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# Need to find the value of x_?, where x_? is x at y=?......

Asked by Ara on 29 Apr 2013

Dear All,

Just a simple question, I need to find x_15, where x_15 is x at y=15. Please imagine x and y as below,

```x       y
0.1     20
0.05    15
0.2     85
```

Thank you!

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Answer by Iman Ansari on 29 Apr 2013

Hi.

```x=[0.1; 0.05; 0.2];
y=[20; 15; 85];
x(y==15)
```

Ara on 29 Apr 2013

Two columns (5357x1 double)like the example x and y. Usually Matlab give me an error but I never faced this problem (no error, no figure..)it's weird. When I plot (el1nov,s4_01nov)the figure show me values for el1nov=15 so I am wondering why can not show me for exact value I want.

Iman Ansari on 29 Apr 2013

See this works:

```s4_01nov=rand([5357 1]);
el1nov=randi(20,[5357 1]);
plot(s4_01nov(el1nov==15)./sin(el1nov(el1nov==15)))
```

but with for example 30, because el1nov doesn't have 30 nothing happened. With nnz(el1nov==15) you can check how many 15 in the el1nov vector:

```nnz(el1nov==15)
nnz(el1nov==30)
plot(s4_01nov(el1nov==30)./sin(el1nov(el1nov==30)))
```
Ara on 29 Apr 2013

Thank you, yes this works. Ok, now by using "nnz" I realize at that value the vector was zero which is not reasonable based on my observation then I will go to recheck that part. So no wonder then why Matlab shows no figure.