Help with if statement in a calculation loop

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Ivan Mich
Ivan Mich am 12 Apr. 2021
Kommentiert: Rik am 12 Apr. 2021
Hello,
I have a problem with a code. I want to calculate some numbers but I want to "limit" my results via a loop.
I want values <1 to be =1, values with >10 to be 10 and the others to be calculated from this equation:
x= x*1.3+0.5. I wrote this code but it is not use
x= 5 + randn * 0.5
if x < 1
x == 1
elseif x > 10
x == 10
else x > 3
x == 1.3* x -0.75
end
Where is the problem?
Thanking you in advance
  2 Kommentare
Nino Wyssen
Nino Wyssen am 12 Apr. 2021
I can see several problems here.
First, Matlab likes the lines to be finished with a semicolon, but this isn't necessary if you don't mind your programm outputting all the operations.
Then, you aren't assigning values. You are comparing them. To assign a new value to a variable you only need one equal sign.
Third, you can't use a condition for you else statment, if you only wnt to execute the command below u need to use another elseif.
From your description I suspect the following code does the right thing for you:
x= 5 + randn * 0.5;
if x < 1
x = 1;
elseif x > 10
x = 10;
else
x = 1.3* x -0.75;
end
even though I think it would be better to use a new variable for your result
x= 5 + randn * 0.5;
if x < 1
y = 1;
elseif x > 10
y = 10;
else
y = 1.3* x -0.75;
end
Ivan Mich
Ivan Mich am 12 Apr. 2021
I am sorry but running this code stil values up to 10 exist.
It is no use

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Akzeptierte Antwort

Stephen23
Stephen23 am 12 Apr. 2021
The simplest and most efficient solution is to use MIN and MAX:
x = 5 + randn(5,7)*1.5;
x = x*1.3 + 0.5
x = 5×7
7.6491 8.4519 8.8820 8.7385 9.8065 8.6945 8.5660 5.7131 13.1416 6.3650 9.1859 6.9860 9.4199 5.4750 8.8947 7.3018 6.4618 7.2714 9.2859 8.7553 5.7112 8.6327 5.6802 3.2153 9.5393 7.2815 7.1600 11.3087 6.1828 6.8579 9.3617 8.8780 4.6976 3.9478 7.3776
x = min(max(x,1),10) % this is all you need.
x = 5×7
7.6491 8.4519 8.8820 8.7385 9.8065 8.6945 8.5660 5.7131 10.0000 6.3650 9.1859 6.9860 9.4199 5.4750 8.8947 7.3018 6.4618 7.2714 9.2859 8.7553 5.7112 8.6327 5.6802 3.2153 9.5393 7.2815 7.1600 10.0000 6.1828 6.8579 9.3617 8.8780 4.6976 3.9478 7.3776

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Rik
Rik am 12 Apr. 2021
Bearbeitet: Rik am 12 Apr. 2021
The problem is that you assume Matlab will process each element of x separately. Matlab will only do that if you use a loop.
An alternative is to use logical indexing to process x as an array.
L=x<1;
x(L)=1;
L=x>10;
x(L)=10;
L=x>3;
x(L)=1.3* x(L) -0.75;
  4 Kommentare
Ivan Mich
Ivan Mich am 12 Apr. 2021
Because values greater than 10 are shown. This is what I mean
Rik
Rik am 12 Apr. 2021
In that case you should use the code Stephen suggested.
If you have a piecewise function you can use the code I suggested.

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