Why does this line of code loop infinitely?

Hi everyone,

Why does this line of code produce an infinite loop upon receiving invalid input, and how do I fix it?

Code:

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for (int i = 0; !i; i = scanf("%lf", &myDouble)) puts("Please enter a double: ");

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I am just trying to prompt the user for input until myDouble is, in fact, a double (i.e. when the scanf() function returns non-zero). I get similar results using a do ... while loop:

Code:

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do {
  puts("Please enter a double: ");
} while (!scanf("%lf", &myDouble));

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You need to clear the bad input or it will be read over and over again.

Code:

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#include <stdio.h>
 
int main() {
    double d;
 
    while (puts("Enter a double: "), !scanf("%lf", &d))
        for (int c; (c = getchar()) != EOF && c != '\n'; ) ;
 
    return 0;
}

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