CODEWITHSUNDEEP
cascii
Apeee
Apeee

Reputation: 75

Low-level detail of char representation on a computer

I read that in C, char is actually an integer because characters are represented as patterns of bits.

So I wonder when you run your program how does the operating system know when to print the bits pattern as a number or a char. What code does determine which is to be printed?

And, where is the ASCII table located, and how does the conversion work?

Upvotes: 3

Views: 528

Answers (4)

alexis
alexis

Reputation: 50190

... characters are represented as patterns of bits.

On a computer, everything is represented as a pattern of bits: characters, integers, real numbers, and executable code. The eight bits 01000001 could represent the integer 65 (though integers normally take up at least 16 bits), or the letter A in the ASCII system. There are elaborate ways of keeping track of what is what, it's one of the main responsibilities of the operating system and of high-level programming languages. printf is one of the rare cases where the programmer has to explicitly tell a function what kind of data is being passed.

I read that in C, char is actually an integer because characters are represented as patterns of bits.

That's not quite right: In C, chars and ints have been distinct types for a very long time. C allows you to use a char as an int if you want, by choice of the language designers: the bit pattern is basically used as the corresponding int. C won't let you use a float as an int since the bit pattern would not give you anything useful (the bit pattern for 1.0 looks nothing like the bit pattern for integer 1).

As for the ASCII table, it's embodied in the design of the hardware and software that displays text, and of the programming language functions that manipulate it. A computer font is a mapping from numbers to shapes. or "glyphs". In the simplest case, it maps numbers in the ASCII range (32-126) to the appropriate glyph. (In reality it's often a more indirect route to the same result). Old computer terminals had the glyphs hardwired, while Windows or X11 applications use software fonts.

As for programming languages, a function like isdigit() or isalpha() just looks up the character code on a table of its properties: isdigit() returns true for the numbers 48-57 (which encode the ASCII digits), and false for all others. No glyphs are involved.

Upvotes: 0

Simon MILHAU
Simon MILHAU

Reputation: 214

If you are running a linux machine just type man ascii in a terminal and you'll see all of the char values (in decimal, octal and hexa). If not, just google ascii table and you'll probably be done.

Upvotes: 3

P.P
P.P

Reputation: 121387

It depends on how you tell your program to interpret the bits in your code. For example,

printf("%d %c", 'a', 97);

This will print: 97 a

Upvotes: 3

Alok Save
Alok Save

Reputation: 206526

How does the operating system know when to print the bits pattern as a number or a char.

It doesn't. You explicitly tell the compiler whether to treat it as a character or integer by specifying the proper format descriptor to printf. And that is the very reason that if there is a mismatch between format descriptor and type of the actual argument then it results in Undefined Behavior.

Upvotes: 4

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