If statement dictating acceptable range is not functioning properly, why?

I'm working on a project to where I have to find pairs of values that provide the same output when inputted into an equation. I want only combinations that provide an output within a defined range, so for example, say I want all the possible combinations of values that sum to 6 +/- 0.5 so combinations that would sum to 5.5, 6 or 6.5 are all acceptable, however if there's a combination that sums to 7, I want that to be excluded. Getting a code working that at least satisfies the process of finding combinations is something I already have, thanks to the gracious people on here in this thread , so my issue is setting the condtions that I just spoke of to exclude certain combinations.

In my test code however, it doesn't seem like this is being done for some reason. I want all combinations that give me a value that is between lim1 and lim2, however when plugging the combinations that are output back into the original equation to find davg, I find that it has cominations that output values way beyond the conditions that I set (for example davg(1)= 89.3, when I set a condition for no combinations that would result in davg being greater than 59.663). So is something off with my if statement, or am I off base here completely? I would appreciate some insight regarding this.

value1= deg2rad(35);

value2=deg2rad(27);

davg_initial= rad2deg(atan((cot(value2)+cot(value1))*0.5));

lim1=(davg_initial-0.5);

lim2= (davg_initial+0.5);

a = [0:0.5:35];

n = length(a);

b = [];

for i = 1:n

value1 = a(i);

for j = 1:n

value2 = a(j);

davg= rad2deg(atan((cot(value2)+cot(value1))*0.5));

if davg<=lim2 && davg>=lim1...

&& value2<value1

b = [b,[value1;value2]];

end

end

end

b'

x=deg2rad(b(1,:));

y=deg2rad(b(2,:));

davg= rad2deg(atan((cot(y)+cot(x))*0.5));

[Steven Lord]

Just a suggestion: rather than repeatedly converting back and forth between degrees and radians I recommend using the degree-based trig functions. Here's a simple example:

format longg
s = sind(35);
s2 = sin(deg2rad(35));
[s; s2] % Compare answers using radians and degrees

ans = 2×1

0.573576436351046 0.573576436351046

c = cosd(35);
c2 = cos(deg2rad(35));
[c; c2]

ans = 2×1

0.819152044288992 0.819152044288992

ct = cotd(35);
ct2 = cot(deg2rad(35));
[ct; ct2]

ans = 2×1

1.42814800674211 1.42814800674211

% Check: cot(x) = cos(x)/sin(x)
check = [ct; c./s]

check = 2×1

1.42814800674211 1.42814800674211

[Aly Osman]

Good suggestion! I've used cosd, sind in the past, but for some reason my mind blanked on using them here since originally I was doing everything in radians and to understand what each number meant I was just doing rad2deg, regardless, I'll switch to this now, much cleaner and shorter! Thank you!

Answer 1

[Geoff Hayes]

@Aly Osman - what are the units for a and so the units for b? In your looping code, that includes the if statement, you do

for i = 1:n
    value1 = a(i);
    for j = 1:n
        value2 = a(j);
        davg= rad2deg(atan((cot(value2)+cot(value1))*0.5));
        if davg<=lim2 && davg>=lim1...
                 && value2<value1 
            b = [b,[value1;value2]];
        end
    end
end

where you take pairs of values from a and pass them into the equation

davg= rad2deg(atan((cot(value2)+cot(value1))*0.5));

If the criteria is met, then you save value1 and value2 to the b array. Once the outer loop completes, then you seem to do a check or validation of the data

x=deg2rad(b(1,:));
y=deg2rad(b(2,:));
davg= rad2deg(atan((cot(y)+cot(x))*0.5));

where you convert each value in b from degrees to radians, and then you pass them into the (almost identical) equation

davg= rad2deg(atan((cot(y)+cot(x))*0.5));

So why is there a conversion from degress to radians of the data in b? Shouldn't you have done that also when extracting value1 and value2 from a?

[Aly Osman]

Wow, I'm a dummy it seems. You are right, value 1 and value 2 are in radians instead of being in degrees, which is what messed everything up. This fixed it and now the numbers are actually within the range I designated in the if statement. Thank you!