在 c 中将 if 语句转换为 switch 语句

Question

I wrote a program that will ask for an integer input (1 - 9999) and will convert the inputted integer into its corresponding word format in English.

And I'm trying to modify it so that switch statements will be used instead of if-statements (where it is applicable).

Example 1: Input number: 2481 Output: two thousand four hundred eighty one

Here is my code:

#include<stdio.h>
#include<conio.h>

main()
{
      int num,thousands,hundreds,tens,ones;
      printf("Enter number (1-9999): ");
      scanf("%d",&num);
      //&& - check if within the range
      //|| - check if outside of the range
      if (num < 1 || num > 9999)
         printf("Invalid number.");
      else
      //if (num > 0 && num < 10000)
      {
      thousands = num / 1000;
      hundreds = num % 1000 / 100;
      tens = num % 1000 % 100 / 10;
      ones = num % 1000 % 100 % 10;
      //if (num / 1000 == 1)
      if(thousands == 1)
                   printf("one thousand ");
      if(thousands == 2)
                   printf("two thousand ");
      if(thousands == 3)
                   printf("three thousand ");
      if(thousands == 4)
                   printf("four thousand ");
      if(thousands == 5)
                   printf("five thousand ");
      if(thousands == 6)
                   printf("six thousand ");
      if(thousands == 7)
                   printf("seven thousand ");
      if(thousands == 8)
                   printf("eight thousand ");
      if(thousands == 9)
                   printf("nine thousand ");
      //if (num % 1000 / 100 == 1)
      if(hundreds == 1)
                   printf("one hundred ");
      if(hundreds == 2)
                   printf("two hundred ");
      if(hundreds == 3)
                   printf("three hundred ");
      if(hundreds == 4)
                   printf("four hundred ");
      if(hundreds == 5)
                   printf("five hundred ");
      if(hundreds == 6)
                   printf("six hundred ");
      if(hundreds == 7)
                   printf("seven hundred ");
      if(hundreds == 8)
                   printf("eight hundred ");
      if(hundreds == 9)
                   printf("nine hundred ");
      //if (num % 1000 % 100 / 10 == 1)
      if(tens == 1)
      {
              //if (num % 1000 % 100 % 10 == 0)
              if(ones == 0)
                      printf("ten ");
              if(ones == 1)
                   printf("eleven ");
              if(ones == 2)
                   printf("twelve ");
              if(ones == 3)
                   printf("thirteen ");
              if(ones == 4)
                   printf("fourteen ");
              if(ones == 5)
                   printf("fifteen ");
              if(ones == 6)
                   printf("sixteen ");
              if(ones == 7)
                   printf("seventeen ");
              if(ones == 8)
                   printf("eighteen ");
              if(ones == 9)
                   printf("nineteen ");
      }
      if(tens == 2)
                   printf("twenty ");
      if(tens == 3)
                   printf("thirty ");
      if(tens == 4)
                   printf("forty ");
      if(tens == 5)
                   printf("fifty ");
      if(tens == 6)
                   printf("sixty ");
      if(tens == 7)
                   printf("seventy ");
      if(tens == 8)
                   printf("eighty ");
      if(tens == 9)
                   printf("ninety ");
      if (tens != 1)
      {
               if(ones == 1)
                   printf("one ");
               if(ones == 2)
                   printf("two ");
               if(ones == 3)
                   printf("three ");
               if(ones == 4)
                   printf("four ");
               if(ones == 5)
                   printf("five ");
               if(ones == 6)
                   printf("six ");
               if(ones == 7)
                   printf("seven ");
               if(ones == 8)
                   printf("eight ");
               if(ones == 9)
                   printf("nine ");
      }
      
      }
      //else
          //printf("Invalid number.");
      getch();
}

However, when I tried substituting the if statements into switch statements it displays a blank. I'm stuck at the thousands place. I'm trying to fix it first before getting to the hundreds, tenths, and ones. Here is what I tried:

#include<stdio.h>
#include<conio.h>

main()
{
    int num,thousands,hundreds,tens,ones;
      printf("Enter number (1-9999): ");
      scanf("%d",&num);
      if (num < 1 || num > 9999)
         printf("Invalid number.");
      else
    {
        thousands = num / 1000;
        switch (thousands)
        {
            case '1': printf("one thousand ");
                      break;
            case '2': printf("two thousand ");
                      break;
            case '3': printf("three thousand ");
                      break;
            case '4': printf("four thousand ");
                      break;
            case '5': printf("five thousand ");
                      break;
            case '6': printf("six thousand ");
                      break;
            case '7': printf("seven thousand ");
                      break;
            case '8': printf("eight thousand ");
                      break;
            case '9': printf("nine thousand ");
                      break;
        }
    }
}

Answer 1

You're not comparing the same values you were before.

In your original code, you were comparing thousands against 0 , 1 , 2 , etc. In your updated code, you're comparing thousands against '0' , '1' , '2' , etc.

Use the same values you had before and you'll get the expected results.

Answer 2

When you write case '1': , you compare to character '1' 's value (converted to an integer), not integer; you should write case 1: .

Answer 3

Single quotes in C are used to define single characters. Since characters size is 1 byte in C, a char can be assigned up to 128 different values (1 byte = 8 bits = 2⁸ = from 0 to 127 ). But how are char represented, according to that integer value?
That is implementation-defined, but usually C uses the seven-bit US ASCII character set to represent char by default (for example, you might change the locale to have it use something different, like utf-8).

Back to your question, in the second snippet you're comparing the value of thousands with the ASCII values of the characters '1' , '2' , '3' , etc.
That means you are not comparing thousands with the value between single quotes, but with the character's ASCII decimal value corresponding to the character, so for example, case '1': equals to case 49:

decimal -   character

[...]

49          1
50          2
51          3
52          4
53          5
54          6

[...]

---

Practical Example

The following program shows just what I've said:

#include <stdio.h>

int main(int argc, char** argv)
{
    char ch = '1';

    printf("size of a char: %ld bytes\n", sizeof(char));
    printf("character representation: %c\n", ch);
    printf("character corresponding value in ASCII table: %d\n", ch);

    return 0;
}

Output:

size of a char: 1 bytes
character representation: 1
character corresponding value in ASCII table: 49