C++ program debug

my assigment need me to use a input file while read the file print out if the text in side are letter (upper case or lower case) or number (odd or even)

I got my own code down there please help

#include

#include

void ShowProgramHeader();

void IsLetter(char Gets);

void IsDigit(int Gets);

using namespace std;

int main()

{

int i = 1;

char Gets[i];

ifstream readFile;

readFile.open("data.txt");

if(!readFile)

{

cout<<"Error the file can't be open!!"<

exit(1);

}

while(readFile)

{

readFile.get(Gets[i]);

if((Gets[i] >= 'A'&& Gets[i] <= 'Z')||(Gets[i] >= 'a'&& Gets[i] <= 'z'))

{

IsLetter(Gets[i]);

}

else if(Gets[i] >= '0'&& Gets[i] <= '9')

{

IsDigit(Gets[i]);

}

i++;

}

readFile.close();

return 0;

}

void Isletter(char Gets)

{

cout<<" "<

if(Gets >= 'A'&& Gets <= 'Z')

cout<

else if(Gets >= 'a'&& Gets <= 'z')

cout<

}

void IsDigit(int Gets)

{

cout<<" "<

if(Gets % 2 == 0)

cout<

else

cout<

}

and this is the error is show:

"IsLetter(char)", referenced from:

clang: error: linker command failed with exit code 1 (use -v to see invocation)