Test if all elements of a vector are equal

Question

I want to test if a non-empty vector contains identical elements. Is this the best way?

count(vecSamples.begin()+1, vecSamples.end(), vecSamples.front()) == vecSamples.size()-1;

It is simple, sure, but inefficient if they are different. Plus you could simplify it further by removing the +/- 1's. Especially since this would blow up on an empty vector. — Ryan Guthrie, Mar 20 '13 at 18:00
Possible duplicate of [C++: Fastest method to check if all array elements are equal](http://stackoverflow.com/questions/14120346/c-fastest-method-to-check-if-all-array-elements-are-equal) — Ciro Santilli OurBigBook.com, Apr 28 '16 at 14:26
Possible duplicate of [Checking if all elements of a vector are equal in C++](https://stackoverflow.com/questions/20287095/checking-if-all-elements-of-a-vector-are-equal-in-c) — Virus_7, Feb 15 '19 at 03:09

score 9 · Answer 1 · answered Mar 20 '13 at 18:01

9

In c++11 (or Boost Algorithm)

std::all_of(vecSamples.begin()+1,vecSamples.end(),
          [&](const T & r) {return r==vecSamples.front();})

answered Mar 20 '13 at 18:01

sbabbi

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I'm aware of the fact that OP is asking about a *non-empty* vector. Even so, I think that adding `!vecSamples.empty() ||` to your expression before the call to `std::all_of()` wouldn't damage your answer. What do you think? – JFMR Jan 27 '19 at 08:57
Simply replacing the `vecSamples.begin()+1` with just `vecSamples.begin()` in your call to `std::all_of()` also makes your code suitable for working with empty vectors, since the predicate won't be evaluated on an empty vector. – JFMR Jan 27 '19 at 09:03

Ed Rowlett-Barbu · Accepted Answer · 2013-03-20T18:30:24.970

As @john correctly points out, your solution iterates over the entire container even if the first two elements are different, which is quite a waste.

How about a purely no-boost no c++11 required solution?

bool allAreEqual = 
  find_if(vecSamples.begin() + 1, 
    vecSamples.end(), 
    bind1st(not_equal_to<int>(), vecSamples.front())) == vecSamples.end();

Stops on first non-equal element found. Just make sure your vecSamples is non-empty before running this.

score 2 · Answer 3 · answered Mar 20 '13 at 17:59

2

Probably not, because it always examines all the elements of the vector even if the first two elements are different. Personally I'd just write a for loop.

answered Mar 20 '13 at 17:59

john

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score 1 · Answer 4 · answered Jan 22 '16 at 23:39

1

If your vector contains at least one element:

std::equal(vecSamples.begin() + 1, vecSamples.end(), vecSamples.begin())

answered Jan 22 '16 at 23:39

Daniel Laügt

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Test if all elements of a vector are equal

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