CODEWITHSUNDEEP
arraysc
Ryan Kafoor
Ryan Kafoor

Reputation: 1

Getting unexpected output in C while accessing array. Getting mostly 0 as output

This is my code

#include<stdio.h>


int main()
{
  int n,a[n],op;
  printf("Enter size of the array\n");
  scanf("%d",&n);
  printf("Enter the array elements\n");
  for(int i=0;i<n;i++)
  {
    scanf("%d",&a[n]);
  }
  printf("Select array element to display from 1 to %d\n",n);
  scanf("%d",&op);
  if(op-1>=0&&op-1<n)
  {
    printf("Element is %d\n",a[op-1]);
  }
  else
  {
    printf("Invalid Entry\n");

  } 
  return 0;
}

Output

Enter size of the array 
5
Enter the array elements
2
5
6
1
5
Select array element to display from 1 to 5
2
Element is 0

Why is this happening? I couldn't figure this out

Upvotes: 0

Views: 60

Answers (3)

paxdiablo
paxdiablo

Reputation: 881113

Consider your code:

int n,a[n],op;

Given that the value of n is undefined at this point, the variable length array (VLA) created by a[n] will be of some arbitrary size. C doesn't magically reach back in time from the point where you request a value of n and adjust the array size :-)

You need to create the VLA after you've set the value of n, reorganising your code at the start of main with something like this:

int n,op;
printf("Enter size of the array\n");
scanf("%d",&n);
int a[n];

That way, n will be a known value when you use it to create the VLA.

Additionally, your scanf("%d",&a[n]); is undefined behaviour because n will always be beyond the end of the array (which has indexes 0 through n-1 inclusive). You should be using i as the index there.

There are other logic issues such as checking that the scanf succeeded and that you didn't enter a non-positive number but that's usually okay for educational code (assuming that's what this is). As you develop as a coder, you will naturally begin to think of these possibilities and harden your code accordingly.

By way of example, this is a better way to enter n, although you could recover from non-numeric data as an optional extra (see here for a nifty line input solution in C):

int n = -1;
while (n < 1) {
    printf("Enter array size (positive integer): ");
    if (scanf("%d", &n) != 1) {
        puts("\nERROR: non-numeric");
        exit(1);
    }
}

Upvotes: 4

programmerc3981143
programmerc3981143

Reputation: 94

The error is in this part of your code:

  int n,a[n],op; 
  
  // int n is not defined as anything so it is a garbage value
  // you specify the size of the array to be n
  // which allocates an undefined size for the array

Also in your first loop you reassign &a[n], which is undefined since it goes beyond the size of the array and even if it were valid it would just reassign a single element, it should be &a[i].

to fix it read the value of n before defining array 'a'. The changes i made are down below:

#include<stdio.h>


int main()
{
  int n; // change
  printf("Enter size of the array\n");
  scanf("%d",&n);

  int a[n],op; // change

  printf("Enter the array elements\n");
  for(int i=0;i<n;i++)
  {
    scanf("%d",&a[i]); // change
  }
  printf("Select array element to display from 1 to %d\n",n);
  scanf("%d",&op);
  if(op-1>=0&&op-1<n)
  {
    printf("Element is %d\n",a[op-1]);
  }
  else
  {
    printf("Invalid Entry\n");

  } 
  return 0;
}

Upvotes: 0

heysujal
heysujal

Reputation: 138

scanf("%d",&a[n]);

should be scanf("%d",&a[i]);

Upvotes: 2

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