111111111111111111111111111111111111111111111111111111"
    "1101111010101101101111101010111111011110101011011011111010101111"