Why fixed value, and 48...?? Help, thank you

[helloea]

void OnStart() 
  { 
   int symbol_0='0'; 
   int symbol_9=symbol_0+9; 
   printf("In a decimal form: symbol_0 = %d,  symbol_9 = %d",symbol_0,symbol_9); 
   printf("In a hexadecimal form: symbol_0 = 0x%x,  symbol_9 = 0x%x",symbol_0,symbol_9); 
  }

2019.07.24 22:56:27.205 P78-Character Constants (EURUSD,D1)     In a decimal form: symbol_0 = 48,  symbol_9 = 57
2019.07.24 22:56:27.206 P78-Character Constants (EURUSD,D1)     In a hexadecimal form: symbol_0 = 0x30,  symbol_9 = 0x39

symbol_0 = 48,  symbol_9 = 57

symbol_0 = 0x30,  symbol_9 = 0x39

[William Roeder]

Why 48? The symbol for the character zero through nine was defined in ASCII around 1972 as 48 through 57. Prior to that, the values where machine/manufacturer specific.
          ASCII - Wikipedia

Now compare the table on the above link to EBCDIC table where half the the codes to not correspond to any character.
         EBCDIC - Wikipedia

[Keith Watford]

helloea:

You have 2 other topics that are based on similar themes

https://www.mql5.com/en/forum/318452

https://www.mql5.com/en/forum/318535

Don't open multiple topics on related subjects. Stick to one.

I have not deleted the other topics as you have received answers.

[helloea]

威廉罗德:

为什么48？ 字符0到9的符号 在1972年的ASCII 中 定义 为48到57.在此之前，机器/制造商特定的值。
          ASCII - 维基百科

现在将上面链接上的表与EBCDIC表进行比较，其中一半代码与 任何 字符 都不对应 。
          EBCDIC - 维基百科

I'm glad to hear from you.I could memorize the answers, but I never understood

[helloea]

Keith Watford:

You have 2 other topics that are based on similar themes

https://www.mql5.com/en/forum/318452

https://www.mql5.com/en/forum/318535

Don't open multiple topics on related subjects. Stick to one.

I have not deleted the other topics as you have received answers.

Thanks for the advice. I'm not going to offend the rules.I will try my best to understand the logic here, but I hope it can be easier to understand the introduction. Thanks again!