getting_user_input.md

#Getting User Input

##getchar() and putchar() getchar() reads the next character from the text stream, which in our examples is the keyboard input. putchar(c) will print out the character c. Example:

int main() {
  char c;
  c = getchar();
  puts("The first character was:");
  putchar(c);
  return 0;
}

This can be used for looping through command line input one character at a time:

int main() {
  int c;
  c = getchar();
  while (c != EOF) {
    putchar(c);
    c = getchar();
  }

  return 0;
}

When running the above type of example, remember that you can trigger an 'EOF' (End Of File symbol) by entering Ctrl-D on a line by itself. On Windows enter Ctrl-Z.

Reference: The C Programming Language section 1.4, p.15

##scanf() Example:

int main() {
  char name[11]; // We want to store 10 characters, plus the string terminator
  puts("Enter your name: ");
  // scanf will save the first 10 characters of input entered into 'name'
  scanf("%10s", name);
  printf("Hello %s!\n", name);

  return 0;
}

Always specify the string length when using scanf().

###Good:

scanf("%10s", name);

###BAD:

// Antipattern!
scanf("%s", name);

###How to break it Try using scanf without a character limit, and giving it more characters than the variable can hold.

It will cause the program to write more data into the character array than it can hold, and this leads to a crash.

##fgets() With fgets(), you must always pass the length of your buffer:

int main() {
  char name[11]; // We want to store 10 characters, plus the string terminator
  puts("Enter your name: ");
  // fgets will save the first 10 characters of input entered into 'name'
  // params: pointer to a buffer, max size, and the source of the data
  fgets(name, sizeof(name), stdin);
  // "stdin" just means the data will come from the keyboard
  printf("Hello %s!\n", name);

  return 0;
}

##scanf() vs. fgets()

	scanf()	fgets()
Setting character limits	scanf gives you the option	fgets requires you pass a limit
Can you enter multiple fields?	Yes, and you may enter complex data strutures too	Only one thing at a time, and only strings
Can the string contain spaces?	No; scanf will stop reading when it hits a space	Yes; fgets reads the whole string

Avoid gets()

The function fgets() is a safer, newer version of gets(), which has no way to limit the character input.

Always prefer fgets() to gets().

Reference: Head First C, p. 66-68