Array outputing garbage

**steals10304** · 08-04-2009

Hello!

I'm new to C and am working on a problem I was hoping to get some help. I am trying to create a survey asking a group of people how they liked a new product on a scale of x to y (these must be defined as Macros and must be able to accept negative numbers). The program outputs garbage values for my negative values instead of 0's. I'm sure it is an easy fix? I would love constructive criticism on my code as a whole too. Thanks. Code is as follows...

Code:

#include <stdio.h>
#include <stdlib.h>
#define MAX_RESPONSE_VALUE 5
#define MIN_RESPONSE_VALUE -5
#define MAX_RESPONDENTS 5
#define ERROR_EXIT 3


int main(void)
{
   int responsevalue[ MAX_RESPONSE_VALUE - MIN_RESPONSE_VALUE + 1 ] = {0};
   int temp = 0;
   int errorcounter = 0;
   int usercounter = 1;
   int printloop;

   do
   {
      printf("PERSON NUMBER %d, How did you like our new product?\n", usercounter);
      printf("Please enter a number on a scale of %d to %d,\n", MIN_RESPONSE_VALUE, MAX_RESPONSE_VALUE);
      printf("Where %d is \"I Hated It!\" and %d is \"I Loved It!\"\n", MIN_RESPONSE_VALUE, MAX_RESPONSE_VALUE);
      printf("(To exit program early, Please enter %d Out Of Range responses.)\n: ", ERROR_EXIT);
      scanf("%d", &temp);
      printf("\nThank you.\n\n");
   
      if (temp >= MIN_RESPONSE_VALUE && temp <= MAX_RESPONSE_VALUE)
      {
         responsevalue[ temp - 1 ]++;
         usercounter++;
      }
      else
      {
         printf("Error: Value out of range.  Try Again.\n\n");
         errorcounter++;
      }
   }
   while (errorcounter < ERROR_EXIT && usercounter <= MAX_RESPONDENTS);
   
      printf("Here are your results!\n\nRATING     # OF RESPONSES\n\n");
      for(printloop = MIN_RESPONSE_VALUE - 1; printloop < MAX_RESPONSE_VALUE; printloop++)
         printf("%d           %d\n", printloop + 1, responsevalue[printloop]);
      

   return(EXIT_SUCCESS);
}

**GL.Sam** · 08-04-2009

No wonder. Do you know it's not allowed to use negative numbers as array indexes?

**GL.Sam** · 08-04-2009

responsevalue[ temp - 1 ]++;

There. Make it temp + 5.

MIN_RESPONSE_VALUE - 1

Same.

**BEN10** · 08-04-2009

Here's an example where you can use negative index using pointer.

Code:

int a[5]={1,2,3,4,5};
int *p;
p=&a[2];
printf("%d",p[-1]); // will print 2

**GL.Sam** · 08-04-2009

using pointer
Yes, pointer, since it is merely translated to *(p - 1). Do you want to raise a discussion about esoteric trivia, or we'd rather stick to good programming style?

Btw, 1[p - 2] is perfectly valid too.

**Kennedy** · 08-04-2009

I think it is more of you spoke in absolutes, thus that has a HIGH probability that you are wrong.

**brewbuck** · 08-04-2009

Originally Posted by GL.Sam

Do you want to raise a discussion about esoteric trivia, or we'd rather stick to good programming style?

The equivalence between a[b] and *(a+b) is not "esoteric trivia" any more than Einstein's equivalence of E = mc^2 is esoteric trivia. It is a critical link between arrays and pointers, and in my experience, it's the piece of information that finally "clicks" the understanding of pointers for a lot of people.

**GL.Sam** · 08-04-2009

Okay, let's rewind it to starting point. First off, when I say a word I'm not trying to make anyone believe it like undoubtedly truth in its last occurence. Perhaps, I should've added clear "In general" to my first sentence. Before anything else I'm trying to help a novice, bearing that intention in mind, I assume that usually there is no need of stating special cases. Topic starter haven't posed a question about relation between arrays and pointers, am I right? In fact, there were no pointer occurences at all. I think he just looked forward for receiving just-at-the-moment help.

**BEN10** · 08-05-2009

I was just giving an example of negative array indices, it's a different fact that it's a good programming style or not, but it's valid too.

**iMalc** · 08-05-2009

Code:

#define MAX_RESPONSE_VALUE 5
#define MIN_RESPONSE_VALUE -5

   int responsevalue[ MAX_RESPONSE_VALUE - MIN_RESPONSE_VALUE + 1 ] = {0};

Okay, the above declares an array of 11 elements, so only indexes 0 to 10 are valid. With me so far?

Code:

      for(printloop = MIN_RESPONSE_VALUE - 1; printloop < MAX_RESPONSE_VALUE; printloop++)
         printf("%d           %d\n", printloop + 1, responsevalue[printloop]);

This for-loop iterates over indexes -6 to 4, and those indexes are used in the array lookup - UH OH!
See the problem? It's called buffer underrun. The fix ... well IMHO the below is probably what you are after:

Code:

      for(printloop = MIN_RESPONSE_VALUE; printloop <= MAX_RESPONSE_VALUE; printloop++)
         printf("%d           %d\n", printloop, responsevalue[printloop - MIN_RESPONSE_VALUE]);

The for loop iterates over -5 to 5 inclusive, and it access array elements 0 to 10

Originally Posted by Gl.Sam

There. Make it temp + 5.

Sounds like you're suggesting hardcoding a 5 in there which would mean that changing the value of the #define wouldn't quite work any more. The correct change for that line is:

Code:

responsevalue[temp - MIN_RESPONSE_VALUE]++;

which as you can see mirrors the above fix to the printf in the for loop.

**steals10304** · 08-05-2009

Worked like a charm!

**roaan** · 08-05-2009

Originally Posted by BEN10

Here's an example where you can use negative index using pointer.

Code:

int a[5]={1,2,3,4,5};
int *p;
p=&a[2];
printf("%d",p[-1]); // will print 2

In the declaration

p = &a[2] is the & sign not redundant. The arrays are supposed to decay into pointers.

**KBriggs** · 08-05-2009

a[2] is the contents of the memory adress a+2
&a[2] is the memory address

ie a[2] is equivelent to *(a+2)
&a[2] is equivelent to a+2

**BEN10** · 08-05-2009

Originally Posted by roaan

In the declaration

p = &a[2] is the & sign not redundant. The arrays are supposed to decay into pointers.

Just imagine, p is a pointer, so it holds address of any variable, and a[2] is the value of the 3rd element, so it cant be p=a[2].(actually it could be, but while doing *p will create undefined behaviour). But in my example it cant be.

Thread: Array outputing garbage

Thread Tools

Search Thread

Display

Array outputing garbage

Thank you Imalc

Similar Threads

from 2D array to 1D array

Outputing an array to a textbox ?

Unknown Memory Leak in Init() Function

Quick question about SIGSEGV

Array Program

Tags for this Thread