Using a single scanf() to take in different variations of user input in C

Question

Basically what my program does is take multiple lines of input from the user and displays a chart when the user quits the program. The issue i'm running into is that there are different variations for how the user can input information, for example: "L 200000" or "J 3.5 12". I'm not having an issue so much as to what the input itself is just how scanf() interprets it. I know that if I did scanf("%s" "%i", &identifier, &number) it would work just fine for the first example but as soon as the user inputs three separate values that single scanf() no longer works. So my question is, how would I get scanf() to ignore the fact there are missing values but still be able to assign what it's given to pre-defined variables? Is there any other methods that would work besides scanf()?

Don't use `scanf` for this. It's just not well suited for such a task. Use alternatives such as `fgets` to read the whole line and then `strtok` to parse each token. Or read the first character which presumably describes the data format that follows and then maybe `scanf` at that point. But would still advise against `scanf` as it can be difficult to get right in any non-trivial use cases. — kaylum, Sep 07 '21 at 21:42
See [What can I use for input conversion instead of scanf?](https://stackoverflow.com/questions/58403537) — Steve Summit, Sep 07 '21 at 22:24
Input the line with `fgets` and apply `sscanf` calls first to convert supposed 3 inputs and second to convert 2 inputs if that fails. How do you know it fails? By testing how many conversions were successful (its function return value), which is *essential* when using any of the `scanf` function family. — Weather Vane, Sep 07 '21 at 22:29
Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. — Community, Sep 13 '21 at 22:06

chux - Reinstate Monica · Answer 1 · 2021-09-08T15:29:35.247

Is there any other methods that would work besides scanf()?

Stop using scanf(). It does not well handle ill-formatted data, leaving stdin is a challenging state for alternative processing.

Use fgets() to read a line of user input into a string – then process the string.

#define EXPECTED_LINE_LENGTH_MAX 100
char buf[EXPECTED_LINE_LENGTH_MAX*2 + 1];  // Be generous
if (fgets(buf, sizeof buf, stdin)) {

Now parse buf for various formats.
Using " %n" to store the offset of the scanning is very useful to detect trailing junk.

  int n = 0;
  int identifier;
  double number
  if (sscanf(buf, "L %d %n", &identifier, &n) == 1 && buf[n] == '\0') {
    puts("Found L data");
  } else if (sscanf(buf, "J %lf %d %n", &number, &identifier, &n) == 2 && buf[n] == '\0') {
    puts("Found J data");
  } else  {
    puts("Fail");
  }
}

Using a single scanf() to take in different variations of user input in C

1 Answers1