If I try to use `sprintf` to format a string as numeric, why does a different number get passed?

When I try to format strings with numeric values using sprintf, I notice that if I forget to change the value that I'm trying to pass from a string to numeric, then a different value is passed.

For example, I'm trying to format a string using the value in C.

C='10';
% Forget to convert to number
sprintf('%d %f', C)

ans =

    '49 48.000000'

If I try to convert to a number using double, I get

double(C)

ans =

    49    48

which matches the sprintf output. If I use str2num, then everything works as I expect.

sprintf('%d %f', str2num(C))

ans =

    '10 '

Why does this happen?

[Stephen23]

"Why does this happen?"

Everything on a computer is stored as a number. The character vector '10' consists of two characters:

• '1' , which is stored as the number 49
• '0' , which is stored as the number 48

DOUBLE gives you those chararacter codes, as does converting them to numeric (basically just a typecast).

Answer 1

[Star Strider]

Using:

C='10';

‘C’ is a character array, not actually a number. The conversion to double results in the ASCII codes for the individual character elements, ‘1’ and ‘0’ so if you instead used the '%s' format the result would be:

C='10';
sprintf('%s',C)

ans = '10'

The char function can help to explain this.

.

Answer 2

[Jérôme]

When you write

C='10';

C is a char array, i.e. an array with two elements of class char, which are '1' and '0'.

These are characters, which are (like any type of variable) encoded with numbers behind.

The encoding used here is probably UTF-8 / Unicode, where '1' is stored as 49 and '0' as 48 (see https://en.wikipedia.org/wiki/List_of_Unicode_characters#Basic_Latin).

In the same way, you would get the following and you can check this value in the link above:

D = 'λ';
double(D)

ans = 955

When you write

sprintf('%d %f', C)

ans = '49 48.000000'

you are telling the function to expect numbers, so it interprets the content stored (49 and 48) as numbers, which is why it shows them. Whereas if you write

sprintf('%c', C)

ans = '10'

you are telling the function to expect characters, so it interprets the content stored (49 and 48) as characters encoded with UTF-8 / Unicode, then it shows you the characters.