How to detect if a array is sorted.

**just_rookie** · 09-08-2012

Hi,all!
Is there a way to detect if a array is sorted or not?

**whiteflags** · 09-08-2012

Loop through the array with the same comparison(s) as a sort. For example, integers stored in ascending order means that the smaller integers will be encountered first. If you find an integer somewhere that is out of order then you know it isn't sorted.

**laserlight** · 09-08-2012

Originally Posted by just_rookie

Is there a way to detect if a array is sorted or not?

whiteflags' suggestion is good for a novice to practice, but once you have that practice, it would be simpler to #include <algorithm> then use the is_sorted generic algorithm.

**rodrigorules** · 09-09-2012

Great suggestion laserlight.

just_rookie if you would like to 'roll your own', just iterate over the array and check if the current element is 'less than' the next element. If you receive true over the entire array, it is sorted.

**iMalc** · 09-09-2012

I thought that the definition of "sorted" implied an obvious simple algorithm for checking it.

You might benefit from learning how to write that.

**just_rookie** · 09-09-2012

Originally Posted by whiteflags

Loop through the array with the same comparison(s) as a sort. For example, integers stored in ascending order means that the smaller integers will be encountered first. If you find an integer somewhere that is out of order then you know it isn't sorted.

my code as follows.I used your method,but it cannot solve this problem:

Code:

:
#include <iostream>
#include <ctime>
#include <cstdlib>

using namespace std;

void displayarr( int arr[],int size )
{
    for ( int i = 0;i < size;i++ )
    {
        cout << arr[i] << "  ";
        }
    }

int main()
{
    int arr[10];
    srand(time(NULL));
    for ( int i = 0;i < 10;i++ )
    {
        arr[i] = rand() % 100;
    }
    for ( int i = 0 ;i < 9;i++ )
    {
    if ( arr[ i ] < arr[ i+1 ] )
        {
       cout << "Sorted!" << endl;
    displayarr(arr,10);
        }
    }
}

**just_rookie** · 09-09-2012

Originally Posted by laserlight

whiteflags' suggestion is good for a novice to practice, but once you have that practice, it would be simpler to #include <algorithm> then use the is_sorted generic algorithm.

whiteflags' suggestion is good for me,because I didn't learn generic algorithm.Thank you for your reply!

**whiteflags** · 09-09-2012

The biggest mistake you made was writing "Sorted!" and displaying the array each time around the loop. You won't know until you've compared each element in turn once. Your if statement is fine, but you don't do anything if the if condition is false, which is a mistake.

**just_rookie** · 09-09-2012

Originally Posted by whiteflags

The biggest mistake you made was writing "Sorted!" and displaying the array each time around the loop. You won't know until you've compared each element in turn once. Your if statement is fine, but you don't do anything if the if condition is false, which is a mistake.

hi,someone send me a piece of code as follows,but i can not understand this code well,so please do me a favor!Thank you!

Code:

#include <cstdlib>
#include <ctime>
#include <iostream>
using namespace std;
int main()
{
 int i;
 int p[4];
 cout << "The original array is:" << endl;
 srand(time(NULL));
 for(i=0;i<4;i++)
 {
 p[i] = rand()%10;
 cout << p[i] << "  ";
}
 for(i=0;i<3;i++)
 {
 if(p[i]>p[i+1])
 {
 break;
 }
 }
 if(i==3)
 {
 printf("Ascending array！\n");
 }
 else
 {
 for(i=0;i<3;i++)
 {
 if(p[i]<p[i+1])
 {
 break;
 }
 }
 if(i==3)
 {
 printf("Descending array！\n");
 }
 else
 {
 printf("Unsorted array！\n");
 }
 }
 free(p);
 return 0;
}

**laserlight** · 09-09-2012

If I were you, I would write a function like this:

Code:

bool array_is_sorted(int arr[], int size)
{
    // ...
}

Then call it from the main function.

Originally Posted by just_rookie

hi,someone send me a piece of code as follows,but i can not understand this code well,so please do me a favor!Thank you!

Concentrate on your own code. You are not that far away from a working solution.

**rodrigorules** · 09-09-2012

just_rookie, it looks like the logic you have in your previous post's code is correct.

What is the problem?

**just_rookie** · 09-10-2012

Originally Posted by rodrigorules

just_rookie, it looks like the logic you have in your previous post's code is correct.

What is the problem?

My problem is why we need the "if(i==3)" statement?

**laserlight** · 09-10-2012

Originally Posted by just_rookie

My problem is why we need the "if(i==3)" statement?

That is one way to check if all elements of the array have been checked. The array has 4 elements, so you only need to check for 3 elements (since you are checking the current and next). However, if you write the function that I suggested, you don't need to do this since you would just return true after the loop ends.

**just_rookie** · 09-10-2012

Originally Posted by laserlight

That is one way to check if all elements of the array have been checked. The array has 4 elements, so you only need to check for 3 elements (since you are checking the current and next). However, if you write the function that I suggested, you don't need to do this since you would just return true after the loop ends.

But when i =2,we can get "if(p[2]>p[3])…" or "if(p[2]<p[3])…",so i mean that we already checked "p[3]",why we need to do it again?

**whiteflags** · 09-10-2012

You actually never check p[3] because p[3] is outside the valid range of array elements. The only reason the if (i==3) is necessary is because the implementation you were gifted is so ad hoc that there is no other way to determine the results. You were doing better on your own. You were also given fairly blunt hints in the thread, so I earnestly believe you have this.

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