Getting wrong printf output for floats

Question

In the following:

float n1= 3.0;
double n2 = 3.0;
long n3 = 2000000000L;
long n4 = 1234567890L;
printf("%f %Lf %ld %ld\n", n1, n2, n3, n4);

3.000000 1.200000 2000000000 1234567890

Why is 1.2 printed for the second value and not 3.0? I suppose maybe I'm screwing up the float prints -- with float, double, and long double, or what's the reason why the above prints an incorrect result? Is this the correct way to print all decimal-types?

float n1= 3.0F;
double n2 = 3.0;
long double n3 = 3.0L;
printf("%f %f %Lf\n", n1, n2, n3);

Joshua · Accepted Answer · 2021-01-16 21:26:44Z

4

Becaue %Lf isn't the specifier for double, it's the specifier for long double. float promotes to double automatically; hence %f covers both float and double. But long double needs its own.

You kinda got lucky here. On some platforms, the rest of the arguments would be misaligned.

answered Jan 16, 2021 at 21:26

Joshua

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7 Comments

carl.hiass Over a year ago

are all non-qualified decimal literals parsed as double ? For example, if I enter in 2.0, would that be double unless doing 2.0L (long) or 2.0F (float) ?

Joshua Over a year ago

@carl.hiass: The rule in play is you passed a float as an argument to a variadic function. There are no vardiac floats. 2.0 would be a float in a context that allowed it to be.

carl.hiass Over a year ago

ok, but when I enter a decimal literal in VS Code it displays it as a double unless I qualify it. For example: gyazo.com/e78a218fdcd29c240019b8ff4038e11a

Joshua Over a year ago

@carl.hiass: A difference that makes no difference. On all modern platforms, all floating point arithmetic is done by casting all arguments to long double, performing the calculation, and casting them back.

jwdonahue Over a year ago

@Joshua, pretty sure that not all modern platforms cast to long double, given that some do have higher precision long doubles than doubles, and the cost of those higher precision operations can be substantially higher than doubles, in terms of power consumption, memory/registers and execution time. Think IoT. Your statement is likely true for most PC's and Servers.

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