CODEWITHSUNDEEP
cscanf
Prasanth Sai
Prasanth Sai

Reputation: 77

Can anyone explain what this does: 'scanf("%d%*c",&n)'

i wrote a program for sorting the names of persons and i ask the user to give the number of persons that he want, and i stored it by using the below given line and it worked properly but, i din't understand the exact working of it. Can anyone explain it clearly?

scanf("%d%*c",&n);

Upvotes: 2

Views: 4065

Answers (3)

chux
chux

Reputation: 153348

%*c consumes the first char after the number (if there was one), be it a '\n', some other white-space, alpha, anything including '\0'.

The '*' in %*c, directs scanf() to consume, but not save that char.

If no number was scanned via "%d", maybe due to invalid input, then "%*c" does nothing as the preceding directive failed and "%*c" is not evaluated.

Using scanf("%d%*c",&n); has weak error handling. For robust code, use a helper function. Needless to say, this is far more complicated, but unfortunately C does not provide simple and robust reading of even simple things like int.

// Read 1 line, decode an `int`, save in *n
// Return EOF, 0:Invalid,  1:Success
int ReadInt(int *n) {
  int i;
  char buf[50];
  if (fgets(buf, sizeof buf, stdin) == NULL) {
    return EOF;  // EOF occurred
  }
  char *endptr;  
  errno = 0;
  long num = strtol(buf, &endptr, 10);
  if (buf == endptr || (*endptr != '\n' && *endptr != '\0')) {
    return 0;  // failed conversion
  }
  if (errno || num < INT_MIN || num > INT_MAX) {
    return 0;  // range error
  }
  if (n != NULL) {
    *n = (int) num;
  }
  return 1;
}

Even this could be expanded to get for excessively long lines.

Upvotes: 0

Gopi
Gopi

Reputation: 19864

%*c is used to ignore the newline space or any other special characters. So in this scanf() after entering the integer let's say you have a newline character in the buffer which will be gobbled by this %*c.

If you are not doing this and just after this scanf() if you have say

scanf("%c",&c); /* where c is a char */

Then you will be wondering that scanf() will never wait for you to enter a character instead it picks a newline character.

Upvotes: 5

Eugene Sh.
Eugene Sh.

Reputation: 18299

From here: scanf format string

"An optional asterisk (*) right after the percent symbol denotes that the datum read by this format specifier is not to be stored in a variable. No argument behind the format string should be included for this dropped variable."

Upvotes: 0

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