How can I check if vector elements are in order consecutively?

Question

I need to check if in my vector the elements are in order consecutively?

for(i=1; i<=K; i++)
    if(v[i]=v[i+1]-1)

If the statement would be true I want to return the biggest integer.

ex. 4 5 6 7

You want to check its sorted or you want the biggest element (even if its not sorted)? — Asha, Feb 03 '16 at 16:12
Please correct your question. Do you want to find out if your vector is sorted or do you want the largest number ? It is ambiguous at the moment. — gusaki, Feb 03 '16 at 16:16
Here is a link for your future questions: http://www.google.com — LogicStuff, Feb 03 '16 at 16:21
@schizoid Please don't post pseudo code here but a [MCVE], and exactly state what your problems or errors are. — πάντα ῥεῖ, Feb 03 '16 at 16:52

score 15 · Accepted Answer · answered Feb 03 '16 at 16:13

15

There's an algorithm for that: std::is_sorted:

if (std::is_sorted(v.begin(), v.end()) {
    return v.back(); // the largest element would be the last one
}
else {
    // ?? 
}

answered Feb 03 '16 at 16:13

Barry

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@schizoid That has nothing to do with using `std::is_sorted` – juanchopanza Feb 03 '16 at 16:18
@schizoid: So instead of fixing that major problem, you gave up and want to live on with a workaround? That's bad. It **must** work with a standard-library solution. – Christian Hackl Feb 03 '16 at 16:23
@ChristianHackl i have no idea how to fix it – supreme_tg Feb 03 '16 at 16:25
@schizoid Did you `#include `? – NathanOliver Feb 03 '16 at 16:27
@schizoid: Search Stackoverflow for people who've had the same problem in the past, and reuse the solution. – Christian Hackl Feb 03 '16 at 16:27
i put include algorithm and all that – supreme_tg Feb 03 '16 at 16:30
but it's a problem if I work with the classic vector? – supreme_tg Feb 03 '16 at 16:31
@schizoid: There is no such thing as a "classic vector". There is `std::vector` or `std::array`. If you are talking about raw arrays, then yes, it is a problem. – Christian Hackl Feb 03 '16 at 16:32
@ChristianHackl i didn't declare it as a container,I tried to,but it won't let me read it from file,I got an error "operator>>" something like that – supreme_tg Feb 03 '16 at 16:34
@schizoid: You cannot successfully use C++ without a decent textbook (which should include a section on simple text I/O and `operator>>`). Seriously, it's **hopeless** without a book. Please have a look here: http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list – Christian Hackl Feb 03 '16 at 16:37
@ChristianHackl algorithms work fine with raw arrays – Galik Feb 03 '16 at 16:37
@Galik: Judging from the OP's question and comments, the fact that arrays work with algorithms won't be much of a help. – Christian Hackl Feb 03 '16 at 16:38
@Barry I'm also having an issue where, even though I've included "#include " and "using namespace std;" I receive the error: "is_sorted() was not declared in this scope". Any ideas? – CCD Apr 10 '17 at 23:44

Christian Hackl · Answer 2 · 2017-10-11T09:31:10.850

I need to check if in my vector the elements are in order

Use C++11's std::is_sorted algorithm:

#include <iostream>
#include <vector>
#include <algorithm>

int main()
{
    std::vector<int> v1 { 4, 5, 7, 6 };
    std::vector<int> v2 { 4, 5, 6, 7 };

    using std::begin;
    using std::end;

    std::cout << std::is_sorted(begin(v1), end(v1)) << "\n";
    std::cout << std::is_sorted(begin(v2), end(v2)) << "\n";
}

If the statement would be true I want to return the biggest integer.

That's a job for std::max_element:

#include <iostream>
#include <vector>
#include <algorithm>

int main()
{
    std::vector<int> v1 { 4, 5, 7, 6 };
    std::vector<int> v2 { 4, 5, 6, 7 };

    using std::begin;
    using std::end;

    std::cout << *std::max_element(begin(v1), end(v1)) << "\n";
    std::cout << *std::max_element(begin(v2), end(v2)) << "\n";
}

Note that std::max_element does not require its input to be sorted.

Or if it's sorted anyway, just use v1.back().

score 2 · Answer 3 · answered Feb 03 '16 at 16:13

2

I would use:

is_sorted(begin(v), end(v));

If the sequence is not sorted then use:

max_element(begin(v), end(v));

to get the max.

answered Feb 03 '16 at 16:13

Jorge González Lorenzo

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score 2 · Answer 4 · answered Feb 03 '16 at 16:14

2

Use std::is_sorted algorithm of STL.

answered Feb 03 '16 at 16:14

Asha

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score 1 · Answer 5 · answered Feb 03 '16 at 16:12

1

bool test(vector<int> v) {
    for(int i = 0; i < v.size()-1; i++)
        if(v[i] > v[i+1])
            return false;
    return true;
}

If the vector is in order, so the biggest is the last one, that is, v[v.size()-1].

answered Feb 03 '16 at 16:12

Paulo

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score 1 · Answer 6 · answered Feb 03 '16 at 16:17

From the question it seems you are interested if the vector elements are consective integers. If that is the case you can use a flag to indicate if the condition holds and do a single iteration:

bool consecutive = false;
for (int i = 1; i < v.size(); ++i) {
  if (v[i] != v[i - 1] + 1) {
    consecutive = false;
    break;
  }
}
if (consecutive) {
   cout << v.back() << endl; // the last element is the biggest.
}

However your question title suggests you are interested in checking if the vector is sorted. If that is the case you can use the built-in function std::is_sorted that is present since c++11.

score 1 · Answer 7 · answered Feb 03 '16 at 16:33

Traverse through the vector starting from the second element and keep a flag variable initialized to 0, for each iteration check the following condition :-

if(v[i]<v[i-1])

if this condition is true, then set the flag to 1 and break out of the loop. After the control moves out of the loop, check the value of flag. If flag is 1, then the vector elements are not in order. Otherwise, if the flag is 0, then the vector elements are in order and you have to print the largest element which is v[v.size()-1].

How can I check if vector elements are in order consecutively?

7 Answers7

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