为什么像（0 <a <5）这样的条件总是正确的？

Question

I implemented the following program in C

    #include <stdio.h>
    int main() 
    {
       int a  = 10 ; 
       if(0 < a < 5) 
       {
          printf("The condition is true!") ; 
       }
       return 0 ; 
    }

Why does the condition 0<a<5 always return true ?

Answer 1

Unlike Python (which has operator chaining ), C evaluates the condition as:

(0 < a) < 5

The result of (0 < a) is either 0 or 1, both of which are less than 5, so the overall condition is true.

In C, a range test must be written:

0 < a && a < 5

Note that the Python script:

for a in range(-1,7):
  if 0 < a < 5:
    print a, " in range"
  else:
    print a, " out of range"

produces the output:

-1  out of range
0  out of range
1  in range
2  in range
3  in range
4  in range
5  out of range
6  out of range

The 'equivalent' C program using the same if condition would, of course, produce the answer 'in range' for each value.

Answer 2

Because 0 < a evaluates to 1 .

Use:

a > 0 && a < 5

if you want to test if a is greater than 0 and lower than 5 .

Answer 3

Since 0 is less than 10 , 0 < a would always evaluates to 1 , which is less than 5 , making 0 < a < 5 always true . Change your condition to

if(0 < a && a < 5) {...}     

Answer 4

  if(x<y<z)

It actually is valid syntax, but it doesn't do what you want.

Realize that x<y returns a bool, ie true or false. You then compare it against whatever value "z" has. So the intention of " x<y<z " looks wrong.

It can be:

if(x < y && y < z)