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POC++ Type casting from double to const int does not work properly
 

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  1. POC++ Type casting from double to const int does not work properly
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    <p>I have a variable of type const int, but the parameters that it's dependent upon are of type double. When I try to cast this down from a 'double' to a 'const int', it doesn't work properly. For example, when N should be 991, it is entered as 990. I've tried several methods and only one has worked, but I'm not sure if this method is going to work all the time. Here are some methods that I have tried:</p> <p>First method:</p> <pre><code>const int N = (Ls-1)/dx + 1; </code></pre> <p>Second Method:</p> <pre><code>const int N = static_cast&lt;const int&gt;((Ls-1)/dx) + 1; </code></pre> <p>Third Method:</p> <pre><code>double Z = (Ls-1)/dx + 1; const int N = Z; </code></pre> <p>Fourth Method (only working method):</p> <pre><code>double Z = (Ls-1)/dx; const int N = Z + 1; </code></pre> <p>Please note that dx is a value such that the remainder of (Ls-1)/dx will always be zero (i.e. it's always an integer value). Can anyway explain why the other methods aren't working so that I may understand type casting better?</p> <p>EDIT: As requested, I'm uploading the entire code to show how everything is working:</p> <pre><code>#include &lt;iostream&gt; #include &lt;math.h&gt; #include &lt;stdio.h&gt; #include &lt;fstream&gt; #include &lt;cmath&gt; #include &lt;algorithm&gt; #define pi 3.14159265 using namespace std; //Define Fluid Properties double rho_L = 998; //Liquid Density double rho_LG = 828.9; //Liquid-Gas Density Ratio double mu_L = 0.000798; //Liquid Viscosity double mu_LG = 40.24; //Liquid-Gas Viscosity Ratio double sigma = 0.0712; //Surface Tension double nu_G = (mu_L/mu_LG)/(rho_L/rho_LG); //Define Injector Properties double Uinj = 56.7; //Injection Velocity double Dinj = 0.0998; //Injector Diameter double theta = 15.0*pi/180.0; //Spray Cone Angle double L = 500.0*Dinj; //Atomization Length double Ls = L/Dinj; //Normalized Atomization Length //Define Solver Parameters double K = 5294; //Viscous Dissipation Coefficient double Eps = pow(10,-5); //Residual Error double dx = 0.0001; //Step Size double Ui = 10; //Initial Guess //const int Z = static_cast&lt;const int&gt;((Ls-1)/dx + 1) + 1; const int N = (Ls-1)/dx + 1;//Z; double deriv (double U, double X, double delta, double m) { double dudx; dudx = -(1.0/delta)*(1.0/U)*(U - sqrt(1.0 - U)/sqrt(m*X*X))*(U - sqrt(1.0 - U)/sqrt(m*X*X)); return (dudx); } int main() { //Declare Variables int max_step; double ERR; int step; double DEN; double SMD; double m; double Ug; double Re; double Cd; double delta; double K1; double K2; double K3; double K4; //Allocate Memory From Heap double *U = new double [N]; double *X = new double [N]; //Initialize Vectors and Variables DEN = 0.5*rho_L - (4.0/3.0)*K*(mu_L)/(Uinj*Dinj*Dinj)*L; m = 4.0/rho_LG*tan(theta)*tan(theta); for (int i = 0; i &lt; N; i++) { X[i] = 1.0 + dx*i; } U[0] = 1.0; max_step = 1; ERR = 1; step = 0; while(abs(ERR) &gt; Eps &amp;&amp; step &lt; max_step) { //Calculate Ug Ug = sqrt(1.0 - (Ui/Uinj))/sqrt(m*Ls*Ls)*Uinj; //Calculate SMD SMD = 6.0*sigma/(DEN*(Uinj*Uinj - Ui*Ui)); //Calculate Re # and Drag Coefficient Re = abs(Ui-Ug)*SMD/nu_G; if(Re &lt;= 0.01) { Cd = (0.1875) + (24.0/Re); } else if(Re &gt; 0.01 &amp;&amp; Re &lt;= 260.0) { Cd = (24.0/Re)*(1.0 + 0.1315*pow(Re,(0.32 - 0.05*log10(Re)))); } else { Cd = (24.0/Re)*(1.0 + 0.1935*pow(Re,0.6305)); } //Determine New U delta = (4.0/3.0)*(1.0/Cd)*(rho_LG)*(SMD/Dinj); //RK4 for (int i = 0; i &lt; N-1; i++) { K1 = deriv(U[i],X[i],delta,m); K2 = deriv(U[i]+0.5*dx*K1,X[i]+0.5*dx,delta,m); K3 = deriv(U[i]+0.5*dx*K2,X[i]+0.5*dx,delta,m); K4 = deriv(U[i]+dx*K3,X[i+1],delta,m); U[i+1] = U[i] + dx/6.0*(K1 + 2.0*K2 + 2.0*K3 + K4); //if(i &gt;= 0 &amp;&amp; i &lt;= 3) //cout &lt;&lt; i &lt;&lt; " " &lt;&lt; K1 &lt;&lt; " " &lt;&lt; K2 &lt;&lt; " " &lt;&lt; K3 &lt;&lt; " " &lt;&lt; K4 &lt;&lt; " " &lt;&lt; U[i] &lt;&lt; endl; } ERR = abs(U[N-1]*Uinj - Ui)/Ui; Ui = U[N-1]*Uinj; step = step + 1; } SMD = 6.0*sigma/(DEN*(Uinj*Uinj - Ui*Ui)); cout &lt;&lt; "U = " &lt;&lt; Ui &lt;&lt; endl; cout &lt;&lt; "SMD = " &lt;&lt; SMD &lt;&lt; endl; cout &lt;&lt; "DEN = " &lt;&lt; DEN &lt;&lt; endl; cout &lt;&lt; "Ug = " &lt;&lt; Ug &lt;&lt; endl; cout &lt;&lt; "m = " &lt;&lt; m &lt;&lt; endl; cout &lt;&lt; "delta = " &lt;&lt; delta &lt;&lt; endl; cout &lt;&lt; "Re = " &lt;&lt; Re &lt;&lt; endl; cout &lt;&lt; "Cd = " &lt;&lt; Cd &lt;&lt; endl; cout &lt;&lt; "U* = " &lt;&lt; U[N-1] &lt;&lt; endl; cout &lt;&lt; "Error = " &lt;&lt; ERR &lt;&lt; endl; cout &lt;&lt; "step = " &lt;&lt; step &lt;&lt; endl; //Output Data Into Text File ofstream outputdata("result-500-15.txt"); for (int i = 0; i &lt; N; i++) { outputdata &lt;&lt; X[i] &lt;&lt; " " &lt;&lt; U[i] &lt;&lt; '\n'; } outputdata.close(); delete [] U; delete [] X; return 0; } </code></pre>
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    1. USuser1562781
    1. USuser1562781
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    1. COIt sounds like it might be a roundoff issue -- perhaps `(Ls-1)/dx` is actually (say) 990.999999999999, so it's effectively 991, but downcasting will truncate it to 990. What happens if you try `static_cast<const int>((Ls-1)/dx + 0.01) + 1`?
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      1. POC++ Type casting from double to const int does not work properly
      1. USruakh
    2. COCurrently, dx = 0.1, and Ls-1 = 99, so (Ls-1)/dx should be 990, but maybe it can't do 0.1 in binary? If I add 0.01, it does work, but will it always?
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      1. POC++ Type casting from double to const int does not work properly
      1. USuser1562781
    3. COActually, now that I think about it, rounding shouldn't be a problem because including the '+1' should fix any rounding issues.
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      1. POC++ Type casting from double to const int does not work properly
      1. USuser1562781
 

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