How to use lower_bound(upper_bound) to find position of any number in array?

Question

For example, I have an array with elements 1,7,9,23,34,47,67,89,123,234,345,567. I need to know the position of 123.

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std::find Ought to work just fine. You can subtract std::begin(array) to get an (numerical) index rather than an iterator if that's what you need. – Justifier

Have you tried anything? please provide a Minimal, Complete, and Verifiable example – Osbourne

@Justifier with std::find you have the same problem. The function returns that iterator, not the position. So you still have to do the calculation in User_Targaryen's answer. – Cobham

@George, I think that the OP provided a complete minimal example (it doesn't look like they've edited it since your comment either). – Cobham

@Cobham All I can see is "For example, I have an array with elements 1,7,9,23,34,47,67,89,123,234,345,567. I need to know the position of 123.". The sequence bit doesn't count, it needs to be a code example, a C++ code example really from the tag. – Osbourne

Together with the title of the question, it seems pretty explicit: std::lower_bound on an array to get the index instead of the element. Sure, the OP's question could be a bit better, but not enough to require revision. – Cobham

Answer 1

Declare and initialize the array with the elements. Then create a vector of ints. Use upper_bound() on the vector. Here is an example:

#include<iostream>
#include<algorithm>
#include<vector>
using namespace std;

int main() {
    int arr[] = {1,7,9,23,34,47,67,89,123,234,345,567};
    int len = sizeof(arr)/sizeof(arr[0]);
    vector<int> v(arr,arr+len);

    vector<int>::iterator upper;
    upper = upper_bound(v.begin(), v.end(), 123);
    cout<<(upper-v.begin())<<endl;  // Output: 9

    return 0;
}

Hope it helps!!

Zeus

You can use std::upper_bound and std::lower_bound directly on the array. There's no need for that vector. – Amputee

std::upper returns the first value greater than the sought one. If, for instance, you had sought for value 122 you would still get position 9 (or rather, 8), even though the value you looked for wasn't found. – Tugman

@Cobham No, it's actually as I wrote it. Have a look: en.cppreference.com/w/cpp/algorithm/upper_bound You're thinking of std::lower_bound which returns an iterator to either the value you're looking for or the one preceding it. For both std::lower_bound and std::upper_bound end() is returned if no suitable value could be found. – Tugman