how does scanf() check if the input is an integer or character?

Question

I am wondering how does the standard C library function scanf() check if the input is an integer or a character when we call scanf("%d",&var) when a character itself is just a number? I know that when it encounters a non-integer it puts it back into the input buffer and returns a -1 but how does it know that the input is not an integer?

Answer 1

The input is always a string. If scanf is expecting an integer (because you passed it "%d"), it attempts to convert the string to an integer for you.

Answer 2

You're correct in that each character is really represented as an 8-bit integer. The solution is simple: look at that number, and see if it is in the range 48-57, which is the range of SCII codes for the characters '0' - '9'.

Starting on line 1315 of the scanf() source code we can see this in action. scanf() is actually more complicated, though - it also looks at multi-byte characters to determine the numeric value. Line 1740 is where the magic happens and that character is actually converted into a number. Finally, and possibly this is the most useful, the strtol() function does the looping to perform that conversion.

Answer 3

In scanf statement you are mentioning %d which is holding integer only so by default it understood that the variable as integer

Answer 4

Basically, the scanf function matches regular expressions based on the conversion specifier you pass. If you specify %d, that tells scanf to match input against a regular expression that's one or more characters between '0' and '9' (optionally with a leading + or - character). It then converts that sequence of characters to the equivalent integer value.

A very simplistic version might look something like this:

while (isdigit(c = fgetc(stream))
  val = val * 10 + valueOf(c); 
ungetc(c, stream);

where isdigit is a standard library function that returns true (non-zero) if the character value represents a decimal digit, and valueOf is a user-defined function that maps the character representing an integer ('0' - '9') to the equivalent integer value (0 - 9) (I'm not aware of a standard library function that does that for individual character values). Why not just subtract '0' from c to get the equivalent integer value? Depending on the encoding, it's not guaranteed that all decimal integer characters will be laid out in order (All The World Is Not ASCII); best to delegate the actual conversion to a function that's aware of the current encoding.

Answer 5

It tries to parse a valid decimal integer from the beginning of the stream, in practice optional + or - and then sequence of one or more digits.

Answer 6

It basically uses a loop like:

while (isdigit((ch=getchar()))
    // ...

Answer 7

You misunderstand the %d format specifier. It does not return the ASCII value of the entered character (you can use %c for that); it returns the integer that the characters represent. So, on an input character of '9', %d returns the value 9 - not the ASCII value of '9'.

(Well, %d actually looks at a sequence of characters, so if the input is '9' followed by '0' followed by ' ', it'll interpret that as 90).