1. Compile in VS2017 environment. When using scanf, write scanf as scanf_s and you can input from the keyboard.
When using VS2017, the debug window will flash and you need to write a header file #include <; stdlid.h> , and then in return 0; Before writing system ("pause"); This phenomenon can be avoided. Three, the following is a few classic examples 1. Can receive keyboard characters, if it is lowercase, then output uppercase; If it is uppercase, output lowercase; If it is a number, it is not printed

#include <stdio.h>
#include <stdlib.h>

int main()
{
	int ch = 0;
	while ((ch = getchar()) != EOF)
	{
		if (ch >= 65 && ch <= 90)
		{
			ch = ch + 32;
			putchar(ch);
		}
		else if (ch >= 97 && ch <= 122)
		{
			ch = ch - 32;
			putchar(ch);
		}
		else if (ch >= '0' && ch <= '9')
		{
			;
		}
		else
		{
			putchar(ch);
		}
	}
	system("pause");
	return 0;
}

2. For example
Output a diamond

#include<stdio.h>
#include<stdlib.h>
int main()
{
     int line = 0;
     int i = 0;
     scanf_s("%d", &line);
     for (i = 0; i < line; i++)
     {
          int j = 0;
          for (j = 0; j < line-1-i ; j++)
          {
               printf(" ");
          }
          for (j = 0; j < 2 * i + 1; j++)
          {
               printf("*");
          }
          printf("\n");
     }
     for (i = 0; i < line-1; i++)
     {
          int j = 0;
          for (j = 0; j <=i; j++)
          {
               printf(" ");
          }
          for (j = 0; j < (line-1-i)*2 - 1; j++)
          {
               printf("*");
          }
          printf("\n");
     }
     system("pause");
     return 0;
}