Thread: Strcmp Question - C

Hello,

I'm trying to quickly learn C as I have a program is write in about 2 weeks using it at uni. Its client/server stuff.

To the point I can get this string compare code to work. The client is sending a value like "1" or "2" up to "10" but the server only responds with the text "One". Even if a input a letter like "h" or a number like "23" it responds with "One". I'm pretty sure the problems with the code bellow. Everything else seems fine.

Code:

     if(strcmp(echoBuffer, "1"))
     {
         strcpy(echoBuffer, "One ");
     }
     else if(strcmp(echoBuffer, "2"))
     {
         strcpy(echoBuffer, "Two ");
     }
     else if(strcmp(echoBuffer, "3"))
     {
         strcpy(echoBuffer, "Three ");
     }
    else  if(strcmp(echoBuffer, "4"))
     {
         strcpy(echoBuffer, "Four ");
     }
     else if(strcmp(echoBuffer, "5"))
     {
         strcpy(echoBuffer, "Five ");
     }
     else if(strcmp(echoBuffer, "6"))
     {
         strcpy(echoBuffer, "Six ");
     }
     else if(strcmp(echoBuffer, "7"))
     {
         strcpy(echoBuffer, "Seven ");
     }
     else if(strcmp(echoBuffer, "8"))
     {
         strcpy(echoBuffer, "Eight ");
     }
     else if(strcmp(echoBuffer, "9"))
     {
         strcpy(echoBuffer, "Nine ");
     }
     else
     {
         strcpy(echoBuffer, "Ten ");
     }

I tried some stuff like if(strcmp(echoBuffer, "1") == 0) but that would just give an answer of 2 etc..

echoBuffer is just a number like 1, 2 10 depending on client input etc..

Any ideas?

Thanks

In C what you put inside the if brackets can be false (=0) or true (!=0) so you need to put a '!' in this way:

Code:

if (!strcmp(str1, str2));
//or
if (strcmp(str1, str2)==0);

what you wrote is exactly the opposite, so if the string echoBuffer is different from the string "1" it will write "One ";
If you are sure you are going to compare a prefixed number of characters, use tha strncmp(), that takes a parameter more which specifies the upper bound of the characters to be compared.

So what is in echoBuffer -- print it out and be sure.

A first caveat: I don't know stink about client/server code.

However:

In C "1" is not the same as '1', and neither "1" nor '1', is the same as 1.

"1" is a string of one letter, followed by an end of string char: '\0'. The end of string char appended to it, elevates the char (or chars), from just char(s), into a string that C will recognize.

'1' is a single char, and what it's followed by, is irrelevant.

1 is an integer, and should be separated from other char's or numbers, if it is not a part of it.

So I have no idea why you're trying to do a string comparison with what you describe in your problem description, as a number.

Treat a number as a number, a char as a char, and a string as a string, please.

I got it working with strncmp. I have one more quick noob question though.

I want the server to validate the user input so if there is more than one character in echoString the message "message too big" is displayed. I've got to code bellow to display a message if a letter is input however if a number greater to 9 such as 10, 21 it would display 1 or 2 respectively. Its just using the if statements to its textual value and ignoring the if statement to detect its size.

echoBuffer is defined at 32 bits long. I'm 99% sure strlen will only pick up the length of characters entered by the user in echoBuffer[32] as if it isnt present in the send socket, only one character is sent instead of several.

I've tried it as length == 1.

Code:

    int length = strlen(echoBuffer);
    
    
    if(length != 1)
    {
	    if(strncmp(echoBuffer, "1", 1) == 0) 
	     {
		 strcpy(echoBuffer, "One ");
	     }
	     else if(strncmp(echoBuffer, "2", 1) == 0)
	     {
		 strcpy(echoBuffer, "Two ");
	     }
	     else if(strncmp(echoBuffer, "3", 1) == 0)
	     {
		 strcpy(echoBuffer, "Three ");
	     }
	    else  if(strncmp(echoBuffer, "4", 1) == 0)
	     {
		 strcpy(echoBuffer, "Four ");
	     }
	     else if(strncmp(echoBuffer, "5", 1) == 0)
	     {
		 strcpy(echoBuffer, "Five ");
	     }
	     else if(strncmp(echoBuffer, "6", 1) == 0)
	     {
		 strcpy(echoBuffer, "Six ");
	     }
	     else if(strncmp(echoBuffer, "7", 1) == 0)
	     {
		 strcpy(echoBuffer, "Seven ");
	     }
	     else if(strncmp(echoBuffer, "8", 1) == 0)
	     {
		 strcpy(echoBuffer, "Eight ");
	     }
	     else if(strncmp(echoBuffer, "9", 1) == 0)
	     {
		 strcpy(echoBuffer, "Nine ");
	     }
	     else
	     {
		 strcpy(echoBuffer, "300 ");
	     }
     }
     else
     {
		 strcpy(echoBuffer, "Message too big");
     }

    recvMsgSize = strlen(echoBuffer);

Thanks for any tips. I'm working from tcp/ip sockets and c/the c programming language books to do this.