CODEWITHSUNDEEP
c
Jason Sahota
Jason Sahota

Reputation: 35

How would I check if an input from the user is a float?

I want to write an if statement where if the user input is a float then X happens but I don't actually know how to translate this in to code. The code is below so if anyone could help me that would be great.

// declare a variable
float i = 0;

// ask for an input
scanf( "%f", &i );

// if the input is a float
if() <--- DON'T KNOW WHAT TO PUT HERE
{
// print normally
printf();
} 

// in all other cases
else 
{
// print an error message
printf();
}

Sorry if this is a really simple question but I'm very new to learning C.

Upvotes: 0

Views: 105

Answers (3)

selbie
selbie

Reputation: 104504

Read it as a string first:

char message[100+1];
char *end = NULL;
double result = 0.0;
scanf("%100s", message);

Then try a safe conversion to double:

result = strtod(message, &end);
if ((end == message) || errno == ERANGE)) {

    // not a floating point value

}
else {
    // is a floating point value - stored in result
}

Upvotes: 1

chux
chux

Reputation: 153348

To test if user input is a float

Step 1: Read input as a string using fgets() @DeiDei. Best to avoid scanf().

char buf[1000];
if (fgets(buf, sizeof buf, stdin) == NULL) {
  puts("End-of-file or rare input error occurred");
  return;
}

Could test (not shown) if buffer is such that use input was too large. IMO, consider that a hostile attack.

Now parse the string;

errno = 0;
char *endptr;
float f = strtof(buf, &endptr);
if (buf == endptr) {
  puts("No conversion");
  return;
}

while (isspace((unsigned char) *endptr) endptr++;
if (*endptr) {
  puts("trailing junk");
  return;
}

if (errno == ERANGE) {
  puts("Out of `float` range.");  // too big or maybe too small
  // suggest continue on and use the +/- HUGE_VALF or wee number. 
} else if (errno) {
  puts("Implementation additional error.");
  return; 
}

// use `f`

Upvotes: 1

Chris Dodd
Chris Dodd

Reputation: 126193

You check the return value of the scanf call -- it tells you how many conversions succeeded:

if (scanf("%f", &i) == 1) {
    // the input is a float
} else {
    // all other cases -- print an error message
}

Upvotes: 1

Related Questions