问题
For example, what happens if I say:
for(i = 2; i < 2; i++)
Obviously, this is a useless for loop, but maybe i = a, and a is set by something else. So what happens in this case?
回答1:
Neither iteration of the loop will be executed.
In fact this loop (provided that the condition has no side effects)
for(i = 2; i < 2; i++) { /* ... */ }
is equivalent to this statement
i = 2;
回答2:
The condition of a for loop is checked before every iteration, including the first one; so your loop's body will never be executed.
回答3:
The way for loop works is it checks for condition (in your case i < 2) and executes whatever is between { } or whatever code on following lines
As you are initializing i to 2 the condition fails immediately and nothing executes.
Essentially whatever code that is inside of for loop never executes.
回答4:
In a for loop, the condition is evaluated before the first iteration. This means that in your example, the content of the loop would not be executed because i is already greater than or equal to 2.
Example code path:
- Set
i = 2. - Check if
i < 2. - Exit loop because step 2 evaluated to false.
i would still be modified, however, as the variable initialization (i.e. i = 2) still occurs before the condition is checked.
来源：https://stackoverflow.com/questions/42708412/in-c-what-happens-when-the-condition-for-a-for-loop-arent-met-at-the-beginning