How to find the position of a number in an array?

If I have a vector, a = [7 8 8 2 5 6], how do I compute the positions of the value 8?

I expect 2 and 3 or (1,2) and (1,3).

Answer 1

[Walter Roberson]

You can use the “find” function to return the positions corresponding to an array element value. For example:

a = [7 8 8 2 5 6]; 
linearIndices = find(a==8) 
 
linearIndices = 
     2     3 

To get the row and column indices separately, use:

[row,col] = find(a==8) 
 
row = 
     1     1 
 
col = 
     2     3 

If you only need the position of one occurrence, you could use the syntax “find(a==8,1)”. You can also specify a direction if you specifically want the first or last occurrence, such as “find(a==8,1,’first’). For more information on these options, see “find”.

[Arvind Andrew Das]

i did the find function, but it gives me the ans vector with all of the values of the array and the results are in true or false for my condition( 0 & 1 ).

[Walter Roberson]

find() can never return 0. Perhaps you are looking at ans for a different operation?

Answer 2

[Bhagyesh Shiyani]

what if i want both 8 positions, any code?

[Florian Reinbold]

Hi Bhagyesh

i would suggest this one:

[val, idx] = find(a==8);

It seems to make a great job! :)

Cheers

Florian

[Walter Roberson]

This will not return value and index, it will return row and column numbers.

Answer 3

[Ujjawal Barnwal]

a = [7 8 8 2 5 6];

position = find(a==8)

Answer 4

[Sorne Duong]

a = 1, 3, 6, 9, 10, 15

We know the fourth value is 9, but how to find the fourth value in MATLAB?

[Walter Roberson]

a = [1, 3, 6, 9, 10, 15]

a = 1×6

1 3 6 9 10 15

a(4)

ans = 9

Unless you mean how to predict the 4th value -- that is, how to find a rule or formula for the values.

Legendré (and others) proved that for any finite set of finite values, that there is a polynomial that goes through all of the values exactly (to within computation error.) So you can construct a polynomial that goes through all of the points. There are an infinite number of expressions that can be used... but because any list of values can be fit, if what you were given was

a = [1, 3, 6, X, 10, 15]

and you were asked to figure out what X is, then the technique of constructing polynomials cannot decide -- there are an infinite number of polynomials that go through (1,1), (2,3), (3,6), (5,10), (6,15) and an infinite number of them have a different value at X = 4. There is a polynomial for [1,3,6,-8,10,15]. There is a polynomial for [1,3,6,10,10,15]. There is a polynomial for [1,3,6,72432015,10,15] .

Because of this, if the question is to figure out what X is given a = [1, 3, 6, X, 10, 15] then the answer has to be "It could be any finite number".

[Reem RA]

Hi Walter, I used the find function to find "positions" of values in a table colum. How do I get the values of those positions (values on int_t_03)? my code is bellow:

% determine the significant time interval for the bikes (3 sec before/after)
tstart_bikes_T_03 = min(bikes_T_03(:,4:4))-75
tend_bikes_T_03 = max(bikes_T_03(:,4:4))+75

% find road users that intersect with bikes time interval
int_t_03 = find(T_03(:,4)>= tstart_bikes_T_03 & T_03(:,4)<= tend_bikes_T_03);

[Torsten]

Try

values = T_03(int_t_03,4)

[Reem RA]

Thanks it worked!!!!

Answer 5

[Nilesh Kumar Bibhuti]

BRO , THIS WILL GIVE U THE DESIRED OUTPUT . HAPPY CODING :)

#include <stdio.h>
#define MAX_SIZE 100 // Maximum array size
int main()
{
    int arr[MAX_SIZE] ,brr[MAX_SIZE] ;
    int size, i, toSearch, found ,k=0;
    
    /* Input size of array */
    printf("Enter size of array: ");
    scanf("%d", &size);
    /* Input elements of array */
    printf("Enter elements in array: ");
    for (i = 0; i < size; i++)
    {
        scanf("%d", &arr[i]);
        }
    
    printf("\nEnter element to search: ");
    scanf("%d", &toSearch);
    
    for (i = 0; i < size; i++)
        {
            if (arr[i] == toSearch)
            {
                brr[k] = i+1 ;
                printf("%d ",brr[k]);
                k++ ;
                }
            }
            return 0;
            }

[Walter Roberson]

In C, int size should really be size_t size and your scanf() should be using %lu instead of %d . i and k should also be size_t

You should be checking the return status of each scanf() call .

Also

int main()

should be

int main(void)

unless you are using K&R C from before C was standardized.

The user is expecting the positions to be returned, rather than displayed.

You probably shouldn't be assuming integer for the array, but that would be an acceptable limitation if specifically documented.

You do not use or initialize found.