c++ array illegal or legal?

[weirdo123]

Hi i'm a beginner in c++, recently i found out there's legal and illegal array practice. i wanted to know if this code here is illegal or legal.
i have a display of 1-5 with arr[5] ={5,5,5,5,5} and the user will input a location, the location input eg: 1 then the array will be arr[0] since it start with 0..

[Zhuge]

If the user inputs 5, line 10 will access arr[5] which is illegal. Can you explain a bit more carefully what you are trying to do? Your code doesn't seem to match up to your explanation of what you want.

[weirdo123]

umm if the user input 5, line 10 will access arr[4] sry that was typo and im sorry if this is abit confusing..i cut up abit of my original code to have a more simpler code since i wanted to know if its illegal or not only..

[keskiverto]

Lets assume that the array is at start equal to

{ 6, 7, 8, 9, 10}

As user I choose location '2', so the first thing is to assing 0 to that position:

{ 6, 0, 8, 9, 10}

Please show what else should happen to the array.

[weirdo123]

{6, 0, (8+1), (9+1), (10+5)}

[JayhawkZombie]

So you're changing the value of the input - 1 index (remember we count from 0 in C++) and adding...what to the other values? Is (10+5) supposed to be (10+1)?

You can do that with a little loop. You already know where that loop has to start.
Hint: the user gave it to you

[weirdo123]

(10+5)

[JLBorges]

#include <iostream>

int main()
{
    const int N = 5 ;
    int array[5] { 3, 5, 4, 2, 0 } ;

    // print out the contents of the array
    for( int value : array ) std::cout << value << ' ' ;
    std::cout << '\n' ;

    int location_chosen_by_user = 3 ; // one-based
    const int actual_loc = location_chosen_by_user - 1 ; // zero-based

    if( actual_loc > 0 && actual_loc < N ) // if within bounds of the array
    {
        int seed = array[actual_loc] ; // save the seed
        array[actual_loc] = 0 ; // aet item to zero

        for( int i = actual_loc + 1 ; i < N ; ++i ) // for the remaining positions
        {
            ++array[i] ; // add one to the element
            --seed ; // and reduce seed by one
        }

        // at the end, if the user did not choose the last location,
        // add the residue left over in seed to the last location
        if( location_chosen_by_user != N ) array[N-1] += seed ;
    }

    // print out the contents of the modified array
    for( int value : array ) std::cout << value << ' ' ;
    std::cout << '\n' ;
}

http://coliru.stacked-crooked.com/a/43ed77211decfcf1

[weirdo123]

i

[JLBorges]

If the number of elements in the array is N, arr[i] is fine if i >= 0 && i < N,
otherwise it is undefined behaviour. The valid positions in the array are 0 to N-1, inclusive.

arr[location-1] is fine if and only if location >= 1 && location <= N.
arr[location+1] is fine if and only if location >= -1 && location <= (N-2).

[48T7M4Gy]

Rather than the smoke and mirrors approach of picking up a snippet here and snippet there the best way is to get a comprehensive understanding of arrays as a fundamental building block and spend a half an hour or so with a tutorial like http://www.cplusplus.com/doc/tutorial/arrays/

Straight away the pictorial demonstrations show how memory is set out and all the other aspects and beyond.