POJ 表达式求值

表达式求值.

程序套用的是poj 2252Equation Solver的标程，写的相当漂亮，简洁

所有的表达式求值问题都可以用这种方法解决

算法：ELL(1)-Parser

2106 Boolean Expressions

范式：

Expression := Term { ('+' | '-') Term }

Term       := Factor { '*' Factor }

Factor     := Number | 'x' | '(' Expression ')'

Number     := Digit | Digit Number

Digit      := '0' | '1' | ... | '9'

#include<cstdio>
using namespace std;

char expr[210];
int p;

bool expression();

bool factor()
{
    bool t;
    if(expr[p]=='(')
    {
        p++;
        t=expression();
        p++;
    }
    else if(expr[p]=='!')
    {
        p++;
        t=!factor();
    }
    else
        t=(expr[p++]=='V'?true:false);
    return t;
}

bool term()
{
    bool ret=factor();
    while(expr[p]=='&')
    {
        p++;
        ret=ret&factor();
    }
    return ret;
}

bool expression()
{
    bool ret=term();
    while(expr[p]=='|')
    {
        p++;
        ret=ret|term();
    }
    return ret;
}

void Process()
{
    int i,j;
    for(i=j=0;expr[i]!='\0';i++)
        if(expr[i]!=' ')
            expr[j++]=expr[i];
    expr[j]='\0';
}
int main()
{
    int T=0;
    while(gets(expr))
    {
        Process();
        p=0;
        //puts(expr);
        printf("Expression %d: %s\n",++T,expression()?"V":"F");
    }
    return 0;
}

POJ  1472 Instant Complexity

与上题稍有不同的是需要用结构体来表示一元多项式

范式：

< Program > ::= "BEGIN" < Statementlist > "END" 

< Statementlist > ::= < Statement > | < Statement > < Statementlist > 

< Statement > ::= < LOOP-Statement > | < OP-Statement > 

< LOOP-Statement > ::= < LOOP-Header > < Statementlist > "END" 

< LOOP-Header > ::= "LOOP" < number > | "LOOP n" 

< OP-Statement > ::= "OP" < number >

#include<cstdio>
#include<cstring>
#include<cstdlib>
using namespace std;

struct term
{
    int a[11];
};

char s[22];

term Statementlist();

term OP()
{
    term t;
    char v[22];
    int i;
    for(i=0;i<11;i++)
        t.a[i]=0;
    scanf("%s%s",v,s);
    t.a[0]=atoi(v);
    return t;
}

term LOOP()
{
    char v[22];
    scanf("%s%s",v,s);
    int i,k;
    term t=Statementlist();
    if(v[0]=='n')
    {
        for(i=9;i>=0;i--)
            t.a[i+1]=t.a[i];
        t.a[0]=0;
    }
    else
        for(k=atoi(v),i=0;i<11;i++)
            t.a[i]*=k;
    scanf("%s",s);
    return t;
}

term Statement()
{
    if(!strcmp(s,"LOOP"))
        return LOOP();
    return OP();
}

term Statementlist()
{
    term t2,t=Statement();
    if(!strcmp(s,"END"))
        return t;
    t2=Statementlist();
    for(int i=0;i<11;i++)
        t.a[i]+=t2.a[i];
    return t;
} 

term Program()
{
    scanf("%s",s);
    scanf("%s",s);
    return Statementlist();
}

int main()
{
    int T,k,i;
    term t;
    scanf("%d",&T);
    for(k=1;k<=T;k++)
    {
        char sign[3]="";
        t=Program();
        printf("Program #%d\nRuntime = ",k);
        for(i=10;i>=0;i--)
            if(t.a[i])
            {
                printf("%s",sign);
                if(t.a[i]>1||i==0)
                    printf("%d",t.a[i]);
                if(t.a[i]>1&&i>0)
                    putchar('*');
                if(i>1)
                    printf("n^%d",i);
                if(i==1)
                    putchar('n');
                strcpy(sign,"+");
            }
        if(!strcmp(sign,""))
            putchar('0');
        puts("\n");
    }
    return 0;
}