C++: temperature converter - why calculation return 0

Question

Why is k value returns 0? Please help.

double fah, kel;

fah = std::atof(input.c_str());  //convert string input to a double & assign value to be fah degree 
kel = (double)((f + 459.67) * (5/9)); //calculate fah temp to kelvin

k value returns 0 when I add "5/9" to the calculation.

score 4 · Answer 1 · answered Nov 27 '13 at 20:15

4

In C++, 5/9 == 0 because of integer division.

Use 5.0/9.

answered Nov 27 '13 at 20:15

Matt

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score 2 · Answer 2 · answered Nov 27 '13 at 20:15

2

The problem is integer division. The result of this:

5/9

is 0. You should use a floating point type:

5/9.0 // 9.0 is a double.

answered Nov 27 '13 at 20:15

juanchopanza

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Why is only one of those two number needs to be in a decimal form? – user2925557 Nov 27 '13 at 20:19
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@user2925557 it is enough for one of the operands of the division operator `/` to be a floating point (FP) type for the operation to be calculated in FP (and the result of the operation to be FP) – juanchopanza Nov 27 '13 at 20:23
@user2925557 an operation of `int/double` or `double/int` is automatically promoted to a `double` result. For readability, it is often good to write both as a double - though, that is not required. – Zac Howland Nov 27 '13 at 20:25

score 0 · Answer 3 · answered Nov 27 '13 at 20:17

0

Expression 5 / 9 has integral type that is its result is equal to 0. Change (5/9) at least to (5.0/9)

answered Nov 27 '13 at 20:17

Vlad from Moscow

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Zac Howland · Answer 4 · 2013-11-27T20:37:16.050

This is an integer division issue. Additionally, you simplify the equation a bit:

Dk = C*Df + C*B ===> Dk = C*Df + A

where Dk is degrees Kelvin, Df is degrees Farenheit, C is the 5/9 constant, and A is C*Kelvin Offset (also constant). Which would make your code:

const double FtoKMultiplier = 5.0/9.0; // done at compile time
const double FtoKOffset = 459.67 * FtoKMultiplier; // also done at compile time
double fah = std::atof(input.c_str());
double kel = FtoKMultipler * fah + FtoKOffset; // single multiplication and addition at runtime

C++: temperature converter - why calculation return 0

4 Answers4