How-To Make an Compiler to Check IF and ELSE Counts

This program is a basic way to calculate the "Nested If-Else" in a program. This takes input from Console and puts that into a buffer, however you can take input from a file as : yyin();

Initial : Lex and Yacc are pre-installed in Linux. If you haven't install Flex and Bison in Windows, refer to This Tutorial - How to Compile Lex and Yacc

Step -1: Make a file named as if_else.l. Copy the below code and paste it in that file.

IF ELSE.L

1:  %{  
2:  #include"y.tab.h"  
3:  %}  
4:  %%  
5:  "if" {return IF;}  //Token for If statements
6:  "else" {return ELSE;}  //Token for Else Statements
7:  [sS][0-9]* {return S;}  //Statement symbol token
8:  "<"|">"|"="|"!="|"<="|">=" {return RELOP;}  //Relational Operator
9:  [0-9]+ {return NUMBER;}  //Number
10:  [a-zA-Z][a-zA-Z0-9_]* {return ID;}   //Ids
11:  \n {;}  
12:  . {return yytext[0];}  //Character Not Desfined
13:  %%  

Step-2: After completing step 1, you've to create a file in yacc readable format i.e. if_else.y and copy and paste the below code.

IF ELSE.Y

1:  %token IF RELOP S NUMBER ID ELSE  
2:  %{  
3:  int count=0;  
4:  %}  
5:  %%  
6:  stmt:if_stmt {printf("Nested : %d\n",count);};  
7:  if_stmt:IF'('cond')'if_stmt {count++;}  
8:  |IF'('cond')'S' 'ELSE' 'if_stmt {count++;}  
9:  |IF'('cond')'ELSE' 'if_stmt {count++;}  
10:  |S;  
11:  cond:x RELOP x;  
12:  x:ID  
13:  |NUMBER  
14:  ;  
15:  %%  
16:  int yywrap(){return 1;}  
17:  int yyerror(char *ch)  
18:  {  
19:  return 1;  
20:  }  
21:  int main(){  
22:  printf("Enter the Statement : \n");  
23:  yyparse();  
24:  return 1;  
25:  }  

Now as you've got lex and yacc files, let's compile a compiler.

root@user~:#lex if_else.l
root@user~:#yacc -d if_else.y
root@user~:#gcc lex.yy.c y.tab.c
root@user~:#./a.out

These code will help you in achieving result.

Enter the input as:

if(----)S
if(----)S else S
if(----)S else if(----)S
...........and so on.

How it works:

Consider an input:

if(----)S else if(----)S

now,

we have :

1:stmt:if_stmt {printf("Nested : %d\n",count);};  
2:if_stmt:IF'('cond')'if_stmt {count++;}  
3:  |IF'('cond')'S' 'ELSE' 'if_stmt {count++;}  
4:  |IF'('cond')'ELSE' 'if_stmt {count++;}  
5:  |S;  
6:  cond:x RELOP x;  
7:  x:ID  
8:  |NUMBER  

Starting from "stmt":

stmt                                                                      //start symbol
if_stmt                                                                  //rule 1
IF'('cond')'S' 'ELSE' 'if_stmt                                //rule 3
IF'('cond')'S' 'ELSE' 'IF'('cond')'if_stmt               //rule 1
IF'('cond')'S' 'ELSE' 'IF'('cond')'S                        //rule 5
IF'('x RELOP x')'S' 'ELSE' 'IF'('x RELOP x')'S      //rule 6
IF'('ID RELOP ID')'S' 'ELSE' 'IF'('ID RELOP ID')'S  //rule 7

Now, we have got tokens at the input places, that means our derivation is correct.