How to use the operation of C language ll and &&&

The && and || operators have short-circuit evaluation characteristics: &&: When the first operand is false, it returns false, and the second operand calculation is skipped. ||: When the first operand is true, return true, skip the second operand calculation. The short circuit feature can avoid meaningless calls to expressions that produce side effects, improve efficiency, and prevent null pointer errors. However, side effects should be taken into account when using it, and short circuit should be disabled if necessary.

Logic and short circuit evaluation in C language: In-depth discussion && and ||

Many beginners are confused by the logical operators && (logical and) and || (logical or) in C language, not only their basic usage, but more importantly the short-circuit evaluation mechanism behind them. This article will explain these two operators in an easy-to-understand manner and share some pitfalls and solutions I encountered in my programming career.

The goal of this article is to give you a thorough understanding of how && and || work, and how to use them safely and effectively, avoiding some common mistakes. After reading it, you will be able to confidently write C code containing logical operations and easily troubleshoot related issues.

First, let's review the Boolean logic. In C, non-zero values ​​are considered true, while zero values ​​are considered false. The && operator returns true only when both operands are true, otherwise it returns false; the || operator returns true as long as one operand is true, and returns false only when both operands are false. This looks simple, right? But the key lies in their "short-circuit" characteristics.

Both && and || operators have short-circuit evaluation characteristics. This means that when the compiler performs logical operations, it does not always calculate all operands. For && , if the first operand is false, the result of the entire expression must be false, and the compiler will directly skip the calculation of the second operand. Similarly, for || , if the first operand is true, the result of the entire expression must be true, and the compiler will also skip the calculation of the second operand.

Let's take a look at some examples, feel the charm of short circuit evaluation, and also take a look at some potential traps:

 <code class="c">#include <stdio.h> int func1() { printf("func1 called\n"); return 0; // 假} int func2() { printf("func2 called\n"); return 1; // 真} int main() { if (func1() && func2()) { printf("Both functions returned true\n"); } else { printf("At least one function returned false\n"); } if (func2() || func1()) { printf("At least one function returned true\n"); } else { printf("Both functions returned false\n"); } return 0; }</stdio.h></code>

Run this code and you will find func2 is called only once. This is because in the case of && , func1 returns false, func2 will not be executed. In the second if statement, func2 returns true, func1 will not be executed.

This short-circuit feature is very useful when dealing with functions or expressions that may have side effects. For example, avoid dereferences to null pointers:

 <code class="c">int* ptr = NULL; if (ptr != NULL && *ptr > 10) { // 安全的操作}</code>

If ptr is a null pointer, then ptr != NULL is false. The short-circuit evaluation mechanism will prevent access to *ptr and avoid program crashes. If there is no short circuit, the code will try to access the null pointer, causing the program to crash.

However, short-circuit evaluation can also bring some unexpected results, especially when you rely on side effects of operands. For example, if you want both functions to be called regardless of their return value, then you cannot rely on short circuit evaluation.

Performance-wise, short circuit evaluation usually improves efficiency because it reduces unnecessary calculations. However, this improvement is usually trivial unless your operands are very complex or time-consuming.

In summary, understanding the short-circuit evaluation mechanisms of && and || is essential for writing efficient and safe C code. Remember, when using these operators, always pay attention to the possible impact of short-circuit evaluation to avoid potential errors. Only by practicing more and thinking more can you truly master the mystery behind these seemingly simple operators.

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