Trying to sort out user input error in C

Question

Hiya, I'm trying to make a small loop which presents an error when the user inputs something apart from a float and gives them another opportunity. Here's what I've got so far.

printf("Enter a value for x:   ");
while (scanf("%lf", &x_temp) != 1) {
    printf("ERROR: Input real number\n");
    printf("Enter a value for x:   ");
    scanf("%lf", &x_temp);
}

But this just runs through the loop without giving the user another chance to enter another number:

user@user-vm:~/Desktop/Exercise_0$ ./a.out 
Enter a value for x:   a
ERROR: Input real number
Enter a value for x:   ERROR: Input real number
Enter a value for x:   ERROR: Input real number
Enter a value for x:   ERROR: Input real number
Enter a value for x:   ERROR: Input real number
Enter a value for x:   ERROR: Input real number
Enter a value for x:   ERROR: Input real number
Enter a value for x:   ERROR: Input real number
Enter a value for x:   ERROR: Input real number

Anyone got any ideas? Cheers guys

StilesCrisis · Accepted Answer · 2010-12-15 01:16:13Z

1

Try this:

for (;;)
{
    printf("Enter a value for x:   ");
    if (scanf("%lf", &x_temp) == 1) 
        break;
    printf("ERROR: Input real number\n");
}

answered Dec 15, 2010 at 1:16

StilesCrisis

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1 Comment

JMzance Over a year ago

This works but you have to input the value twice for some reason hmmm Cheers

Jim Lewis · Accepted Answer · 2010-12-15 01:15:30Z

1

You're not using the return value of the second scanf call anywhere. So it's probably succeeding, but then you immediately do another scanf at the top of the loop, after the input has already been consumed.

answered Dec 15, 2010 at 1:15

Jim Lewis

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Comments

Jonathan Wood · Accepted Answer · 2010-12-15 01:16:01Z

0

I'm not sure what the behavior is, but it appears you have an extra scanf(). The one that is part of the while condition will repeat each time through the loop. I don't see a need for the second one at the bottom of the loop.

answered Dec 15, 2010 at 1:16

Jonathan Wood

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