How do I find a particular value in an array and return its index

Question

Pseudo Code   int arr  5       4  1  3  2  6    x   x   find 3  arr      x would then return 2

User · Accepted Answer

The syntax you have there for your function doesn't make sense (why would the return value have a member called arr?).

To find the index, use std::distance and std::find from the <algorithm> header.

int x = std::distance(arr, std::find(arr, arr + 5, 3));

Or you can make it into a more generic function:

template <typename Iter>
size_t index_of(Iter first, Iter last, typename const std::iterator_traits<Iter>::value_type& x)
{
    size_t i = 0;
    while (first != last && *first != x)
      ++first, ++i;
    return i;
}

Here, I'm returning the length of the sequence if the value is not found (which is consistent with the way the STL algorithms return the last iterator). Depending on your taste, you may wish to use some other form of failure reporting.

In your case, you would use it like so:

size_t x = index_of(arr, arr + 5, 3);

User · Answer

Here is a very simple way to do it by hand   You could also use the  lt algorithm gt   as Peter suggests    include  lt iostream gt  int find int arr    int len  int seek        for  int i   0  i  lt  len    i                if  arr i     seek  return i            return -1    int main         int arr  5       4  1  3  2  6        int x   find arr 5 3       std  cout  lt  lt  x  lt  lt  std  endl

User · Answer

include  lt vector gt   include  lt algorithm gt   int main          int arr 5     4  1  3  2  6        int x   -1       std  vector lt int gt  testVector arr  arr   sizeof arr    sizeof int           std  vector lt int gt   iterator it   std  find testVector begin    testVector end    3        if  it    testVector end                     x   it - testVector begin                return 0      Or you can just build a vector in a normal way  without creating it from an array of ints and then use the same solution as shown in my example

User · Answer

If the array is unsorted  you will need to use  linear search

User · Answer

the fancy answer  Use std  vector and search with std  find  the simple answer  use a for loop

User · Answer

We here use simply linear search  At first initialize the index equal to -1   Then search the array   if found the assign the index value in index variable and break  Otherwise  index   -1      int find int arr    int n  int key            int index   -1          for int i 0  i lt n  i                       if arr i   key                          index i              break                             return index           int main          int arr  5       4  1  3  2  6        int n    sizeof arr  sizeof arr 0        int x   find arr  n  3       cout lt  lt x lt  lt endl      return 0

User · Answer

int arr 5     4  1  3  2  6   vector lt int gt  vec  int i  0  int no to be found   cin  gt  gt  no to be found   while i    4        vec push back arr i        i       cout  lt  lt  find vec begin   vec end   no to be found  - vec begin

[c++] How do I find a particular value in an array and return its index?

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