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

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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).

Risposta accettata

Walter Roberson
Walter Roberson il 15 Feb 2018
Modificato: MathWorks Support Team il 27 Feb 2020
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.
  2 Commenti
Arvind Andrew Das
Arvind Andrew Das il 19 Lug 2022
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
Walter Roberson il 20 Lug 2022
find() can never return 0. Perhaps you are looking at ans for a different operation?

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Più risposte (4)

Bhagyesh Shiyani
Bhagyesh Shiyani il 5 Dic 2019
what if i want both 8 positions, any code?
  2 Commenti
Florian Reinbold
Florian Reinbold il 15 Gen 2020
Hi Bhagyesh
i would suggest this one:
[val, idx] = find(a==8);
It seems to make a great job! :)
Walter Roberson
Walter Roberson il 15 Gen 2020
This will not return value and index, it will return row and column numbers.

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Ujjawal Barnwal
Ujjawal Barnwal il 7 Giu 2020
a = [7 8 8 2 5 6];
position = find(a==8)

Sorne Duong
Sorne Duong il 21 Lug 2021
a = 1, 3, 6, 9, 10, 15
We know the fourth value is 9, but how to find the fourth value in MATLAB?
  4 Commenti

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Nilesh Kumar Bibhuti
Nilesh Kumar Bibhuti il 15 Ott 2021
Modificato: Walter Roberson il 15 Ott 2021
#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;
  1 Commento
Walter Roberson
Walter Roberson il 15 Ott 2021
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 .
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.

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