一个简单的语法制导翻译器

最新推荐文章于 2020-01-08 10:38:15 发布

转载最新推荐文章于 2020-01-08 10:38:15 发布 · 163 阅读

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CC 4.0 BY-SA版权

原文链接：http://www.cnblogs.com/lovehyy/articles/3568098.html

本文介绍了一个逐步完善的中缀表达式转后缀表达式的翻译器实现过程，从基本加减运算开始，逐步增加了对数字、括号及乘除的支持。

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刚刚开始学习编译原理，重新开始写一个翻译器，从最简单的开始写一个最简单的例子，将中缀表达式改写成后缀表达式（单个数字）

#include<stdio.h>

char lookhead;
int main()
{
    void term();
    lookhead=getchar();
    term();
    while(1)
    {
        if(lookhead == '+')
        {
            lookhead=getchar();
            term();
            putchar('+');
        }
        if(lookhead == '-')
        {
            lookhead=getchar();
            term();
            putchar('-');
        }
    }
    
return 0;    
}

void term()
{
    if(lookhead>='0'&&lookhead<='9')
    {
        putchar(lookhead);
        lookhead=getchar();
    }
    else
    {
        printf("error");
    }
}

改进版

#include<stdio.h>

char lookhead;
int main()
{
    char myget();
    void term();
    void rest();
    lookhead=myget();
    term();
    rest();
return 0;    
}

char myget()
{
    char mylook;
    mylook = getchar();
    for(;;mylook = getchar())
    {
        if(mylook ==' '||mylook =='\t')
        {
            ;
        }
        else
        {
            return mylook;
        }
    }
    
}

void rest()
{
    while(1)
    {
        if(lookhead == '+')
        {
            lookhead=myget();
            term();
            putchar('+');
            putchar(' ');
            continue;
        }
        if(lookhead == '-')
        {
            lookhead=myget();
            term();
            putchar('-');
            putchar(' ');
            continue;
        }

    }
}
void term()
{
    int n;
    if(lookhead>='0' && lookhead<='9')
    {
        n=0;
        do
        {
            n = n*10 + lookhead - '0';
            lookhead=myget();
        }while(lookhead >= '0' && lookhead<='9');
        printf("%d ",n);
    }
    else
    {
        printf("error");
    }
}

改进版2（加入括号和乘除）

#include<stdio.h>

char lookhead;
int main()
{
    char myget();
    void term();
    void factor();
    void expr();
    lookhead=myget();
    factor();
    term();
    expr();
return 0;    
}

char myget()
{
    char mylook;
    mylook = getchar();
    for(;;mylook = getchar())
    {
        if(mylook ==' '||mylook =='\t')
        {
            ;
        }
        else
        {
            return mylook;
        }
    }
    
}

void expr()
{
    
    
        if(lookhead == '+')
        {
            lookhead=myget();
            expr();

            term();
            putchar('+');
            putchar(' ');
        }
        if(lookhead == '-')
        {
            lookhead=myget();
            expr();

            term();
            putchar('-');
            putchar(' ');
        }
        term();

    
}
void term()
{
    
    
        if(lookhead == '*')
        {
            lookhead=myget();
            term();
            putchar('*');
            putchar(' ');
            factor();
        }
        if(lookhead == '/')
        {
            lookhead=myget();
            term();
            putchar('/');
            putchar(' ');
            factor();
        }
        factor();

    

}
void factor()
{
    int n;
    if(lookhead>='0' && lookhead<='9')
    {
        n=0;
        do
        {
            n = n*10 + lookhead - '0';
            lookhead=myget();
        }while(lookhead >= '0' && lookhead<='9');
        printf("%d ",n);
    }
    if(lookhead == '(')
    {
        lookhead =myget();
        do
        {
            expr();

        }while(lookhead != ')');
        lookhead =myget();
    }
}

转载于:https://www.cnblogs.com/lovehyy/articles/3568098.html