CODEWITHSUNDEEP
c++
N. F.
N. F.

Reputation: 901

Why doesn't setPrecision automatically use the least available precision?

I have a text file which contains some values in each of the line like:

xxx.xxxxxx

I am reading each of the line and converting them to str using strod() and storing them in a temporary double variable.

when I print the value of that double variable, it outputs more digits than there should be after point.

The expected output should be:

xxx.xxxxxx

but I am getting

xxx.xxxxxxxxxxxxxxxx

using setprecision(20). Any idea why is this happening?. Also how can I read what's exactly in the file?

Upvotes: 0

Views: 185

Answers (2)

James Kanze
James Kanze

Reputation: 154047

If you ask for 20 digits precision in output, you're going to get it. You seem to be surprised that you're getting something you explicitly requested. Anything other than 20 digits precision would surprise me.

As for what you read, typically, a number with 6 decimal places has no exact representation in double; strtod() will return the closest approximation.

I would suggest not trying to use double until you've read and understood http://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html. double is not a real number.

Upvotes: 4

Luchian Grigore
Luchian Grigore

Reputation: 258698

You are reading what's in that file, but you can't store the exact number. Not every number can be precisely represented as a double (or anything else for that matter). Therefore, you'll get an approximation.

Upvotes: 2

Related Questions