Loop that contains arrays

[christianB]

I have an assignment that requires me to write code that compares two arrays and confirms if every element in the first is equal to every element in the second. this is my code.

#include <stdio.h>

#define DIM 16

int main() {
   int ndx, vals[DIM], vals2[DIM];

   printf("Enter %d numbers: ", DIM);
   for (ndx = 0; ndx < DIM; ndx++)
      scanf("%d", &vals[ndx]);

   printf("And another %d numbers: ", DIM);
   for (ndx = 0; ndx < DIM; ndx++)
      scanf("%d", &vals2[ndx]);
   
   for (ndx = 0; vals[ndx] == vals2[ndx]; ndx++){}
   if (ndx == DIM + 2)
	   printf("Equal\n");
   else
	   printf("Not equal\n");

}

This code works (I came up with everything after the 2nd scanf). I get no compile errors and I pass all test cases. However, my professor's auto grader is not accepting it and it is not saying why. Can someone tell me what is wrong with my code? I don't see what it is. Thank you for your help.

[Matticus]

for (ndx = 0; vals[ndx] == vals2[ndx]; ndx++){}
   if (ndx == DIM + 2)
	   printf("Equal\n");
   else
	   printf("Not equal\n");

First of all, I don't think the braces after the "for()" statement are intended to be like that.

Second of all, you seem to have the logic all skewed. This is how I read it:

(1) nxd = 0
(2) if ndx == 16+2, print "equal" - otherwise, print "not equal"
(3) increment ndx
(4) if the values in the two arrays are not equal, the "for()" loop ends - otherwise, loop.

You aren't basing the "equal"/"not equal" output on the equality of the array indices, but on the equality of your counter variable to a seemingly arbitrary value.

Also, the "for()" loop is not guaranteed to stop before you overrun the bounds of your arrays; i.e., "ndx" can in some situations run beyond the value of DIM.

[TheBigH]

Why is it "DIM + 2" instead of just plain DIM?

[christianB]

those braces are unnecessary i guess. so you're saying i'm not asking for "equal/not equal" based on weather or not the indices are equal, but only if my counter gets up to a number I want it to?

my logic was:

1. nxd = 0
2. if val[nxd] == val2[nxd], increment ndx.
3. repeat 1 and 2, until val[ndx] != val[ndx], stop the loop.
4. if at the point the loop stops, ndx is not equal to the size of the NMD,
that is, if I didn't get 16 "val[nxd] == val2[ndx] is true" instances, then the arrays were not equal.

it seemed right and i got it to work how I wanted, but it seems a little strange to me now.

it's DIM + 2, because when all of the indices of both arrays are equal, "ndx" goes up to 18. I don't know why it goes up to 18 though.

[TheBigH]

As Matticus says, you are looking at "array elements" out past the ends of your arrays once ndx exceeds DIM-1. There is no way of knowing what is in those memory locations. Maybe they contain the same information for a few more tries, but this is a matter of pure chance. It doesn't work on my machine for instance.

[AndrewHunter]

1. I see what you were thinking but your implementation is flawed. You should never just allow the program to go marching into memory it doesn't own, aka unchecked incrementing past the bounds of your array. You are just asking for problems. You could do it in one line like:

for(ndx=0;(vals[ndx] == vals2[ndx]) && ndx < DIM;ndx++);

This way you ensure you do not pass the bounds of your array.

Why is it DIM+2? Well when you access memory that isn't yours you are into undefined behaviour and anything can happen. Don't do it.

[christianB]

so because I didn't have "ndx < DIM" as a condition in the loop it was going on past the 15th index of both the arrays? It was just my luck that those values happened to be equal huh? I understand though. Originally, i had the if statement in the loop, but that would just give me a whole column of "Equal"s, so I was just trying to find a way around that. Thanks again for the help.

[AndrewHunter]

You had a good idea, just missing some pieces. It is all part of the learning process.

[christianB]

so this should work right?

#include <stdio.h>

#define DIM 4

int main() {
   int ndx, vals[DIM], vals2[DIM];

   printf("Please enter 16 numbers.\n");
   for (ndx = 0; ndx < DIM; ndx++){
	   scanf("%d", &vals[ndx]);
   }

   printf("And another 16 numbers.\n");
   for (ndx = 0; ndx < DIM; ndx++){
	   scanf("%d", &vals2[ndx]);
   }

   for(ndx=0; (vals[ndx] == vals2[ndx]) && ndx < DIM; ndx++);

   if (vals[ndx] == vals2[ndx])
	   printf("Equal\n");
   else
	   printf("Not equal\n");

}

[Matticus]

   for(ndx=0; (vals[ndx] == vals2[ndx]) && ndx < DIM; ndx++);

   if (vals[ndx] == vals2[ndx])
	   printf("Equal\n");

You've already taken care of the array comparison in the "for()" loop. Now check the "ndx" value in the "if()" statement. If the arrays were all equal, then "ndx" would have incremented all the way to DIM. If a set of array indices were not equal, then the loop would have broken and "ndx" would be less than DIM.

[AndrewHunter]

Uh....no. Let's look:

#define DIM 4

int main() {
   int ndx, vals[DIM], vals2[DIM];

   printf("Please enter 16 numbers.\n");

You only are capable of taking in 4 numbers but you ask the user for 16.

//this will stop looping if either: values are not equal or you are at the last number of your array
for(ndx=0; (vals[ndx] == vals2[ndx]) && ndx < DIM; ndx++);
//that means you need to do if(ndx==DIM)....to continue with your logic
   if (vals[ndx] == vals2[ndx])
	   printf("Equal\n");
   else
	   printf("Not equal\n");

[christianB]

of course. i'm just being redundant. it works now. thanks for all the help. I changed DIM to 4 because I was tired of typing 16 numbers over and over to test it. 16 is supposed to be a %d that reads in DIM. I changed it back though.

[AndrewHunter]

of course. i'm just being redundant. it works now. thanks for all the help. I changed DIM to 4 because I was tired of typing 16 numbers over and over to test it. 16 is supposed to be a %d that reads in DIM. I changed it back though.

No, your code wouldn't have worked; it isn't redundant. You are trying to access outside of your array again. Think about how the for statement would actually terminate. Here is a simple example to demonstrate the problem, can you guess what value ndx is?

#include <stdio.h>

int main(void){

	const int DIM = 10;
	int ndx = 0;

	for(ndx=0;ndx < DIM;ndx++);

	printf("%d",ndx);
	getchar();
	return(0);
}

Remember, in C arrays begin with zero, so if you have val[10] you can only access val[0] through val[9].

[CommonTater]

Hard way...

   for(ndx=0; (vals[ndx] == vals2[ndx]) && ndx < DIM; ndx++);

   if (vals[ndx] == vals2[ndx])
	   printf("Equal\n");
   else
	   printf("Not equal\n");

Easy way...

   for(ndx=0; ndx < DIM; ndx++)
     if (vals[ndx] != vals2[ndx])
       { printf("Not Equal\n");
         return 0; }

    printf("Equal\n");

    return 0; }