Programming Forums > Application Development > C

pulpfiction · Mar 28th, 2006, 10:58 PM

I'm getting a strange behavior on this little script.

I'm trying to implement a hash table, so I have a hash function which should always return the same numeric result for a given string.

Problem is, depending on the parent function which is calling hash(), it returns different results for the same string. If I call it twice inside the same function it works ok.

Now the really strange thing (well, I don't know, maybe I'm doing something wrong - I'm new to C) is this behavior depends on the machine I'm running the script. At university machines, running Windows, I got the error. Then I tried at home on Windows and Linux, same thing. I then asked for a friend to try for me... it worked!

http://francisco.datazero.net/myhash_fail.jpg
http://francisco.datazero.net/myhash_ok.jpg

You're encouraged to test it on your machine too to see if I'm going crazy.

This is the code (quite simple script):

(Toggle Plain Text)

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

#define HASHELEMENTS 100
#define MAXLEN 100

struct htab {
    struct htab *child;
    struct htab *parent;
    unsigned int key;
    char value[MAXLEN];
};

struct htab *hashtab[HASHELEMENTS];

unsigned int hash (char *string) {
    unsigned int hashval;

    while (*string != '\0')
        hashval += *string++;

    return hashval % HASHELEMENTS;
}

// temporary debug functions
int testhash1(char *test1) {
    unsigned int hashval;

    return hash(test1);
}
int testhash2(char *test2) {
    unsigned int hashval;

    return hash(test2);
}

int main(){
    char string[MAXLEN];
    unsigned int hashval;
    struct htab *curhash;

    do {
        printf("enter value to hash: ");
        scanf("%s",&string);
        printf("hash1: %d\n",testhash1(string));
        printf("hash2: %d\n",testhash2(string));
    } while (1); // use ctrl+c to exit
}

Does anyone have any idea what's happening?

The Dark · Mar 28th, 2006, 11:43 PM

(Toggle Plain Text)

unsigned int hash (char *string) {
    unsigned int hashval = 0;

    while (*string != '\0')
        hashval += *string++;

    return hashval % HASHELEMENTS;
}

You have an uninitialised variable. The value that hashval starts with depends on what is currently in memory. Add in the code I put in red, above and you should be fine.

Mar 28th, 2006, 10:58 PM	#1
pulpfiction Newbie Join Date: Mar 2006 Posts: 1 Rep Power: 0	Strange behavior: getting different results from the same input on a function. WTH? I'm getting a strange behavior on this little script. I'm trying to implement a hash table, so I have a hash function which should always return the same numeric result for a given string. Problem is, depending on the parent function which is calling hash(), it returns different results for the same string. If I call it twice inside the same function it works ok. Now the really strange thing (well, I don't know, maybe I'm doing something wrong - I'm new to C) is this behavior depends on the machine I'm running the script. At university machines, running Windows, I got the error. Then I tried at home on Windows and Linux, same thing. I then asked for a friend to try for me... it worked! http://francisco.datazero.net/myhash_fail.jpg http://francisco.datazero.net/myhash_ok.jpg You're encouraged to test it on your machine too to see if I'm going crazy. This is the code (quite simple script): (Toggle Plain Text) #include <stdio.h> #include <stdlib.h> #include <string.h> #define HASHELEMENTS 100 #define MAXLEN 100 struct htab { struct htab child; struct htab parent; unsigned int key; char value[MAXLEN]; }; struct htab hashtab[HASHELEMENTS]; unsigned int hash (char string) { unsigned int hashval; while (string != '\0') hashval += string++; return hashval % HASHELEMENTS; } // temporary debug functions int testhash1(char test1) { unsigned int hashval; return hash(test1); } int testhash2(char test2) { unsigned int hashval; return hash(test2); } int main(){ char string[MAXLEN]; unsigned int hashval; struct htab curhash; do { printf("enter value to hash: "); scanf("%s",&string); printf("hash1: %d\n",testhash1(string)); printf("hash2: %d\n",testhash2(string)); } while (1); // use ctrl+c to exit } #include <stdio.h> #include <stdlib.h> #include <string.h> #define HASHELEMENTS 100 #define MAXLEN 100 struct htab { struct htab child; struct htab parent; unsigned int key; char value[MAXLEN]; }; struct htab hashtab[HASHELEMENTS]; unsigned int hash (char string) { unsigned int hashval; while (string != '\0') hashval += string++; return hashval % HASHELEMENTS; } // temporary debug functions int testhash1(char test1) { unsigned int hashval; return hash(test1); } int testhash2(char test2) { unsigned int hashval; return hash(test2); } int main(){ char string[MAXLEN]; unsigned int hashval; struct htab curhash; do { printf("enter value to hash: "); scanf("%s",&string); printf("hash1: %d\n",testhash1(string)); printf("hash2: %d\n",testhash2(string)); } while (1); // use ctrl+c to exit } Does anyone have any idea what's happening?

Mar 28th, 2006, 11:43 PM	#2
The Dark Expert Programmer Join Date: Jun 2005 Posts: 852 Rep Power: 4	(Toggle Plain Text) unsigned int hash (char string) { unsigned int hashval = 0; while (string != '\0') hashval += string++; return hashval % HASHELEMENTS; } unsigned int hash (char string) { unsigned int hashval = 0; while (string != '\0') hashval += string++; return hashval % HASHELEMENTS; } You have an uninitialised variable. The value that hashval starts with depends on what is currently in memory. Add in the code I put in red, above and you should be fine.

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Programming Forums > Application Development > C

Strange behavior: getting different results from the same input on a function. WTH?