Question

使用matlab 如何用分段线性插值求某个函数值（已附C的编写好的语句）不可以调用函数！

已知:
x=0.0,0.1,0.195,0.3,0.401,0.5时,
f(x)分别=0.39894,0.39695,0.39142,0.38138,0.36812,0.35206
一个C程序如下：
求得f(x)的分段线性插值结果.

#include <stdio.h>

void main()
{
double x[6] = {0.0, 0.1, 0.195, 0.3, 0.401, 0.5};
double f[6] = {0.39894,0.39695,0.39142,0.38138,0.36812,0.35206};
double input;
double output;
scanf("%lf", &input);
int i;
for (i = 0; i < 5; i++)
{
if (x[i] <= input && x[i + 1] >= input)
{
output = (f[i] * x[i] + f[i + 1] * x[i + 1]) / (x[i] + x[i + 1]);
break;
}
}
printf("%lf\n", output);
}

但是不会matlab 不知道要怎么改一下，要求不可以调用matlab里面的线性插值的函数呢

Answer 1

clear
clc

x=[0.0, 0.1, 0.195, 0.3, 0.401, 0.5];
f=[0.39894,0.39695,0.39142,0.38138,0.36812,0.35206];

x_in = str2double(inputdlg('Enter X data:','Input Data',1,{'0.2'}));

for i=1:5
if x(i) <= x_in && x(i + 1) >= x_in
f_out = (f(i) * x(i) + f(i+1) * x(i+1)) / (x(i) + x(i+1));
break;
end
end

fprintf('f(%f)=%f\n',x_in,f_out)

还可以这样写代码，完全利用matlab的矩阵优势,而且没有调用任何函数：

clear
clc

x=[0.0, 0.1, 0.195, 0.3, 0.401, 0.5];
f=[0.39894,0.39695,0.39142,0.38138,0.36812,0.35206];

x_in = str2double(inputdlg('Enter X data:','Input Data',1,{'0.2'}));

index=(x(1:end-1) <= x_in & x_in <= x(2:end)) * (1:5)' * [1,1] + [0,1]; %下标数值
f_out = f(index) * x(index)' / sum(x(index));

fprintf('f(%f)=%f\n',x_in,f_out)