How can i plot with if statement ?

>>  x=[10 20 30 40 50 ] ;
>> if x==10
y=((10*10^6)*.1523)./ x ;
end
if x==20 
y=((10*10^6)*.601)./ x ;
end
if x==30
y=((10*10^6)*1.914)./ x ;
end
if x==20 
y=((10*10^6)*.601)./ x ;
end
if x==30
y=((10*10^6)*1.914)./ x ;
end
if x==40
y=((10*10^6)*2.73)./ x ;
end
if x==50
y=((10*10^6)*5.5547)./ x ;
end

Answer 1

[Star Strider]

Try this:

x=[10 20 30 40 50 ] ;
for k = 1:numel(x)
    if x(k)==10
        y(k)=((10*10^6)*.1523)./ x(k) ;
    end
    if x(k)==20
        y(k)=((10*10^6)*.601)./ x(k) ;
    end
    if x(k)==30
        y(k)=((10*10^6)*1.914)./ x(k) ;
    end
    if x(k)==20
        y(k)=((10*10^6)*.601)./ x(k) ;
    end
    if x(k)==30
        y(k)=((10*10^6)*1.914)./ x(k) ;
    end
    if x(k)==40
        y(k)=((10*10^6)*2.73)./ x(k) ;
    end
    if x(k)==50
        y(k)=((10*10^6)*5.5547)./ x(k) ;
    end
end

figure
plot(x, y)
grid

It is necessary to index into both ‘x’ and ‘y’ to do what you want.

[Guillaume]

Note (to the OP) that 10*10^6 is better written as 10e6.

A much better design of the above would be:

LUT = [10, 0.1523;   ...Look Up Table for x and corresponding coefficiet
       20, 0.601;
       30, 1.914;
       40, 2.73;
       50, 5.5547];
x = [10 20 30 40 50];
for k = 1:numel(x)
    [found, whichrow] = ismember(x(k), LUT(:, 1));
    if found
        y(k) = 10e6 * LUT(whichrow, 2) / x(k);
    else
        error('don''t know how to calculate y(k)');
    end
end
figure;
plot(x, y);
grid;

So now if you want to add/remove x values and coefficients you only have one line of a matrix to edit instead of having to copy-paste/delete several lines of codes. It's also a lot clearer what the coefficient are and it avoids having twice the same tests (e.,g. the 20 and 30 test that was repeated originally).