Answer.

In the C language, comparison operators (==, !=, <, >, <=, >=) and the assignment operator (=) differ in both their semantics and priorities. Historically, errors between these operations have led to bugs in software: for example, mixing = and == often became a cause of hard-to-trace errors.

Problem: The main difficulty arises from the low priority of the assignment operator (=) compared to comparison operators. Additionally, assignment returns a value (rvalue), which allows for writing expressions like while(x = y), which sometimes leads to undesirable or non-obvious consequences.

Solution: It is necessary to clearly distinguish between == and =, understand their priorities in the chain of expressions, and use parentheses and linters to track such errors. In complex expressions, always leave parentheses for clarity.

Example code:

int a = 5, b = 3;
if (a = b) {  // mistake: assignment, not comparison
    printf("a == b\n");
}

Correct:

int a = 5, b = 3;
if (a == b) {
    printf("a == b\n");
}

Key features:

The assignment operator (=) returns the assigned value and can be used in complex expressions.
Comparison operators always return 0 (false) or 1 (true).
The priority of the assignment operator is lower than that of comparison operators.

Trick questions.

What is the difference between '==' and '=' in C, and what happens if they are confused in conditions?

== is the comparison operator, = is the assignment operator. If = is used instead of ==, the variable receives the assigned value, and the condition checks this value as a boolean. This is a common cause of bugs.

Can you write chains of assignments, such as a = b = c = 0? What happens then?

Yes, in C, the assignment operator works from right to left. First, 0 will be assigned to c, then this value is assigned to b, then to a. All variables will receive 0.

Example code:

int a, b, c;
a = b = c = 0;

Why is the expression 'if (a = 0)' not the same as 'if (a == 0)'?

In the expression if (a = 0), 0 is assigned to a. The condition is always false (since the result of the assignment is 0), and not "equality check". You need to write if (a == 0).

Typical mistakes and anti-patterns

Using = instead of == in conditions.
Complex nested assignments without parentheses and comments.
Expecting a bool result from the assignment operator.

Real-life example

Negative case

A programmer writes a while loop (x = data[i]) and expects the condition to trigger when x is zero. In fact, the loop only ends when data[i] is equal to 0 in value, rather than when x matches data[i].

Pros:

Allows for concise assignment and checking chains in one line.

Cons:

Hard-to-localize errors, especially if just one = is missed.
The error has syntactic correctness but not the intended semantics.

Positive case

Strict separation of expressions, explicit comparison and assignment. Use of linters to check the code.