Is it possible to have code like this?

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1.00/5 (1 vote)

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can you please tell me why is it possible to have codes like this?

#include <iostream>

using namespace std;

int main()
{
    int arr[4][4];
    int a, b;
    int temp;
    cout << "Enter four numbers: \n";
    for (a=0; a<4; a++)
    {
        cin >> arr[a][a];  >especially in this case is it alright to have code like this and why? because i was so confuse why is it okay to have a code like that  
    }
    cout << endl;
    cout <<"Even numbers: \n";
    for (a=0;a<4;a++)
    {
        if (arr[a][a]%2==0)
        {
            cout << arr[a][a]<<endl;
        }
    }
    cout << endl;
    cout << "Highest to lowest: "<< endl;
    for (a=0; a<4;a++)
    {
        for (b=a+1;b<4;b++)
        {
            if (arr[a][a]<arr[b][b])
            {
                temp=arr[a][a];  >and also can you please explain how sorting works? 
                arr[a][a]=arr[b][b];
                arr[b][b]=temp;
            }
        }
    }
    for(a=0;a<4;a++)
    {
        if (arr[a][a]%2==0)
        {
            cout<<arr[a][a]<<endl;
        }
    }
    return 0;
}
I'm sorry if I ask too much because I was a beginner in programming :< thank you

What I have tried:

I tried this code but I didn't expect it will run

Posted 6-Mar-20 22:44pm

Member 14765979

Updated 7-Mar-20 1:40am

v2

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Patrice T 7-Mar-20 6:45am

Where did you get that code from ?

jusjua 7-Mar-20 7:14am

I just made it by myself because this was a lab activity at my school

3 solutions

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phil.o · Answer 1 · 2020-03-06T22:56:00

Solution 1

It is perfectly valid to use the same value for both dimensions of a bidimensional array. Provided this value is a valid index into both dimensions.
Here, a variable gets successive values 0, 1, 2 and 3; this results in accessing respectively a[0][0], a[1][1], a[2][2] and a[3][3]. Why do you think this should not run?

Posted 6-Mar-20 22:56pm

phil.o

Comments

Member 14765979 7-Mar-20 5:02am

because I didn't expect it will run

phil.o 7-Mar-20 5:08am

Yes, that is what you said. My question is: why don't you expect it to run? Why do you think this is incorrect?

jusjua 7-Mar-20 5:15am

ohh, because I'm a beginner and i always think that 2d array is only compatible in this code like this arr[a][b], so when I try the code arr[a][a] I was confused at the same time happy because the program accept my code

phil.o 7-Mar-20 5:42am

a and b are just variables; logically, arr[a][a] and arr[a][b] are equivalent whenever a == b. Here, using the same value for both dimensions just means that you are accessing the elements in the diagonal of the square matrix represented by the array (see solution 2 for a graphical representation).
The only constraint to a and b is that they have to be a valid index into the array. Specifically, they must be greater than or equal to zero, and lower than the number of elements in relevant dimension.

Member 14765979 7-Mar-20 17:46pm

so is it okay to code like this in cin>>arr[a][a]?

phil.o 8-Mar-20 5:58am

As long as a is a valid index into both dimensions, it is okay.

Member 14765979 8-Mar-20 9:34am

thank you so much :D

OriginalGriff · Answer 2 · 2020-03-06T23:16:00

Just because you allocate a 4 x 4 array (16 elements) that doesn't mean you have to use them - the compiler doesn#t know (or care) that you don't, as long as all the elements you try a=to access are within the 4 x 4 block:

0, 0  0, 1  0, 2  0, 3
1, 0  1, 1  1, 2  1, 3
2, 0  2, 1  2, 2  2, 3
3, 0  3, 1  3, 2  3, 3

And the reason it doesn't care is simple: it has no idea if you have finished your code, or you have written a bit of it, are testing that it works, and will expand the number of elements you use later with additional code when it's OK so far. That's a reasonable way to do things - I do it all the time, write a bit, check it, write some more, check that, repeat.

So yes, your code could be simpler:

C++

int arr[4];
    int a, b;
    int temp;
    cout << "Enter four numbers: \n";
    for (a=0; a<4; a++)
    {
        cin >> arr[a];
    }
    cout << endl;
    cout <<"Even numbers: \n";
    for (a=0;a<4;a++)
    {
        if (arr[a]%2==0)
        {
            cout << arr[a]<<endl;
        }
    }
    cout << endl;

But it's perfectly valid to write more complex code that does the same thing!

Greg Utas · Answer 3 · 2020-03-07T01:40:00

Solution 3

You also asked about sorting. The algorithm you're using is called a bubble sort[^]. The linked article has links to other ones that describe other sorting techniques.

Posted 7-Mar-20 1:40am

Greg Utas