Question

I was writing a program to calculate negative powers of 2.
I used the following two code snippets:

cout.precision(3);
cout << scientific << pow(2.0, p) << endl;

AND

ans = pow(2.0, p);
printf("%.3e\n", ans);

For p = -8271, the cout gives the right answer (1.517e-2490) but I get a widely different answer for the printf (6.929e-310). Why does this discrepancy occur?

I use Codeblocks on Ubuntu.

Answer 1

I bet that’s because ans is a long double, but you didn’t tell printf to expect a long double. The format code you want is %.3Le assuming that’s the case.

The g++ compiler even has a warning to detect format/parameter mismatches (I think it comes with -Wall) but I always prefer iostreams because they’re type safe like this.

All this is of course assuming that p is also long double, causing the compiler to pick the long double version of pow.