Spot the NaN

Can someone help me in spotting the NaN error? I need a pair of fresh eyes. The NaN error is DP. DP depends on P, MEAN_EQ, and STAND_EQ. But for some reason, Matlab shows P=39, MEAN_EQ=0, or STAND_EQ=0 in the workspace.

clc

clear all

format long

%% Definitions

STEP=1000;

START=61000;

END=300000;

DP=0;

DQ=START;

Q=END;

STD_SD = 7000; STD = 21000;

u = 60000;

STD_u = STD/sqrt(40);

while (DQ < Q)

MEAN1=1;

while MEAN1 < END;

    MEAN_EQ = (1/(STD_u*sqrt(2*pi)))*exp(-0.5*((MEAN1-u)/STD_u)^2); 
    STAND=1;
    P=0;
    while STAND < END;        
        STAND_EQ = (1/(STD_SD*sqrt(2*pi)))*exp(-0.5*((STAND-STD)/STD_SD)^2); 
        a = pi/(sqrt(6)*STAND);
        b = MEAN1 - (0.5772/a);
        if(Q > DQ) 
            P = (a*((exp(-a*(DQ-b)))*(exp(-exp(-a*(DQ-b)))))*(1000000*0.005*((DQ-60000)^0.7)));
        else
            P = (a*((exp(-a*(DQ-b)))*(exp(-exp(-a*(DQ-b)))))*0);
        end
        DP = DP + P * STEP * MEAN_EQ * STEP * STAND_EQ * STEP; 
        STAND = STAND + STEP; 
    end
    MEAN1 = MEAN1 + STEP;   
end

end

Answer 1

[Oleg Komarov]

Happens that P becomes NaN, and consequently DP, in:

exp(-a*(DQ-b)) * exp(-exp(-a*(DQ-b))) = inf * exp(-inf) = inf * 0 = NaN

When

a  = 1.2825
DQ = 61000
b  = 62001

I suggest to:

1. Put a counter = 0, at the beginning of the second while statement, incrementing it at the end
2. Set up a conditional break point on the counter (left click next to the line number, then right click --> set/modify condition: isnan(DP).
3. After running the script, when it stops, remember the value of the counter and set condition to counter == value - 1. After re-executing it should stop on the values which generate the NaN as above.

Oleg