c - c中常数的默认数据类型是什么？

Question

float x=2, y=2.6;

if(x==2); 
if(x==2.0) //both these conditions evaluate to be true

但是，

 if(y==2.6); 

评估为假

谁能解释一下常量的默认数据类型是什么，为什么我会得到这样的结果

Answer 1

float x=2, y=2.6;
//           ^^^ double, internally converted to float during the initialization
//      ^ int, internally converted to float during the initialization

if (x == 2) // compare double converted from float (x) with double converted from int (2)
if (x == 2.0) // compare double converted from float (x) with double (2.0)

if (y == 2.6) // compare double converted from float (y) with double (2.6)

注意最后一个条件:最初 2.6 (a double) 被转换为 float；将该 float 转换回 double 并不能保证相同的值。

(double)(float)2.6 != 2.6

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大概是这样的:

                 2.6 is 0b10.1001100110011001100110011001100110011001100110011001
          (float)2.6 is 0b10.10011001100110011001100
  (double)(float)2.6 is 0b10.1001100110011001100110000000000000000000000000000000
          difference is                             ^ approx. ~0.000000095367