[討論] 牛頓法的求值與繪圖問題>"<

[gfs777 (成人展三萬嫖妓)]

是這樣的>"<  這個問題是老師後天考試前發下來   給大家作為範例參考的題目
如下:
================================================================
Newton's method is an efficient algorithm for finding approximations to the

zeros (or roots) of a real-valued function. The algorithm can be written

as follows



Xn+1 = Xn - [f(Xn) / f'(Xn)]



Newton’s method iteratively approximates the root(s) for function f(x)

until Xn+1 and Xn difference is smaller than real number e

for example |Xn+1  - Xn| <e

 (e沒給 我自設  1e-4)

write a script that that performs the Newton method to find a root for

y = xn^3 - 5*xn^2 - xn + 5

Plot the Xn on the y-axis with number of iterations on the x-axis.


===========================================================

目前我寫成這樣:

xn = input('first x?\n');
y = xn^3 - 5*xn^2 - xn + 5;
yplum = 3*xn^2 - 10*xn - 1;
cnt = 1;
e = 1e-4;


while cnt<=10000;
    xn1 = xn - y / yplum
    if abs(xn1 - xn)<= e
        fprintf('converged')
        break;
    end
    xn = xn1
    cnt = cnt + 1
end

===========================================================


問一下這樣是對的嗎?
xn = -1.6062e+005  這答案是對的嗎?    我起始x = 50
        xn =  -2.9412e+005                   起始x = 90
        xn =  -4.9415e+005                   起始x= 150

跑幾次都不一樣><


另外畫圖要怎麼畫? 跪求~感謝><

那時候作業有個神人教我用clf  hold figure等

現在我又不會用了....當時畫出來的都跟同學不一樣><




--
※ 發信站: 批踢踢實業坊(ptt.cc)
◆ From: 140.112.76.121
→ DKer:迴圈很有問題  自己想像模擬一下馬上就會看出來了
→ DKer:你在迴圈裡沒有重新計算xn,y,yplum  跑再多次都一樣
→ DKer:然後記得每一行結尾要加 ;
→ gfs777:感謝大大。目前解答改成
xn = input('first x?\n');
cnt = 1;
e = 1e-4;

while cnt<=10000;
    y = xn^3 - 5*xn^2 - xn + 5;
    yplum = 3*xn^2 - 10*xn - 1;
    xn1 = xn - y / yplum
    if abs(xn1 - xn)<= e
        fprintf('converged')
        break;
    end
    xn = xn1
    cnt = cnt + 1
end

預祝大大事事順利
※ 編輯: gfs777          來自: 140.112.183.66       (11/15 15:04)