scanf and wrong input makes output wierd

0
votes

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?

2
cscanf
Read the manual page for scanf - it returns a value! – Ed Heal
@EdHeal , But that won't help in this case as both the scanfs will return 1. Checking this is good,though... – Spikatrix
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
Perhaps a better pattern for scanf is required – Ed Heal
Always check the return value of scanf, eg: if (scanf("%d", &a) != 1) /* error */; – pmg

2 Answers

1
votes

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.

1
votes

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.