Thread: How to handle inputs if the user inputs the wrong thing

Did a search, nothing really came up so here goes..

I have a long floating input in my program where the user needs to input a value with a decimal point.

Code:

printf("Please enter the altimeter setting (xx.xx): ");
scanf("%lf", &s);

And they input 29.92 or whatever..

Problem is let's say they input 29,92, or the letter "a", or anything else besides a number and a decimal. That basically sends the program into an uncontrollable loop of sorts and crashes.

How do I handle incorrect inputs like that? I have a simple while loop to keep unwanted numbers from being entered but I'm not sure about characters and symbols and such..

I think pretty much the only way would be to read input as c-string, and take it from there with validation.

Cprogramming.com FAQ > How do I get a number from the user (C)

Originally Posted by Xupicor

I think pretty much the only way would be to read input as c-string, and take it from there with validation.

I see you brought a can, and lots of worms!

Validating stupid user input is tough, because those stupids are so damn clever!

Agree 100% with X man, above ^^^. Use fgets(), and then test the string buffer contents, from element 0 to the first end of string char. (there may be other junk char's beyond that, depending)

Besides just being 100% more robust, fgets() won't let the user's bad input, start the endless looping you can see with scanf(), when it's expecting a number, and the user enters a letter instead.

(Although you can stop that endless looping by adding a getchar() immediately after the scanf(), inside the loop.)

But fgets(myBuffer, sizeof(myBuffer), stdin) is soooo good, you'll love it. Just don't do it in public, eh?

Ok. This is bringing back memories of the class I took now.. I remember reading that FAQ before and most of it going right over my head.

I'm going to come back to what you guys said tomorrow after I get some sleep.

One more thing: The FAQ (linked above) does give some explanation of strtol(), and that is the method I would recommend. When I want an integer from the user, I always use a combo of fgets() + strtol(). If the FAQ is not quite enough for you, in terms of how it works, read this also. Of course, you don't want to get an integer, but a float. However, if you can use strtol() properly, then it is trivial to use strtod() to get a double. See the man pages for strtol() and strtod():

• strtol() (Linux man page)
• strtol() (Posix man page)
• strtod() (Linux man page)
• strtod() (Posix man page)

It's a bit of thinking to understand it all, but in the end you will have a rock solid way of getting numeric user input, and once you have hammered out the code, it is easy enough to re-use it elsewhere.