string comparing

This is a discussion on string comparing within the C Programming forums, part of the General Programming Boards category; I canít get my head around this problem. In the following code I want to enter a string that is ...

  1. #1
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    Question string comparing

    I canít get my head around this problem. In the following code I want to enter a string that is a filename, for example hello.txt, myprog.bin assuming that they both exist on my c drive. However I want the program to accept .bin, .txt files only. If you donít enter the 2 extensions I mentioned an error is displayed. Im unsure on how to do the decision making part with the file extensions. I think there should be a for loop to loop around string or strcpy command used within the program to compare extensions but not to sure. Iíve attached a piece of code that relates to it. Any help would be grateful.


    #include <stdio.h>
    #include <conio.h>
    #include <string.h>

    main()
    {
    char string[10];
    int result;

    printf("Enter filename : "); //enter filename - example hello.txt would be valid
    gets(string); //get file name


    if (string != //unfinished here, unsure how to test condition
    printf("Limit your entries to .bin, .txt extensions only"); //error message
    else
    printf("You entered %s with the correct extension\n", string); //otherwise prints correct text to screen
    getch();
    }

  2. #2
    the Corvetter
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    Must use strcmp (or testing each array element). When you use the array ("string"), you are testing the addresses. You must do something like:
    Code:
    if (strcmp(string1, string2) == 0) // the strings are equal
        // ...etc...
    Yes, if the strings are equal, strcmp returns 0.
    1978 Silver Anniversary Corvette

  3. #3
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    I was playing with your code, and to my shock, I couldn't get it to work, here's what I've got.

    Code:
    #include <stdio.h> 
    #include <conio.h> 
    #include <string.h> 
    
    
    int getStringLength(char inputString[BUFSIZ])
    {
     int i = 0;
     do
     {
      i++;
     }
     while(inputString[i]);
     return i;
    }
    
    int validExtension(char fileName[BUFSIZ])
    {
     int fileLength = getStringLength(fileName);	
    
     char goodext1[4] = ".txt";
     char goodext2[4] = ".bin";
     char extension[4];
     extension[0] = fileName[fileLength-4];
     extension[1] = fileName[fileLength-3];
     extension[2] = fileName[fileLength-2];
     extension[3] = fileName[fileLength-1];
    
     printf("%s",extension);
     if(strcmp(goodext1,extension) || strcmp(goodext2,extension))
     {
      return 0;
     }
     else
     {
      return -1;
     }
    }
    
    int main(void) 
    { 
    char string[BUFSIZ]; 
    int result; 
    
    printf("Enter filename : "); //enter filename - example hello.txt would be valid 
    gets(string); //get file name 
    
    
    if (!validExtension(string))  
    printf("Limit your entries to .bin and .txt extensions only"); //error message 
    else 
    printf("You entered %s with the correct extension\n", string); //otherwise prints correct text to screen 
    getch(); 
    return 0;
    }

  4. #4
    the Corvetter
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    Well, just glancing at your code for a couple of seconds, I founds some stuff (maybe you intended, maybe not). You use the constant looking constant BUFSIZE, but I don't see it declared anywhere.

    Also, you may be a little confused with the return value of strcmp here:
    Code:
    if(strcmp(goodext1,extension) || strcmp(goodext2,extension))
    Are you trying to test if these strings are alike? Because, if two strings are the same, strcmp returns 0, not a non-zero number. So, if you want to test if they are equal, it should be:
    Code:
    if(!strcmp(goodext1,extension) || strcmp(!goodext2,extension))
    Or more easier readably:
    Code:
    if((strcmp(goodext1,extension) == 0) || (strcmp(goodext2,extension)==0))
    Tell me if this is the problem. If not I'll study the code a little harder.
    1978 Silver Anniversary Corvette

  5. #5
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    nope that ain't the problem..

  6. #6
    Banned Troll_King's Avatar
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    strcmp will not help you in this case. You need to search the string for the substring extensions.

    Code:
    #include<stdlib.h>
    #include<stdio.h>
    #include<string.h>
    
    int main()
    {
    	char a[30];
    
    	do
    	{
    		printf("Enter file path: ");
    		fgets(a,30,stdin);
    		a[strlen(a) - 1] = '\0';
    
    	}
    	while ( strstr(a,".txt")  == NULL && strstr(a,".bin") == NULL );
    
    	printf("\n%s",a);
    	return 0;
    }
    Last edited by Troll_King; 01-20-2002 at 12:11 PM.

  7. #7
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    yeah but then if it's called a.txt.exe ...

  8. #8
    Banned Troll_King's Avatar
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    Nope, not sure what you are talking about Brian, this is for a datafile, and 'a' is an array variable name. If you study the code carefully you can figure it out.
    Last edited by Troll_King; 01-20-2002 at 12:10 PM.

  9. #9
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    if they try to open "hello.txt description.exe", it will think it is correct, it should only test the last 4 chars

  10. #10
    Banned Troll_King's Avatar
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    If the file has a .txt or .bin anywhere than the path is accepted. If they actually screwed up and have a file called C:\directory\textfile.txt.exe than it will accept that but when you try to open that file with:

    FILE *fptr = fopen(a,"r");
    if(fptr == NULL) exit(1);

    Than fopen wil catch it so long as the file pointer is checked. If you want to check the last 4 characters in the filepath than use a pointer, but that is ugly code

    //point to the end of a
    char *ptr = a + strlen(a);
    //loop backwards 4 characters and validate.
    // *(--ptr);
    Last edited by Troll_King; 01-20-2002 at 01:38 PM.

  11. #11
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    another stab at it

    Try this

    Code:
    //trap the input in the char array 
    
    
    //get the index of where the extension begins
    iExtPosition = strlen(szInputString) - 4;
    
    if( strcmp( &(szInputString[iExtPosition]), ".txt") == 0 )
        //this is a txt file
    else if( strcmp( &(szInputString[iExtPosition]), ".bin") == 0)
       //this is a bin file
    else 
       //it is neither type of file
    zMan

  12. #12
    the Corvetter
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    Re: another stab at it

    Good looking code! I agree with your algorithm. Good job!
    1978 Silver Anniversary Corvette

  13. #13
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    Brian,

    One thing I noticed:
    Code:
    char goodext1[4] = ".txt";
    char goodext2[4] = ".bin";
    You are writing beyond these two char arrays. They should both be declared as having 5 chars instead of 4.

    Another thing:
    You are treating "extension" as if it is null terminated, which it is not.
    Code:
     extension[0] = fileName[fileLength-4];
     extension[1] = fileName[fileLength-3];
     extension[2] = fileName[fileLength-2];
     extension[3] = fileName[fileLength-1];
    
     printf("%s",extension);
     if(strcmp(goodext1,extension) || strcmp(goodext2,extension))
     ....
    Either declare extension as "char extension[5]" and add a null "extension[4] = '\0' "
    -or-
    printf("%4.4s", extension) and use strncmp's instead of strcmp's

    You need to also use && instead of || since one of the strcmp's will return a 1 if the other returns a 0.

    Hope this helps.

  14. #14
    B26354 Deckard's Avatar
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    You are writing beyond these two char arrays.
    Actually, the initialization is legal, but you are correct: there is no room for the null zero. The quickest fix here is:

    Code:
    char goodext1[] = ".txt";
    char goodext2[] = ".bin";
    Cheers,
    Jason Deckard

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