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

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Arnab Pal
Arnab Pal 2018년 2월 15일
댓글: Walter Roberson 2021년 10월 15일 4:40
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).

채택된 답변

Walter Roberson
Walter Roberson 2018년 2월 15일
편집: MathWorks Support Team 2020년 2월 27일
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.

추가 답변(4개)

Bhagyesh Shiyani
Bhagyesh Shiyani 2019년 12월 5일
what if i want both 8 positions, any code?
  댓글 수: 2
Walter Roberson
Walter Roberson 2020년 1월 15일
This will not return value and index, it will return row and column numbers.

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Ujjawal Barnwal
Ujjawal Barnwal 2020년 6월 7일
a = [7 8 8 2 5 6];
position = find(a==8)

Sorne Duong
Sorne Duong 2021년 7월 21일
a = 1, 3, 6, 9, 10, 15
We know the fourth value is 9, but how to find the fourth value in MATLAB?
  댓글 수: 1
Walter Roberson
Walter Roberson 2021년 7월 21일
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".

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Nilesh Kumar Bibhuti
Nilesh Kumar Bibhuti 2021년 10월 15일 3:03
편집: Walter Roberson 2021년 10월 15일 4:29
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;
}
  댓글 수: 1
Walter Roberson
Walter Roberson 2021년 10월 15일 4:40
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.

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