Using scanf to read in certain amount of characters in C?

Question

I am having trouble accepting input from a text file. My program is supposed to read in a string specified by the user and the length of that string is determined at runtime

Answer 1

Normally, you'd use something like "%4c" or "%4s" to read a maximum of 4 characters (the difference is that "%4c" reads the next 4 characters, regardless, while "%4s" skips leading whitespace and stops at a whitespace if there is one).

To specify the length at run-time, however, you have to get a bit trickier since you can't use a string literal with "4" embedded in it. One alternative is to use sprintf to create the string you'll pass to scanf:

char buffer[128];

sprintf(buffer, "%%%dc", max_length);
scanf(buffer, your_string);

I should probably add: with printf you can specify the width or precision of a field dynamically by putting an asterisk (*) in the format string, and passing a variable in the appropriate position to specify the width/precision:

int width = 10;
int precision = 7;
double value = 12.345678910;

printf("%*.*f", width, precision, value);

Given that printf and scanf format strings are quite similar, one might think the same would work with scanf. Unfortunately, this is not the case--with scanf an asterisk in the conversion specification indicates a value that should be scanned, but not converted. That is to say, something that must be present in the input, but its value won't be placed in any variable.