Last C programming lesson. Why is the string disappearing?

Question

The last lesson features the following code:

#include <stdio.h>

#define TO_STR(x) #x

int main() {

printf("%s\n", TO_STR( 123\\12 ));

return 0;
}
Output: 123\12

If I put only a single backslash between the numbers as in the following code, the output changes

#include <stdio.h>

#define TO_STR(x) #x

int main() {

printf("%s\n", TO_STR( 123\12 ));

return 0;
}
Output: 123

Could anyone tell me why only 123 appears and the number 12 disappears?

Answer 1

The number after a single backslash gets interpreted as an octal number for an ASCII character code. In this case \12 becomes the linefeed control character.

Try changing it to TO_STR( 123\101 ) and see what happens.

Answer 2

Escape sequences in C

https://en.wikipedia.org/wiki/Escape_sequences_in_C