Multiple Data Compression and Encryption using iterative Technique. B105

Hello everyone i got stuck in compression4 function,I got index value for each character now i have to convert that index value into string to perform bit manipulation operation.i am getting error like invalid operand.
I am using sprintf() for converting the integer into string

Sprintf(arraywhereIstoring , type of value to be converted \n , index value);
I am using this syntax but giving error
If anybody have suggestions for this please discuss.

Thank you