Function to identify upper/lower case letters C

I'm trying to create a function that will identify whether the first letter input is upper or lower case then output the rest of the string in that same case(upper/lower).

For example, "Hi there" would become "HI THERE".

I'm not familiar with fgets. Once I run it I can input and press enter and the program doesn't run. I'm not getting any compiler errors. I believe I went wrong in the void shift function.

Also, I know gets is not recommended, is fgets similar? Or is it better to use scanf?

#include <stdio.h>
#include <ctype.h>
void shift (char *my_string); // Function declaration

int main()
{
  char inputstring[50];

  printf("Enter a string\n");
  char *my_string = inputstring;
  shift(my_string); // Function
}

void shift (char *my_string) // Function definition
{
  int i =0;
  char ch;

  for(i=0; i<50; i++)
    fgets(my_string, 50, stdin);

  while(my_string[i])
  {
    if(ch>='A' && ch<= 'Z') // When first char is uppercase
    {
      putchar (toupper(my_string[i]));
      i++;
    }
    else if (ch>='a' && ch <= 'z') // When first char is lowercase
    {
      putchar(tolower(my_string[i]));
      i++
    }
  }
  return;
}

Solution

You don't need to call fgets() fifty times. It reads a line from stdin and writes it to my_string. It seems you only want to read one line, not fifty (and keep only the last one). The 50 is the maximum number of characters (minus one) that will be read and written to the buffer. This limit is to prevent buffer overflow. See fgets().

Try removing the for loop on the line before the fgets() call. Also, you don't need the my_string in main(). The corrected code:

#include <stdio.h>
#include <ctype.h>
void shift (char *my_string);//function declaration

int main()
{
  char inputstring[50];

  printf("Enter a string\n");
  shift(inputstring);
}

void shift (char *my_string) //function definition
{
  int i;
  char ch;

  if ( fgets(my_string, 50, stdin) == NULL )
    return;

  ch = my_string[0];
  for ( i=0; my_string[i]; i++ )
  {
    if(ch>='A' && ch<= 'Z') //when first char is uppercase
    {
        putchar (toupper(my_string[i]));
    }
    else if (ch>='a' && ch <= 'z')//when first char is lowercase
    {
        putchar(tolower(my_string[i]));
    }
  }
  return;
}

Edit: Added ch initialization, pointed out by @thurizas. Changed while loop to for loop. Added check to return value of fgets() as suggested by @JonathanLeffler. (See his comment about the buffer size.)