Incompatible size error for display function

Hi everyone, I am currently beginning to learn MATLAB for university. I was playing around with it and I got this error:

This is my code:

%Calculating approximate maximum backhand range based off forehand range

f = input('What is your max forehand range in meters? ');

b = f + 5;

disp('You can throw a forehand about ' + f + ' meters which means you can throw a backhand about ' + b + " meters.")

What is the issue here and how do I fix it? Thank you in advance for your help

Answer 1

[dpb]

f=10; b=f+5;
disp("You can throw a forehand about " + f + " meters which means you can throw a backhand about " + b + " meters.")

You can throw a forehand about 10 meters which means you can throw a backhand about 15 meters.

The "+" operator is only defined for strings, not char variables...

The char() string is an array of char so trying to add

msg1='You can throw a forehand about ';
whos msg1

  Name      Size            Bytes  Class    Attributes

  msg1      1x31               62  char               

msg1+f

ans = 1×31

99 121 127 42 109 107 120 42 126 114 124 121 129 42 107 42 112 121 124 111 114 107 120 110 42 107 108 121 127 126

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whos ans

  Name      Size            Bytes  Class     Attributes

  ans       1x31              248  double              

results in an array in which the value of f has been added to the value of the char() string -- which is an integer which matches the specific character in the ASCII table; internally they're still just numbers; it's only knowing to display as a char() variable that makes it look like text instead...

msg2='meters which means you can throw a backhand about ';
whos msg2

  Name      Size            Bytes  Class    Attributes

  msg2      1x50              100  char               

Now you see that msg1 and msg2 aren't the same length and therefore, can't be added. Of course, even if they just happened to have the same length having added 10 to the first message, it wouldn't be at all what was intended...

char(msg1+f)

ans = 'cy□*mkx*~r|y□*k*py|orkxn*kly□~*'

The olden way of doing this before the strings class was introduced, one did the above with explicit formatting as

msg=sprintf('You can throw a forehand about %f meters which means you can throw a backhand about %f meters.',f,b);
disp(msg)

You can throw a forehand about 10.000000 meters which means you can throw a backhand about 15.000000 meters.

[Walter Roberson]

The "+" operator is only defined for strings, not char variables...

A+B is defined for char() A and char() B. It is defined as double(A)+double(B) -- which requires that A and B are arrays of compatible size. Implicit expansion of size applies.

The problem with A+B for char() A and char() B is that most of the time A and B are not compatible sizes. Character vectors are treated as vectors of individual characters.

string() entries are treated differently than character vectors.

'abc'+5

ans = 1×3

102 103 104

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char(ans)

ans = 'fgh'

"abc"+5

ans = "abc5"

char(ans)

ans = 'abc5'

[Ben]

Ah okay I see, so basically basically because the two pieces of text are different sizes vector-wise they can't be added together. The way you fix that is by creating strings, which will store the text in the same vector size no matter how many characters the text has.

[Walter Roberson]

Right.