str2poly.m

matlab7.0从入门到精通(光盘内容)

function Y=str2poly(X)
%X是字符串形式的多项式
%Y是行向量形式的多项式
%输入格式检查
if(ischar(X)==0) 
    disp('输入错误：输入X必须是一个字符串！');
end;
%用正则表达式寻找'+'或者'-'的下标位置
index=regexp(X,'\+|\-');
%多项式的项数
L=length(index);
%用于存储多项式每一项信息的单元字符串矩阵
term=cell(1,L+1);
term(1)=cellstr(X(1:(index(1)-1)));    
for i=1:L-1,
    term(i+1)=cellstr(X(index(i):(index(i+1)-1)));
end;
term(L+1)=cellstr(X(index(L):end));
%如果第一项为空，则删除该项
if(isempty(char(term(1))))
    term(1)=[];
%多项式的项数减一   
    L=L-1;
end;  
%多项式系数矩阵
coefficient=[];
%多项式幂次矩阵，它与多项式系数矩阵一一对应
power=[];
for i=1:L+1,
%单项多项式字符串表达式    
    substring=char(term(i));
%用正则表达式，寻找字符串'x^'，由于'^'是正则表达式中的特殊字符，多以用'\^'来表示    
    index2=regexp(substring,'x\^');
    if(isempty(index2)==0),
%如果匹配上后        
        if(index2(1)==1),
%单项多项式字符串为'x^*'形式            
            coefficient=[coefficient 1];
            power=[power str2num(substring((index2(1)+2):end))];
        elseif(index2(1)==2),   
%单项多项式字符串为'+x^*'或者'-x^*'，'3x^*'形式            
           if(substring(1)=='+'),
               coefficient=[coefficient 1];
               power=[power str2num(substring((index2(1)+2):end))];
           elseif(substring(1)=='-'),
               coefficient=[coefficient -1];
               power=[power str2num(substring((index2(1)+2):end))];
           else
               coefficient=[coefficient str2num(substring(1:(index2(1)-1)))];
                power=[power str2num(substring((index2+2):end))];
           end;          
        else
%单项多项式字符串为'2.2x^*'            
            coefficient=[coefficient str2num(substring(1:(index2(1)-1)))];
            power=[power str2num(substring((index2+2):end))]; 
        end;
    else
%单项多项式字符串不含'x^'        
%用正则表达式，寻找字符串'x'
        index2=regexp(substring,'x');
        if(isempty(index2)==0),
%如果匹配上'x'            
            if(index2(1)==1),
%单项多项式字符串为'x*'形式                
                coefficient=[coefficient 1];
                power=[power 1];               
            elseif(index2(1)==2), 
%单项多项式字符串为'+x*'或者'-x*'，'3x*'形式                 
               if((substring(1)=='+')==1),
                   coefficient=[coefficient 1];
                   power=[power 1];
               elseif(substring(1)=='-'),
                   coefficient=[coefficient -1];
                   power=[power 1];
               else
%单项多项式字符串为'2.2x*'                    
                   coefficient=[coefficient str2num(substring(1:(index2-1)))];
                    power=[power 1];
               end;              
            else
                
                coefficient=[coefficient str2num(substring(1:(index2-1)))];
                power=[power 1]; 
            end;
        else
%单项多项式字符串不含'x^','x',则断定它是常数项
           coefficient=[coefficient str2num(substring)];
           power=[power 0]; 
        end;   
    end;
end;

N=max(power)+1;
Y=zeros(1,N);
for i=1:N,
    index3=find(power==(N-i));
    Y(i)=sum(coefficient(index3));
end;