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一、词法分析程序的功能
输入一段字符串，从字符串表示的源程序中识别出具有独立意义的单词符号，根据扫描到单词符号的第一个字符的种类，拼出相应的单词符号。
二、符号与种别码对照表
|
单词符号 |
种别码 |
单词符号 |
种别码 |
|
begin |
1 |
: |
17 |
|
if |
2 |
:= |
18 |
|
then |
3 |
< |
20 |
|
while |
4 |
<= |
21 |
|
do |
5 |
<> |
22 |
|
end |
6 |
> |
23 |
|
l(l|d)* |
10 |
>= |
24 |
|
dd* |
11 |
= |
25 |
|
+ |
13 |
; |
26 |
|
- |
14 |
( |
27 |
|
* |
15 |
) |
28 |
|
/ |
16 |
# |
0 |
三、用文法描述词法规则
<字母>：A→a|b|c|…|X|Y|Z
<数字>：B→0|1|2|…|9
<整数常数>：Z→C|ZC
C→0|1|2|...|9
<标识符>: I→X|IX|ID
X→a|b|c|…|X|Y|Z|_
D→0|1|2|...|9
<关键字>: K→const|var|procedure|begin|end|odd|if|then|call|while|do|read|write
<运算符>: Y→+|-|*|/|=|#|<|<=|>|>=|!=
<界符>: J→(|)|,|;|.
四、c语言源代码和运行结果截图
#include<stdio.h> #include<string.h> #include<math.h> char prog[80]; char token[8]; char ch; int syn,p,m,n; double sum; int count; int isSignal; int isDecimal; double decimal; int isExp; int index; int isNegative; double temp; int temp2; void scanner(); char *rwtab[6]={"begin","if","then","while","do","end"}; void main() { p=0; count=0; isDecimal=0; index=0; printf("请输入字符串:\n"); do{ ch=getchar(); prog[p++]=ch; }while(ch!=‘#‘); p=0; do{ scanner(); switch(syn) { case 11: if(isDecimal==0) { printf("(%2d,%8d)\n",syn,(int)sum); break; } else if(isExp==1) { printf("(%2d,%10.5e)\n",syn,sum); isExp=0; isDecimal=0; break; } else if(isDecimal==1) { printf("(%2d,%8.4f)\n",syn,sum); isDecimal=0; break; } case -1: printf("input error\n"); break; default: printf("(%2d,%8s)\n",syn,token); } }while(syn!=0); } void scanner() { sum=0; decimal=0; m=0; for(n=0;n<8;n++) token[n]=NULL; ch=prog[p++]; while(ch==‘ ‘) ch=prog[p++]; if(((ch>=‘a‘)&&(ch<=‘z‘))||((ch>=‘A‘)&&(ch<=‘Z‘))) { while(((ch>=‘a‘)&&(ch<=‘z‘))||((ch>=‘A‘)&&(ch<=‘Z‘))||((ch>=‘0‘)&&(ch<=‘9‘))) { token[m++]=ch; ch=prog[p++]; } token[m++]=‘\0‘; p--; syn=10; for(n=0;n<6;n++) if(strcmp(token,rwtab[n])==0) { syn=n+1; break; } } else if((ch>=‘0‘)&&(ch<=‘9‘)) { IsNum: if(isSignal==1) { } while((ch>=‘0‘)&&(ch<=‘9‘)) { sum=sum*10+ch-‘0‘; ch=prog[p++]; } if(ch==‘.‘) { isDecimal=1; ch=prog[p++]; while((ch>=‘0‘)&&(ch<=‘9‘)) { temp=(ch-‘0‘)*pow(0.1,++count); decimal=decimal+temp; ch=prog[p++]; } sum=sum+decimal; } if(ch==‘e‘||ch==‘E‘) { isExp=1; ch=prog[p++]; if(ch==‘-‘) { isNegative=1; ch=prog[p++]; } while((ch>=‘0‘)&&(ch<=‘9‘)) { index=index*10+ch-‘0‘; ch=prog[p++]; } if(isNegative) sum=sum*pow(0.1,index); else sum=sum*pow(10,index); } if(isSignal==1) { sum=-sum; isSignal=0; } p--; syn=11; } else switch(ch) { case ‘<‘: m=0; token[m++]=ch; ch=prog[p++]; if(ch==‘>‘) { syn=21; token[m++]=ch; } else if(ch==‘=‘) { syn=22; token[m++]=ch; } else { syn=20; p--; } break; case ‘>‘: m=0; token[m++]=ch; ch=prog[p++]; if(ch==‘=‘) { syn=24; token[m++]=ch; } else { syn=23; p--; } break; case ‘:‘: m=0; token[m++]=ch; ch=prog[p++]; if(ch==‘=‘) { syn=18; token[m++]=ch; } else { syn=17; p--; } break; case ‘+‘: temp2=prog[p]; if((temp2>=‘0‘)&&(temp2<=‘9‘)) { isSignal=2; ch=prog[p++]; goto IsNum; } syn=13; token[m++]=ch; break; case ‘-‘: temp2=prog[p]; if((temp2>=‘0‘)&&(temp2<=‘9‘)) { isSignal=1; ch=prog[p++]; goto IsNum; } syn=14; token[m++]=ch; break; case ‘*‘: syn=15; token[m++]=ch; break; case ‘/‘: syn=16; token[m++]=ch; break; case ‘=‘: syn=25; token[m++]=ch; break; case ‘;‘: syn=26; token[m++]=ch; break; case ‘(‘: syn=27; token[m++]=ch; break; case ‘)‘: syn=28; token[m++]=ch; break; case‘#‘: syn=0; token[m++]=ch; break; default: syn=-1; } }
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原文地址：http://www.cnblogs.com/xuyizhu/p/5924490.html
