Distingusih between numbers and alphabet in a string : I am trying to build a TCPIP communication between a server and a client in visual studio , the client will accept a string from the keyboard, and he will send the string to the server only if it is numbers not alphabet I tried the below code but it seems there is something wrong 

while (rc == SOCKET_ERROR);							//try as long till there is connection (no Socket Error)

	printf("conected with %s..\n\n", address);


	do
	{

		printf("Please insert the Blood pressure values for further Diagnostic\n ");
		gets(data);
		char i;
		for (i = data[0]; i <= MAX; i++)
		{

			char n = data[i];
			if ((strlen(data) > MAX) || (strlen(data) == 0)) 
			{

				printf("argument not allowed!\n\n");
				memset(data, '\0', strlen(data));
				continue;

			}
			if ((n >= 'a' && n <= 'z') ||( n >= 'A' && n <= 'Z'))

			{

				printf("you have to enter a number !\n\n");
				memset(data, '\0', strlen(data));					
				continue;
				//next iteration
			
			}
		}