C print long long to float

Question

I have two long long (int) values, and I want to print their division in float format with 3 digits precision. I have already read printf and long double , but all those suggestions lead to nan as the result of printf. E.g.

  long long a = 123123123;
  long long b = 123123124;
  printf("%d\n", b/a);
  printf("%.3f", b/a);

The first print will output 1 (as expected), but the second 0.000 instead of 1.xxx

You must cast either one of a or b to floating point first. Try: a/(double)b. — Dan Byström, May 06 '17 at 08:43
The first one is wrong, too: `%d` takes an `int`, not a `long long int`. You need `%lld` for that. — melpomene, May 06 '17 at 08:44

score 1 · Accepted Answer · edited May 23 '17 at 11:54

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Except for the first one, printf doesn't check the passed arguments or the number of formatting arguments of the string vs the arguments (most compilers can warn you about problems, but that's not guaranteed)

b/a is a long long type.

So even if the first printf kind of works, it would be better with

printf("%lld\n", b/a);

second printf doesn't work at all because argument is interpreted as a double, but not converted (although passing a float would also work: How does printf and co differentiate beetween float and double) , due to the varadic nature of the printf function (you can virtually pass anything as argument, so it's not safe). You have to convert it to double explicitly for instance like this:

printf("%.3f", (double)b/a);

Note that (double)(b/a) would also work but you'd lose the decimal part

edited May 23 '17 at 11:54

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answered May 06 '17 at 08:44

Jean-François Fabre

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printf `%f` takes a `double`, not a `float`. – melpomene May 06 '17 at 08:45
No, printf `%lf` has undefined behavior in C89 and was made equivalent to `%f` in C99. – melpomene May 06 '17 at 08:47
ok, that's for `scanf`. But casting to `float` works, even if it's not optimal: http://stackoverflow.com/questions/6395726/how-does-printf-and-co-differentiate-beetween-float-and-double. However, edited because it's better to cast to `double` – Jean-François Fabre May 06 '17 at 08:49
I used casts for so long.. but still forgot about them :| – joeo May 06 '17 at 08:50

C print long long to float

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