How to solve C++ syntax error: 'expected primary-expression before ':' token'?

How to solve C syntax error: 'expected primary-expression before ':' token'?

In C programming, syntax errors are a common problem. One of the common errors is the "expected primary-expression before ':' token" error message. This error usually occurs when using conditional expressions and the ternary operator. This article will introduce the cause of this error and give the corresponding solution.

First, let’s look at a simple code example:

int main() {
    int x = 5;
    int y = 10;
    int max = (x > y) ? x : y;
    return 0;
}

In the above code, we try to use conditional expressions and ternary operators to compare the sizes of x and y, and compare the The larger value is assigned to the variable max. However, the compiler reported an error "expected primary-expression before ':' token". This is because in C syntax, the "?" and ":" in the ternary operator must be replaced by expressions. When the expression after the question mark is true, the expression after the colon will be executed; otherwise, the expression before the colon will be executed.

To resolve this error, we need to ensure that the conditional expression used in the ternary operator is a legal expression. Common reasons for this error include the following:

1. Missing or incorrect expression: When using the ternary operator, you need to ensure that the expression in the conditional part is a correct expression. If the conditional expression is missing or wrong, the compiler will not recognize it, resulting in the "expected primary-expression before ':' token" error. For example, the conditional expression is missing in the following code, which leads to this error:

int main() {
    int x = 5;
    int y = 10;
    int max = (x >) ? x : y; // 错误：条件表达式缺失
    return 0;
}

2. Missing parentheses: Sometimes, we may ignore the parentheses of the conditional expression in the ternary operator, This will also cause the same error. When using the ternary operator, make sure to use parentheses in the conditional expression to avoid syntax errors. The following is an example of missing parentheses:

int main() {
    int x = 5;
    int y = 10;
    int max = x > y ? x : y; // 错误：缺少条件表达式的括号
    return 0;
}

In order to solve the error in the above two cases, we only need to use a legal expression in the position of the conditional expression and make sure to use the correct grammar.

Also, when we use the ternary operator, we also need to ensure that the correct syntax is used in its syntax context. For example, in the following code, we try to assign the result of a ternary operator to an illegal expression, resulting in an error:

int main() {
    int x = 5;
    int y = 10;
    (x > y) ? x : y = max; // 错误：赋值给非法的表达式
    return 0;
}

To solve this error, we need to make sure that we are using the ternary operator , assign the result to a legal expression, such as a variable.

To sum up, when the C syntax error "expected primary-expression before ':' token" occurs, we should check the following aspects:

1. Make sure to use the ternary operator The conditional expression in is a legal expression and uses correct syntax in its syntax context.
2. Make sure you use the correct brackets to delimit the conditional expression.
3. Ensure that the result in the ternary operator is assigned to a legal expression.

By paying attention to these details, we can avoid this common C syntax error and make our code more stable and reliable.

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