我的程序是:<BR>%画自定义函数<BR>syms x y;<BR>y=input('entre your function:y=')<BR>a=0:0.1:2*pi;<BR>b=a;<BR>for i=1:length(a)<BR> b(i)=eval(y(a(i)));<BR>end<BR>plot(x,y)<BR> 出错提示:??? Index into matrix is negative or zero. See release notes on changes to <BR>logical indices.