scanf and wrong input makes output wierd

Question

I have the following code in C:

#include <stdio.h> #include <stdlib.h> int var(); int var() { return 10; } void main() { int a; double x; a=0; a=var(); printf("hello world, i am a function which returned the value %d",a); printf("\nnow you enter a value:"); scanf("%d",&a); printf("so you entered the value: %d",a); printf("\nnow enter a double value:"); scanf("%lf",&x); printf("The double number till 4 precision is: %0.4lf",x); }

When I input normal integer and double values for the two scanf it runs fine. However I want to make it more robust. If I enter a decimal value for int scanf the code jumps directly to the next printf and skips the scanf for double. It prints the decimal part that I have input in the int as the double value.

eg:

hello world, i am a function which returned the value 10

now you enter a value:44.67

so you entered the value: 44

now enter a double value:The double number till 4 precision is: 0.6700

Any help?

@EdHeal , But that won't help in this case as both the scanfs will return 1. Checking this is good,though... — Spikatrix
– Spikatrix, Commented Apr 11, 2015 at 8:46
When you input 44.67, the first scanf consumes 44. The second scanf sees the .67 and consumes it as it is a valid decimal number . This is why the second scanf gets "skipped" — Spikatrix
– Spikatrix, Commented Apr 11, 2015 at 8:48
Always check the return value of scanf, eg: if (scanf("%d", &a) != 1) /* error */; — pmg
– pmg, Commented Apr 11, 2015 at 10:09

Some programmer dude · Accepted Answer · 2015-04-11 08:38:42Z

It's tricky because a number with fractions (like e.g 12.34) do have a valid integer part, which is read and correctly parsed by the first scanf call.

The simplest solution is to use e.g. fgets to read the input into a buffer, and then use sscanf on that buffer.

score 1 · Accepted Answer · 2015-04-11 10:07:44Z

You should use the function fpurge() to erase any input or output buffered in the given stream (which is stdin in this case). Try this:

#include <stdio.h> #include <stdlib.h> int var(); int main(int argc, char *argv[]) { int a; double x; a = 0; a = var(); printf("Hello world, I am a function which returned the value %d.\n", a); printf("Now you enter a value: "); scanf("%d", &a); printf("So, you entered the value %d.\n", a); printf("Now enter a double value: "); fpurge(stdin); scanf("%lf", &x); printf("The double number till 4 precision is %.4lf.\n", x); return 0; } // End main int var() { return 10; }

Here is the output which I got:

Hello world, I am a function which returned the value 10.
Now you enter a value: 44.67
So, you entered the value 44.
Now enter a double value: 3.14159
The double number till 4 precision is 3.1416.

Thang thanks for the reply. It worked. But one more question. When i used the fflush(stdin) it didnot gave me a desired output but fpurge did ?? why ??
@Magnum Don't use fflush(stdin). Read more here cprogramming.com/faq/cgi-bin/…

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