How to assign a string value - c

Question

Can I do something like the code below to get a persons name into the firstname string?
printf("First Name? ");
scanf("%s", &firstname[11]);

Answer 1

yup. This stores the input in firstname, starting at index 11.

Answer 2

No. You should not do it this way. It is unsafe, in exactly the same way gets is unsafe.
You should instead do something like this. (I am assuming that you really do want to write to firstname starting at character position 11, which is what &firstname[11] does. If firstname is 11 bytes long and you want to write starting at position 0, you would simply use firstname and remove the various occurrences of 11 + below.)
char inbuf[80];
size_t n;
fputs("First Name? ", stdout);
fgets(inbuf, sizeof inbuf, stdin);
n = strlen(inbuf);
if (n == 0) {
fputs("No name entered\n", stderr);
exit(1);
}
if (inbuf[n-1] != '\n') {
fputs("Name too long\n", stderr);
exit(1);
}
inbuf[--n] = '\0';
if (n == 0) {
fputs("No name entered\n", stderr);
exit(1);
}
if (11 + n >= sizeof firstname) {
fputs("Name too long\n", stderr);
exit(1);
}
memcpy(&firstname[11], inbuf, n);
firstname[11 + n] = '\0';
If you reaction is that that looks like a giant pain in the behind, all I can say is, welcome to C programming. All of that is in fact necessary for robustness in the face of arbitrarily malformed input.
You should also reflect upon Falsehoods Programmers Believe About Names and then redesign your database accordingly. (Most importantly in this context: people do not necessarily divide their names into "first" and "last" and "middle" components; many people have first names that require more than 10 bytes to represent.)
EDIT: Bugs in example code should now all be corrected. Serve me right for doing memory arithmetic in my head without testing it.

Answer 3

The short answer to your question is, yes you can.
The long answer (and questions that go with it)...
What's the declaration of firstname?
What's in the first 11 places of filename?
Are you trying to limit the the limit the length of the user input to 11 characters and that was the syntax you thought would work?
I suspect that you are trying to limit the firstname to be at most 11 characters. The way to solve this is:
char firstname[12];
scanf("%11s", firstname);
However, as Zach pointed out, this will be problematic if you have a first name like "Mary Kay". scanf will stop at the first white space and you will end up with a first name that contains just "Mary". The better approach is to use fgets.
fgets(firstname, 12, stdin);
That should work if there is nothing else in a line.
If you expect to read the data from a file and there are other fields in a line, then you will have to deal with that additional complexity. Say you have first name, last name, and age in a line. Your input file could look something like:
John, Deer, 59
Mary Kay, Smith, 42
If your input file contains data like above, you can't use fgets to pick out the first name using fgets(firstname, 12, infile);. You will have to use fgets to read the entire line, then parse the line to extract all the relevant data.