Location of a value in a matrix

4 vues (au cours des 30 derniers jours)
Ismita
Ismita le 6 Déc 2022
Commenté : Ismita le 7 Déc 2022
Say, I have a matrix x = 0:0.5:20. How can I find the exact location (according to the array) of the value of x==9.5? thanks

Réponses (2)

Walter Roberson
Walter Roberson le 6 Déc 2022
x = 0:0.5:20
x = 1×41
0 0.5000 1.0000 1.5000 2.0000 2.5000 3.0000 3.5000 4.0000 4.5000 5.0000 5.5000 6.0000 6.5000 7.0000 7.5000 8.0000 8.5000 9.0000 9.5000 10.0000 10.5000 11.0000 11.5000 12.0000 12.5000 13.0000 13.5000 14.0000 14.5000
find(x == 9.5)
ans = 20
However...
x = 0:0.1:20
x = 1×201
0 0.1000 0.2000 0.3000 0.4000 0.5000 0.6000 0.7000 0.8000 0.9000 1.0000 1.1000 1.2000 1.3000 1.4000 1.5000 1.6000 1.7000 1.8000 1.9000 2.0000 2.1000 2.2000 2.3000 2.4000 2.5000 2.6000 2.7000 2.8000 2.9000
find(x == 0.3)
ans = 1×0 empty double row vector
[found, idx] = ismembertol(0.3, x)
found = logical
1
idx = 4
It is typically (much more often than not) an error to use == to search for a value in a floating point array, with the exception of comparing something that has been extracted from the array. For example,
y = sin(x);
[maxy, idx_oneway] = max(y)
maxy = 0.9996
idx_oneway = 17
idx_anotherway = find(y == maxy)
idx_anotherway = 17
max() and min() and indexing create bit-for-bit identical copies of values (except for weird nan values) so you can be sure that == will work for them. But you should rarely use == to compare non-integers to a list created with the : operator.
  3 commentaires
Walter Roberson
Walter Roberson le 6 Déc 2022
x2 = 0:0.5:20;
find(x2 == 9.5)
ans = 20
In any case, please re-read my response where I say "Don't DO that" -- and I show what to do instead.
Ismita
Ismita le 7 Déc 2022
Thank you so much

Connectez-vous pour commenter.


David Hill
David Hill le 6 Déc 2022
x=0:.5:20;
f=find(x==9.5)
f = 20
x(f)
ans = 9.5000
  1 commentaire
Ismita
Ismita le 6 Déc 2022
Thank you . But I see the exact location -
"x2 = 0:0.5:20;
find(x2 == 9.5)
ans =
1×0 empty double row vector"
Could you please suggest?

Connectez-vous pour commenter.

Catégories

En savoir plus sur Resizing and Reshaping Matrices dans Help Center et File Exchange

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by