1. Every C program has one and only one main function (main), and the program must start with the main () function and end with the main () function.

2. In C language, when e is used to express scientific counting method, it is stipulated that e must be preceded by a number and followed by an integer.

3. Character constants enclosed in single quotation marks can only contain one character; There are no string variables in C language, and only character arrays can be used to store strings.

4. External variables are allocated permanent memory space by the system at compile time, so the type of external variables is not an automatic storage category.

5. Variables defined in a compound statement within a function can only be valid within the scope of the compound statement.

6. Use sizeof(int) to calculate the number of bytes of int data.

1, character constant

In C language,' e' is used as a character constant. It stands for the character' e'. For example, if you need to print the character' e', you can use the printf function and the character constant' e'.

2. Floating decimal notation

"E" stands for the power of 10 in the scientific notation of floating-point numbers. For example, 1.23e5 means 1.23 times 10 to the fifth power. This is a convenient way to represent very large numbers or very small numbers.

3, enumeration types (Enumerations)

In C language, "e" is often used as an identifier of enumeration type, representing a specific enumeration value. Enumeration types are user-defined data types that allow you to define names for a set of related values. In this way, you can use this name to refer to these values, making the code clearer and easier to read.

In C language,' e' is used in many ways, mainly involving mathematical operations, string operations and the call of specific library functions.

Here are some concrete examples:

1,' e' in mathematical operation:' e' in mathematics usually represents the base of natural logarithm and is an irrational number.

2、7 1828。 In c language, you can use M_E to represent' e', which is the math library.