strings, pointers, messy..

[MPrestonf12]

What im trying to do is read a string of numbers in a textbox and if its a digit concatanate them together until I come to a "," then repeat the process. for example.. 56,77,13,66,843 it would read 5(its a digit) then 6(its a digit) then it will hit "," and move ahead to repaet the process. heres what I have anyone know a good functions to use or a few suggestions, thanks alot

char sztext[200];

retval = GetWindowText(gradebx,sztext,200);

char num[200][5];
int i;

for(i=0;i<size;i++)
{
	if(isdigit(sztext[i]))
	{
		strcat(num[i],sztext[i]);
	}
}

i get the error

strcat' : cannot convert parameter 2 from 'char' to 'const char *'
Conversion from integral type to pointer type requires reinterpret_cast, C-style cast or function-style cast
Error executing cl.exe.