Strange output when comparing same float values?

Question

Comparing Same Float Values In C

strange output in comparison of float with float literal

Float addition promoted to double?

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I read the above l

Answer 1

Floating point numbers are never accurate.

This statement is wrong. Some floating point numbers are accurate, such as 1.0, 12345.0, 12345.5, -2.25. All these numbers can be represented as integers didived by a power of 2. All numbers that cannot also are not accurate.

In your specific case, float x = 0.5 results in x having the value 1.00000000 * 2^-1. When you compare this against double 0.5, both operands are converted to double, so the comparison becomes 1.000000000000000 * 2^-1 == 1.000000000000000 * 2^-1, which succeeds.

For float x = 0.1, it looks different. The value is stored as 1.01010101 * 2^-3 (or similar). Note that this is already not precise. When you compare this against double 0.1, the float is extended with zeros at the end, the comparison becomes 1.010101010000000 * 2^-3 == 1.010101010101010 * 2^-3, which fails.