format specifier for double in scanf

Question

I wrote this program for getting a double input:

double n; scanf("%lf",&n); while(fgetc(stdin)!='\n') return 0; printf("%lf",n);

This program is supposed to take double as input. Provided that %lf is the format specifier for double in scanf, I used it. But for inputs of a digit followed by e. ex.(3e), the input is being read without errors. But during processing of the same variable, it is discarding the e and only considering 3 as is shown by the printf statement. What is the cause of such behaviour?

AnT stands with Russia · Accepted Answer · 2014-10-13 06:19:46Z

Following the rationale given in defect report #22 (http://www.open-std.org/jtc1/sc22/wg14/docs/rr/dr_022.html), functions like scanf (with %lf) and strtod should consume as much input as satisfies the expected format for the floating point constant. This means that they should consume the e character in "3e" even though the e character is not followed by the actual exponent value (the exponent value is required to be present by 6.4.4.2).

This applies in equal degree to %lf and %f. In my experiments they behave exactly the same on this input. (Of course, %f requires target variable of type float.)

However, even after consuming both 3 and e from the input stream, I would expect scanf (and strtod) to recognize "3e" as an invalid representation of a floating point number (since, again, 6.4.4.2 requires the exponent value to be present) and act accordingly. In my experiment scanf successfully read 3.0. This I cannot explain yet.

@darknight: Not sure what you mean. I don't see any difference in the behavior for %lf and %f. The standard specifies %lf for double in scanf. %f is for float.

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