1. Components of a C Language Program

• Mobile phones have many functions: “power on”, “power off”, “make a call”, “send a text”, “take a photo”, etc.
• Each function in a mobile phone corresponds to a program segment (function) in a C language program.
• Among many functions, one will always be executed first; multiple functions cannot be executed simultaneously.
• To use a mobile phone, you must first execute the power on function.

• A C language program consists of multiple “functions”, each with its own functionality.
• A program has exactly one main function.
• If a program does not have a main function, it cannot run.
• When the program runs, the system automatically calls the main function, while other functions need to be manually called by the developer.
• The main function has a fixed format and structure.

2. Function Definition Format

• The format of the main function definition:

• int indicates that the function will return an integer value after execution.
• main indicates that the name of this function is main.
• () indicates that this is a function.
• {} indicates the scope of this program segment.
• return 0; indicates that the function returns the integer 0 after execution.

int main() {  // insert code here...  return 0;   }

• The format of other function definitions:

• int indicates that the function will return an integer value after execution.
• call indicates that the name of this function is call.
• () indicates that this is a function.
• {} indicates the scope of this program segment.
• return 0; indicates that the function returns the integer 0 after execution.

int call() {  return 0;   }

3. How to Execute Defined Functions

• The main function (main) will be automatically called by the system, but other functions will not. Therefore, to execute other functions, you must manually call them in the main function.

• call indicates that we find something named call.
• () indicates that the thing we are finding named call is a function.
• ; indicates that the function call statement has been completed.
• Therefore, call(); indicates that we find the call function and execute it.

int main() {       call();  return 0;   }

• How to Output Content to the Screen

• Outputting content is a relatively complex operation, so the system has predefined a function specifically for outputting content called printf. We just need to execute the predefined printf function to output content to the screen.
• Whenever we need to execute a function, we do so by using the function name + parentheses.
• The following code means: when the program runs, the system will automatically execute the main function, and during this process, we manually executed the call function and the printf function.
• From observing the code, we find two problems:
• We did not tell the printf function what content we want to output to the screen.
• We cannot find the implementation code for the printf function.

int call(){  return 0;   }int main(){       call();  printf();  return 0;   }

• How to Tell the printf Function What to Output

• Simply write the content to be output inside the parentheses after the printf function.
• Note: The content written inside the parentheses must be enclosed in double quotes.

printf("hello world\n");

• How to Find the Implementation Code for the printf Function

• Since the printf function is implemented by the system, you must inform the system where to find the implementation code for the printf function before using it.
• #include <stdio.h> is how we tell the system to look in the stdio file for the declaration and implementation of the printf function.

#include <stdio.h>  int call(){  return 0;   }int main(){       call();  printf("hello world\n");  return 0;   }

4. How to Run the Written Program

• Method 1:

• Click the hammer to compile the “source code” into an “executable file”.
• Find the compiled source code, open the terminal (CMD), and run the executable file.

• Method 2

• Click the run button in the Qt development tool directly.

5. Notes on the Main Function and Other Variations

• In C language, every complete statement must end with a semicolon.

 int main(){       printf("hello world\n"); // If there is no semicolon, a compilation error will occur       return 0;   } int main(){       // If there is no semicolon, when multiple statements are merged into one line, the system does not know where one complete statement begins and ends       printf("hello world\n"); return 0;   }

• In C language, except for comments and areas enclosed in double quotes, Chinese characters cannot appear.

 int main(){       printf("hello world\n"); // If the semicolon is a Chinese semicolon, it will cause an error       return 0;   }

• A C language program can only have one main function.

 int main(){       return 0;   }   int main(){ // Compilation error, duplicate definition       return 0;   }

• A C language program cannot exist without a main function.

 int call(){ // Compilation error, because there is only the call function and no main function       return 0;   } int mian(){ // Compilation error, because the name of the main function is misspelled, which is equivalent to not having a main function       return 0;   }

• The int before the main function can be omitted or replaced with void.

 #include <stdio.h>   main(){ // No error       printf("hello world\n");       return 0;   } #include <stdio.h>   void main(){  // No error       printf("hello world\n");       return 0;   } #include <stdio.h>   void main(){  // No error       printf("hello world\n");       return 0;   }

• The return 0 in the main function can be omitted.

int main(){ // No error       printf("hello world\n");   }

• The reason why multiple variations do not cause errors:

• C language was originally just a specification and standard (such as C89, C11, etc.).
• The implementation of the standard requires support and implementation from major vendors.
• However, during the implementation by major vendors, due to issues such as interests and understanding, the standards implemented differ and change.

• Therefore, you will see that different formats are written in different books; some return int, some return void, and some even have no return value.
• Therefore, just remember the most standard writing method, no zuo no die.

#include <stdio.h>  int main(){      printf("hello world\n");      return 0;  }

6. C Language Program Exercises

• Write a C language program that outputs the following content to the screen in at least two ways:

  *** *** *********  *******   ****    **

• Ordinary youth implementation:

 printf(" *** *** \n");   printf("*********\n");   printf(" *******\n");   printf("  ****\n");   printf("   **\n");

• 2B youth implementation:

printf(" *** *** \n*********\n *******\n  ****\n   **\n");

• Artistic youth implementation (pretentious, don’t understand it yet):

 int  i = 0;   while (1) {       if (i % 2 == 0) {           printf(" *** *** \n");           printf("*********\n");           printf(" *******\n");           printf("  ****\n");           printf("   **\n");       }else       {           printf("\n");           printf("   ** ** \n");           printf("  *******\n");           printf("   *****\n");           printf("    **\n");       }       sleep(1);       i++;       system("cls");   }