In C, the main difference between i and i is the timing and performance impact of return value. 1. i is the prefix self-increase, first self-increase, then return the value after self-increase; 2. i is the suffix self-increase, first return to the original value and then increase. Both are usually optimized by the compiler in terms of basic types, but on complex types such as iterators, i may affect efficiency due to the creation of temporary objects. Therefore, it is recommended to use i when you don't care about the return value, especially in performance-sensitive paths or STL iterator operations to use prefixes. Also, avoid modifying the same variable multiple times in a single expression to prevent undefined behavior.

In C, although i (prefix self-increase) and i (suffix self-increase) ultimately add the value of variable i to 1, their behavior is different from the usage scenario. Understanding this is helpful for writing efficient, clear code.

When to use i and when to use i ?

If you only care about the variable itself being eventually added by 1, and not the return value of the expression, the two seem to be interchangeable. However, in actual development, it is more recommended to use i first , especially on loops or performance-sensitive code paths.

Why? Because semantically, i needs to preserve the original value as the result of the expression, which means it may produce a temporary copy. i directly returns the value after increasing, without additional overhead. Although modern compilers will optimize, this difference still exists on some types (such as iterators).

How to choose in a loop?

In common for loops, both writing methods are common:

 for (int i = 0; i < 10; i ) { ... }

 for (int i = 0; i < 10; i) { ... }

In fact, for basic types (such as int ), these two writing methods are almost indistinguishable from modern compilers. But if you are using an iterator when iterating through the container:

 for (auto it = vec.begin(); it != vec.end(); it) { ... }

It is recommended to use it here because there may be performance differences in the iterator type's auto-increment operation, especially in some implementations it will create temporary objects, which will affect efficiency.

Differences in return value behavior

This is the most core difference between the two:

• i : First self-increase, then return the value after self-increase (reference or value)
• i : Save the original value first, then increment it, and finally return the original value (usually a temporary copy)

To give a simple example:

 int i = 5;
int a = i; // a = 6, i = 6

int j = 5;
int b = j ; // b = 5, j = 6

It can be seen that the same is adding 1, but the assignment result is different. Therefore, when there is a dependency in the return value in the expression, pay special attention to which one to choose.

A few tips for practical use

• If you don't care about the return value of the expression, use i to get used to uniformity and avoid potential performance problems.
• In the iterator operation of the STL container, try to use the prefix form.
• Do not modify the same variable multiple times in an expression when writing code, such as int c = ii; which will cause undefined behavior.
• When you encounter similar writing styles when reading other people's codes, don't take it for granted that the order is clear and pay more attention to the context logic.

Basically that's it. These two operators look similar, but are handled well in details, making the code clearer and more efficient.

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