Thread: hello , i need help

Originally Posted by Emad4U

hell guys ,
im new here , i came for a reason , i need help desperately in my project:
i need to read integer numbers , and if read char , error message should show and then ask to re read numbers , but instead i get infinite loop in while !!
how can i do it without using :

fflush malloc realloc math.h ctype.h string.h stdlib.h time.h

!! please help me ty

Hey Emad4u and welcome to the forum

Sure, no worries - what have you come up with so far?

The simplest way is to read in entire lines at once (e.g. with fgets()) and then trying to parse a number from the line with sscanf(). If that doesn't work, just get another line from the user.

You can also do this by calling scanf(), and if that fails then read characters one by one (with e.g. getchar()) until you see a newline, then prompt the user and call scanf() all over again. Basically you're throwing away the characters on the line since they're not digits and allowing the user to try again.

There is a third solution that may have come up in your course, namely using fflush(stdin), but this is not recommended! It's non-standard. It's just a bad idea.

Really the simplest method looks something like this:

Code:

while(scanf("%d", &x) != 1) {
    printf("That doesn't look like an integer to me. Try again.\n");
    while(getchar() != '\n') {}
}

scanf returns the number of format specifiers (%d, %f, etc) that were successfully read, so in this case if it returns 1 then all is well. If it doesn't return 1 then just call getchar() over and over to discard everything up to and including the newline. Note that scanf can also return EOF if the standard input stream ends (e.g. it was redirected from a file and the file has ended, or the user types CTRL-Z/CTRL-D), as can getchar(). Technically this should be handled but I've left it out in this illustrative example.

Finally, here's a thread which discusses other methods of doing this: Safeguarding against non-numerical input