You can use c[i]= '\0' or simply c[i] = (char) 0.

The null/empty char is simply a value of zero, but can also be represented as a character with an escaped zero.

This prints out all ASCII values:

int main()
{
    int i;
    i=0;
    do
    {
        printf("%d %c \n",i,i);
        i++;
    }
    while(i<=255);
    return 0;
}

and this prints out the ASCII value for a given character:

int main()
{
    int e;
    char ch;
    clrscr();
    printf("\n Enter a character : ");
    scanf("%c",&ch);
    e=ch;
    printf("\n The ASCII value of the character is : %d",e);
    getch();
    return 0;
}

There is no particular symbol for whitespace. It is actually a set of few characters which are:

' '      space 
'\t'     horizontal tab 
'\n'     newline
'\v'     vertical tab 
'\f'     form feed 
'\r'     carriage return    

Use isspace standard library function from ctype.h if you want to check for any of these white-spaces.

For just a space, use ' '.

Here are some functions to do the job, which return 0 if the character is not found; hopefully they are self-explanatory:

char const ascii_table[] = " !\"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~";

int char_to_ascii(int ch)
{
    char const *p = strchr(ascii_table, ch);
    return p ? p - ascii_table + 32 : 0;
}

int ascii_to_char(int a)
{
     return (a >= 32 && a < 128) ? ascii_table[a-32] : 0;
}

Expanding this to cover 0-127 instead of 32-127 is left as an exercise to the reader

I just went through the character map to get these. They are all in Calibri.

Number    Name                   HTML Code    Appearance
------    --------------------   ---------    ----------
U+2000    En Quad                       " "
U+2001    Em Quad                       " "
U+2002    En Space                      " "
U+2003    Em Space                      " "
U+2004    Three-Per-Em Space            " "
U+2005    Four-Per-Em Space             " "
U+2006    Six-Per-Em Space              " "
U+2007    Figure Space                  " "
U+2008    Punctuation Space             " "
U+2009    Thin Space                    " "
U+200A    Hair Space                    " "
U+200B    Zero-Width Space       ​      "​"
U+200C    Zero Width Non-Joiner  ‌      "‌"
U+200D    Zero Width Joiner      ‍      "‍"
U+200E    Left-To-Right Mark     ‎      "‎"
U+200F    Right-To-Left Mark     ‏      "‏"
U+202F    Narrow No-Break Space         " "

ASCII 0 is null. Other than that, there are no "nothing" characters in traditional ASCII. If appropriate, you could use a control character like SOH (start of heading), STX (start of text), or ETX (end of text). Their ASCII values are 1, 2, and 3 respectively.

For the full list of ASCII codes that I used for this explaination, see this site