Why we cant reverse a array like this ? what is the problem with the code?

Question

1.00/5 (1 vote)

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#include <iostream>
using namespace std;

int main() {
        int n;
        cin>>n;
        int a[n];
        for(int i=0; i<n; i++){
            cin>>a[i];
        }
        cout<<a[n-1]<<endl;
        cout<<a[0]<<endl;
     for(int i=n-1; i>=0; i++){
         cout<<a[i];
     }
	return 0;
}

What I have tried:

I am not able to figure out why the second loop is printing some garbage values.

Posted 29-Sep-22 7:30am

saurav dadwal

Updated 29-Sep-22 10:57am

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4 solutions

Solution 1

You're starting at the end of the line and incrementing so yeah it will be garbage. You should start at end of line and decrement, as shown in bolded code.

#include <iostream>
using namespace std;

int main() {
        int n;
        cin>>n;
        int a[n];
        for(int i=0; i<n; i++){
            cin>>a[i];
        }
        cout<<a[n-1]<<endl;
        cout<<a[0]<<endl;
     for(int i=n-1; i>=0; i--){
         cout<<a[i];
     }
	return 0;
}

Posted 29-Sep-22 7:47am

Mike Hankey

Comments

CPallini 29-Sep-22 16:50pm

5.

Solution 2

for(int i=n-1; i>=0; i++){
         cout<<a[i];
     }

Shouldn't that be an i--?

Posted 29-Sep-22 7:49am

jeron1

Comments

CPallini 29-Sep-22 16:50pm

5.

Solution 3

Because you are counting up, not down:

for(int i=n-1; i>=0; i++){

So you start with the last element and then move to the non-existent one after that.
Try "--" instead of "++"

To be honest, you would have spotted that for yourself in under a minute if you had run that code in the debugger - I'd strongly recommend that you get used to using that, as it's your best friend in development and it's easier to get used to it with simple code like this than with a 100,000 line behemoth later!

Posted 29-Sep-22 7:49am

OriginalGriff

Comments

CPallini 29-Sep-22 16:51pm

5.

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CPallini · Accepted Answer · 2022-09-29T10:57:00

Solution 4

If you really like to increment...

C++

for (auto it = rbegin(a); it != rend(a); ++it)
  cout << *it << " ";

Posted 29-Sep-22 10:57am

CPallini

Comments

jeron1 29-Sep-22 18:25pm

Nice!