What is wrong with this code? The answer should be 24 for this question right?

Question

int t; 
vector<int>arr={24,434};

for(int i=arr.size()-1;i>=(arr.size()-2);i--)
{

        t=i;
}

cout<<arr[t];

***What is wrong with this code*** Why do you feel something is wrong with this code? Do you face any issues? — kiner_shah, Jan 20 '22 at 08:01
Welcome to Stack Overflow. Please read [the help pages](http://stackoverflow.com/help), take the SO [tour], read [ask], as well as [this question checklist](https://codeblog.jonskeet.uk/2012/11/24/stack-overflow-question-checklist/). Lastly please learn how to [edit] your questions to improve them, like telling us the expected and actual behavior, and what efforts at *debugging* you have made. — Some programmer dude, Jan 20 '22 at 08:02
And where did you get the shown code from? Did you write it yourself? Is it part of some assignment or exercise? An exam perhaps? Or an interview? I wonder because the loop doesn't make much sense, and might actually lead to undefined behavior if there was only a single element in the vector. — Some programmer dude, Jan 20 '22 at 08:04
You could have seen a warning message "comparison of signed and unsigned integer values" thrown by the compiler if you enabled warnings during compilation. — kiner_shah, Jan 20 '22 at 10:34

Vlad from Moscow · Accepted Answer · 2022-01-20T08:27:22.247

This loop

for(int i=arr.size()-1;i>=(arr.size()-2);i--)

is an infinite loop.

When i is equal to 0 it is decremented due to the third expression of the loop and becomes equal to -1.

And then in this condition

i>=(arr.size()-2)

as the operand arr.size()-2 has unsigned integer type with the rank not less than the rank of an object of the type int then due to the usual arithmetic conversions the expression i is converted to the unsigned integer type that corresponds to the type of the expression arr.size()-2 and becomes a very big value due to promoting the bit sign.

That is in this expression

i>=(arr.size()-2)

the left operand can not be a negative value.

In fact this loop

for(int i=arr.size()-1;i>=(arr.size()-2);i--)

does not make a sense because in any case there can be no more than one iteration.

The loop is not infinite, it finishes with int overflow, so UB. — Jarod42, Jan 20 '22 at 11:15

score 0 · Answer 2 · answered Jan 20 '22 at 08:19

Your loop never ends. Because an unsigned integer can never be -1.

Instead, try this smart approach:

#include <iostream>
#include <vector>


int main( )
{
    std::vector<int> arr { 24, 434 };
    std::size_t t { };

    for ( std::size_t idx { arr.size( ) }; idx--; )
    {
            t = idx;
    }

    std::cout << arr[ t ];
}

Output:

As you can see, the loop header does not have an update section, and the condition section also performs the task of decrementing.

What is wrong with this code? The answer should be 24 for this question right?

2 Answers2