Why Do Casting and Variable Assignment Produce Different Results in C# Floating-Point Expressions?

C# Floating Point Expression: Analyzing the Differences between Forced Type Conversion and Variable Assignment

In C#, seemingly simple floating-point arithmetic expressions can lead to unexpected results. This article delves into this issue and elucidates the underlying reasons for the observed differences.

Problem Description

Consider the following code snippet:

<code class="language-csharp">int speed1 = (int)(6.2f * 10);
float tmp = 6.2f * 10;
int speed2 = (int)tmp;</code>

Intuitively, we would expect speed1 and speed2 to have the same value, both representing 6.2f times 10. However, in reality, the two variables have different values:

<code>speed1 = 61
speed2 = 62</code>

This difference raises the question: Why do these seemingly identical operations produce different results?

Explanation

To understand the reasons behind this behavior, one must delve into the subtleties of C#’s floating-point arithmetic.

In the first expression (int)(6.2f * 10), the multiplication result of 6.2f * 10 is treated as a double-precision floating point number (64 bits) before being converted to an integer (32 bits). This conversion truncates the fractional part of the double, resulting in a result of 61.

In the second expression float tmp = 6.2f * 10, the result of the multiplication is stored in a float variable (tmp) with 32-bit precision. When tmp is converted to an integer, the floating point number is rounded to the nearest integer, resulting in 62.

Compiler Optimization

It’s worth noting that the C# compiler optimizes code for performance reasons. In the case of (int)(6.2f * 10), the compiler may choose to keep the intermediate value as a double, resulting in a loss of precision during the cast. However, in the case of float tmp = 6.2f * 10, the compiler must round the result to the nearest float value before storing it in the variable, resulting in a difference in the results.

More insights

To illustrate it more clearly, let us consider the following exercise:

<code class="language-csharp">double d = 6.2f * 10;
int tmp2 = (int)d;
// 计算 tmp2</code>

In this example, the value of tmp2 is 62 because the multiplication result is stored in a double variable before being converted to an integer, and the double data type has sufficient precision to represent 6.2f * 10 without There will be a significant loss of accuracy.

Conclusion

Understanding the properties of floating point arithmetic in C# is critical to avoiding unexpected results. By considering the subtleties of the casting and rounding process, developers can write code that behaves as expected and avoid potential errors.

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