如何解决cat仅打印一个文件描述符
我试图了解文件描述符的工作原理。
$ cat a.txt
Hello
$ cat b.txt
World
$ cat script1.sh
cat a.txt b.txt
$ ./script1.sh
Hello
World
$ cat script2.sh
exec 19<a.txt
exec 20<b.txt
cat <&19 <&20
$ ./script2.sh
World
解决方法
#include<iomanip>
#include<cmath>
using namespace std;
int main()
{
int i,j,k,n,N;
cout.precision(4); //set precision
cout.setf(ios::fixed);
cout << "\nEnter the no. of data pairs to be entered:\n"; //To find the size of arrays that will store x,y,and z values
cin >> N;
double x[N],y[N];
cout << "\nEnter the x-axis values:\n"; //Input x-values
for (i = 0; i < N; i++)
cin >> x[i];
cout << "\nEnter the y-axis values:\n"; //Input y-values
for (i = 0; i < N; i++)
cin >> y[i];
cout << "\nWhat degree of Polynomial do you want to use for the fit?\n";
cin >> n; // n is the degree of Polynomial
double X[2 * n + 1]; //Array that will store the values of sigma(xi),sigma(xi^2),sigma(xi^3)....sigma(xi^2n)
for (i = 0; i < 2 * n + 1; i++)
{
X[i] = 0;
for (j = 0; j < N; j++)
X[i] = X[i] + pow(x[j],i); //consecutive positions of the array will store N,sigma(xi),sigma(xi^3)....sigma(xi^2n)
}
double B[n + 1][n + 2],a[n + 1]; //B is the Normal matrix(augmented) that will store the equations,'a' is for value of the final coefficients
for (i = 0; i <= n; i++)
for (j = 0; j <= n; j++)
B[i][j] = X[i + j]; //Build the Normal matrix by storing the corresponding coefficients at the right positions except the last column of the matrix
double Y[n + 1]; //Array to store the values of sigma(yi),sigma(xi*yi),sigma(xi^2*yi)...sigma(xi^n*yi)
for (i = 0; i < n + 1; i++)
{
Y[i] = 0;
for (j = 0; j < N; j++)
Y[i] = Y[i] + pow(x[j],i) * y[j]; //consecutive positions will store sigma(yi),sigma(xi^2*yi)...sigma(xi^n*yi)
}
for (i = 0; i <= n; i++)
B[i][n + 1] = Y[i]; //load the values of Y as the last column of B(Normal Matrix but augmented)
n = n + 1; //n is made n+1 because the Gaussian Elimination part below was for n equations,but here n is the degree of polynomial and for n degree we get n+1 equations
cout << "\nThe Normal(Augmented Matrix) is as follows:\n";
for (i = 0; i < n; i++) //print the Normal-augmented matrix
{
for (j = 0; j <= n; j++)
cout << B[i][j] << setw(16);
cout << "\n";
}
for (i = 0; i < n; i++) //From now Gaussian Elimination starts(can be ignored) to solve the set of linear equations (Pivotisation)
for (k = i + 1; k < n; k++)
if (B[i][i] < B[k][i])
for (j = 0; j <= n; j++)
{
double temp = B[i][j];
B[i][j] = B[k][j];
B[k][j] = temp;
}
for (i = 0; i < n - 1; i++) //loop to perform the gauss elimination
for (k = i + 1; k < n; k++)
{
double t = B[k][i] / B[i][i];
for (j = 0; j <= n; j++)
B[k][j] = B[k][j] - t * B[i][j]; //make the elements below the pivot elements equal to zero or elimnate the variables
}
for (i = n - 1; i >= 0; i--) //back-substitution
{ //x is an array whose values correspond to the values of x,z..
a[i] = B[i][n]; //make the variable to be calculated equal to the rhs of the last equation
for (j = 0; j < n; j++)
if (j != i) //then subtract all the lhs values except the coefficient of the variable whose value is being calculated
a[i] = a[i] - B[i][j] * a[j];
a[i] = a[i] / B[i][i]; //now finally divide the rhs by the coefficient of the variable to be calculated
}
cout << "\nThe values of the coefficients are as follows:\n";
for (i = 0; i < n; i++)
cout << "x^" << i << "=" << a[i] << endl; // Print the values of x^0,x^1,x^2,x^3,....
cout << "\nHence the fitted Polynomial is given by:\ny=";
for (i = 0; i < n; i++)
cout << " + (" << a[i] << ")" << "x^" << i;
cout << "\n";
return 0;
}
表示“用FD 19替换标准输入”，<&19表示“用FD 20替换标准输入”。这些互相伤害。如果您要读取两个FD，请改为使用<&20。
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