User Input

Functions you will need:

• scanf for numbers
• getchar for characters and other weird characters.
• fgets(char* out_str_reference, int len_of_array, FILE* file) for strings. “get string”
  • f ⇒ “file” (but in this case it’s for anything. stdin is the “file” in our cases)
  • get ⇒ “obvious”
  • s ⇒ “string”

scanf("expected input formatted with format specifiers", many, outputed, pointers);
 
// Here's an example:
 
int x, y;
scanf("x: %d y: %d", &x, &y);
getchar();

If the user doesn’t format it correctly, it just safely aborts and continues. If you want to know how many fields were successfully retrieved, it is the integer than scanf returns.

Why use fgets for strings but not scanf?

? scanf doesn’t work with strings that have spaces since spaces are what scanf looks for to separate each variable. Even if the string you pass is "%d$d", the two integers are separated by a space. scanf DOES work for inputs of 1 word.

char word1[1024], word2[1024];
scanf("%s %s", word1, word2);
// Works as intended if you type Hello World

Why use getchar after scanf?

• When the user presses “enter” it adds some residue to the console. getchar() will simply get rid of that, so it doesn’t mess up future scanf calls.
• Here is an example of a mistake you could make:

char test[2];
int int_test;
 
scanf("%i", &int_test);
fgets(test, 2, stdin); // Remember "2" implies 1 character for the bulk of the vstring and 1 for the delimeter
 
printf("Your number is: %d", int_test);
printf("You typed: %s", test);

These are the values when I type “10” in the terminal

• The \n comes from when I press “enter”. When fgets is called, it reads the enter within the input buffer of stdin and thinks “oh, there’s already an input. I guess we’re done here then!” which is why test is like that.

Input is always in a buffer. When a character is not used, (i.e. the “enter” right after using scanf), it will wait until another input function reads it. Only when the buffer is empty does the user “typing into the console” result in the intended behavior.

Sanity check

Remember, always check that the input is sane. If the user doesn’t type the right thing, it will be junk (i.e. a string when expecting an int, similarly to what happens with printf) To ensure the input makes sense, we do input sanitization.