如何检查用户是否输入了输入

Question

Im using this code to allow the use to enter a numerical value for the variable:

float day;
printf("Day?: ");
scanf("%f",&day);

how can i make the program produce an error message if the user did not enter ANY value for "Day"??

Answer 1

Try this:

if(scanf("%f",&day) != 1)  
    printf("Error"); 

Answer 2

Use fgets()/sscanf()

char buf[100];
fgets(buf, sizeof(buf), stdin);
if (sscanf(buf, "%f", &day) != 1) {
   if (buf[0] == '\n') 
     HandleNothingInput(); 
   else 
     HandleBadInput(); 
   }
}

fgets() typically reads until a Enter or '\\n' is entered. By using sscanf() to parse the buffer, empty inputs ("user did not enter ANY value"), bad input ("abc") are readily detected as the result of scanf() will be 0.

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Alternatives: If you want to insure no additional data is entered, one could use a sentinel:

int ch;     
if (sscanf(buf, "%f %c", &day, &ch) != 1) {

atof() is another approach.