Symbol Table in Compiler Construction

In computer science, a symbol table is a data structure used by a language translator such as a compiler or interpreter, where each identifier in a program’s source code is associated with information relating to its declaration or appearance in the source. A common implementation technique is to use a hash table. There are also trees, linear lists and self-organizing lists which can be used to implement a symbol table. It also simplifies the classification of literals in tabular format. The symbol table is accessed by most phases of a compiler, beginning with the lexical analysis to optimization.

Activity Outcomes:

This lab teaches you

Implementation of symbol table with arrays

Student should have prior knowledge regarding symbol table

Introduction

// Declare an external function

extern double bar(double x);

// Define a public function

double foo(int count)

{

double sum = 0.0;

// Sum all the values bar(1) to bar(count)

for (int i = 1; i <= count; i++) sum += bar((double) i);

return sum;

}

A C compiler that parses this code will contain at least the following symbol table entries:

Stage a1 (apply) Lab Activities:

Activity 1:

Implement symbol table using array data structure

Solution:

using System;

using System.Collections.Generic; using System.ComponentModel; using System.Data;

using System.Drawing; using System.Linq; using System.Text;

using System.Text.RegularExpressions; using System.Threading.Tasks;

using System.Windows.Forms; using System.Collections;

namespace LexicalAnalyzerV1

{

public partial class Form1 : Form

{

public Form1()

{

InitializeComponent();

}

private void btn_Input_Click(object sender, EventArgs e)

{

//taking user input from rich textbox String userInput = tfInput.Text;

//List of keywords which will be used to seperate keywords from variables List<String> keywordList = new List<String>();

keywordList.Add(“int”); keywordList.Add(“float”); keywordList.Add(“while”); keywordList.Add(“main”); keywordList.Add(“if”); keywordList.Add(“else”); keywordList.Add(“new”);

//row is an index counter for symbol table int row = 1;

//count is a variable to incremenet variable id in tokens int count = 1;

//line_num is a counter for lines in user input int line_num = 0;

//SymbolTable is a 2D array that has the following structure

//[Index][Variable Name][type][value][line#]

//rows are incremented with each variable information entry

String[,] SymbolTable = new String[20, 6];

List<String> varListinSymbolTable = new List<String>();

//Input Buffering

ArrayList finalArray = new ArrayList(); ArrayList finalArrayc = new ArrayList();

ArrayList tempArray = new ArrayList(); char[] charinput = userInput.ToCharArray();

9]+)?$”);

//Regular Expression for Variables

Regex variable_Reg = new Regex(@”^[A-Za-z|_][A-Za-z|0-9]*$”);

//Regular Expression for Constants

Regex constants_Reg = new Regex(@”^[0-9]+([.][0-9]+)?([e]([+|-])?[0-9]+)?$”);

//Regular Expression for Operators

Regex operators_Reg = new Regex(@”^[-*+/><&&||=]$”);

//Regular Expression for Special_Characters

Regex Special_Reg = new Regex(@”^[.,’\[\]{}();:?]$”);

for (int itr = 0; itr < charinput.Length; itr++)

{

Match Match_Variable = variable_Reg.Match(charinput[itr] + “”); Match Match_Constant = constants_Reg.Match(charinput[itr] + “”); Match Match_Operator = operators_Reg.Match(charinput[itr] + “”); Match Match_Special = Special_Reg.Match(charinput[itr] + “”);

if (Match_Variable.Success || Match_Constant.Success || Match_Operator.Success || Match_Special.Success || charinput[itr].Equals(‘ ‘))

{

tempArray.Add(charinput[itr]);

}

if (charinput[itr].Equals(‘\n’))

{

if (tempArray.Count != 0)

{

int j = 0; String fin = “”;

for (; j < tempArray.Count; j++)

{

fin += tempArray[j];

}

finalArray.Add(fin); tempArray.Clear();

}

if (tempArray.Count != 0)

{

int j = 0; String fin = “”;

for (; j < tempArray.Count; j++)

{

fin += tempArray[j];

}

finalArray.Add(fin); tempArray.Clear();

}

// Final Array SO far correct

tfTokens.Clear(); symbolTable.Clear();

//looping on all lines in user input

for (int i = 0; i < finalArray.Count; i++)

{

String line = finalArray[i].ToString();

//tfTokens.AppendText(line + “\n”);

char[] lineChar = line.ToCharArray(); line_num++;

//taking current line and splitting it into lexemes by space

for (int itr = 0; itr < lineChar.Length; itr++)

{

Match Match_Variable = variable_Reg.Match(lineChar[itr] + “”); Match Match_Constant = constants_Reg.Match(lineChar[itr] + “”); Match Match_Operator = operators_Reg.Match(lineChar[itr] + “”); Match Match_Special = Special_Reg.Match(lineChar[itr] + “”); if (Match_Variable.Success || Match_Constant.Success)

{

tempArray.Add(lineChar[itr]);

}

if (lineChar[itr].Equals(‘ ‘))

{

if (tempArray.Count != 0)

{

int j = 0; String fin = “”;

for (; j < tempArray.Count; j++)

{

fin += tempArray[j];

}

finalArrayc.Add(fin); tempArray.Clear();

}

if (Match_Operator.Success || Match_Special.Success)

{

if (tempArray.Count != 0)

{

int j = 0; String fin = “”;

for (; j < tempArray.Count; j++)

{

fin += tempArray[j];

}

finalArrayc.Add(fin); tempArray.Clear();

}

finalArrayc.Add(lineChar[itr]);

}

if (tempArray.Count != 0)

{

String fina = “”;

for (int k = 0; k < tempArray.Count; k++)

{

fina += tempArray[k];

}

finalArrayc.Add(fina); tempArray.Clear();

}

// we have asplitted line here

for (int x = 0; x < finalArrayc.Count; x++)

{

Match operators = operators_Reg.Match(finalArrayc[x].ToString());

Match variables =variable_Reg.Match(finalArrayc[x].ToString());

Match digits = constants_Reg.Match(finalArrayc[x].ToString()); Match punctuations =Special_Reg.Match(finalArrayc[x].ToString());

if (operators.Success)

{

// if a current lexeme is an operator then make a token e.g. < op, = >

tfTokens.AppendText(“< op, ” + finalArrayc[x].ToString() + “> “);

}

else if (digits.Success)

{

// if a current lexeme is a digit then make a token e.g. <digit, 12.33 >

tfTokens.AppendText(“< digit, ” + finalArrayc[x].ToString() + “> “);

}

else if (punctuations.Success)

{ // if a current lexeme is a punctuation then make a token e.g. < punc, ; >

tfTokens.AppendText(“< punc, ” + finalArrayc[x].ToString() + “> “);

}

else if (variables.Success)

{

// if a current lexeme is a variable and not a keyword

if (!keywordList.Contains(finalArrayc[x].ToString())) // if it is not a keyword

{

// check what is the category of varaible, handling only two cases here

//Category1- Variable initialization of type digit e.g. int count = 10 ;

//Category2- Variable initialization of type String e.g. String var = ‘ Hello ‘ ;

Regex reg1 = new Regex(@”^(int|float|double)\s([A-Za- z|_][A-Za-z|0-9]{0,10})\s(=)\s([0-9]+([.][0-9]+)?([e][+|-]?[0-9]+)?)\s(;)$”); // line

of type int alpha = 2 ;

Match category1 = reg1.Match(line);

Regex reg2 = new Regex(@”^(String|char)\s([A-Za- z|_][A-Za-z|0-9]{0,10})\s(=)\s[‘]\s([A-Za-z|_][A-Za-z|0-9]{0,30})\s[‘]\s(;)$”); // line of type String alpha = ‘ Hello ‘ ;

Match category2 = reg2.Match(line);

//if it is a category 1 then add a row in symbol table containing the information related to that variable

if (category1.Success)

{

SymbolTable[row, 1] = row.ToString(); //index

SymbolTable[row, 2] = finalArrayc[x].ToString();

SymbolTable[row, 3] = finalArrayc[x -1].ToString(); //type

SymbolTable[row, 4] = finalArrayc[x+2].ToString();//value

SymbolTable[row, 5] = line_num.ToString(); // line number

tfTokens.AppendText(“<var” + count + “, ” + row +”> “);

symbolTable.AppendText(SymbolTable[row, 1].ToString() + ” \t “);

symbolTable.AppendText(SymbolTable[row, 2].ToString() + ” \t “);

symbolTable.AppendText(SymbolTable[row, 3].ToString() + ” \t “);

symbolTable.AppendText(SymbolTable[row, 4].ToString() + ” \t “);

symbolTable.AppendText(SymbolTable[row, 5].ToString() + ” \t “);

row++; count++;

}

//if it is a category 2 then add a row in symbol table containing the information related to that variable

else if (category2.Success)

{

// if a line such as String var = ‘ Hello ‘ ; comes and the loop moves to index of array containing Hello ,

//then this if condition prevents addition of Hello in symbol Table because it is not a variable it is just a string

if (!(finalArrayc[x-1].ToString().Equals(“‘”) && finalArrayc[x+1].ToString().Equals(“‘”)))

SymbolTable[row, 1] = row.ToString(); // index

SymbolTable[row, 2] = finalArrayc[x].ToString(); //varname

SymbolTable[row, 3] = finalArrayc[x-1].ToString(); //type

SymbolTable[row, 4] =finalArrayc[x+3].ToString(); //value

SymbolTable[row, 5] = line_num.ToString(); //line number

tfTokens.AppendText(“<var” + count + “, ” + row + “> “);

symbolTable.AppendText(SymbolTable[row, 1].ToString() + ” \t “);

symbolTable.AppendText(SymbolTable[row, 2].ToString() + ” \t “);

symbolTable.AppendText(SymbolTable[row, 3].ToString() + ” \t “);

symbolTable.AppendText(SymbolTable[row, 4].ToString() + ” \t “);

symbolTable.AppendText(SymbolTable[row, 5].ToString() + ” \t “);

row++; count++;

}

else

{

tfTokens.AppendText(“<String” + count + “, ” +finalArrayc[x].ToString() + “> “);

}

else

{

// if any other category line comes in we check if we have initializes that varaible before,

// if we have initiazed it before then we put the index of that variable in symbol table, in its token

String ind = “Default”; String ty = “Default”; String val = “Default”; String lin = “Default”;

for (int r = 1; r <= SymbolTable.GetLength(0);

r++)

{

//search in the symbol table if variable entry already exists

if (SymbolTable[r,2].Equals(finalArrayc[x].ToString()))

{

ind = SymbolTable[r, 1];

ty = SymbolTable[r, 3];

val = SymbolTable[r, 4];

lin = SymbolTable[r, 5];

tfTokens.AppendText(“<var” + ind + “, ” +ind + “> “);

break;

}

// if a current lexeme is not a variable but a keyword then make a token such as: <keyword, int>

else

{

tfTokens.AppendText(“<keyword, ” + finalArrayc[x].ToString() + “> “);

}

tfTokens.AppendText(“\n”); finalArrayc.Clear();

}

#region Display Symbol Table

for (int j = 0; j < Symboltable.Count; j++)

{

for (int z = 0; z < Symboltable[j].Count; z++)

{ ST.AppendText(Symboltable[j][z] + “\t”); } ST.AppendText(“\n”);

}

#endregion

}

#region Make Entry Symbol Table

void Check_And_Make_Entries()

{

KeyWords.Remove(“begin”); KeyWords.Remove(“end”); KeyWords.Remove(“print”);

KeyWords.Remove(“if”); KeyWords.Remove(“else”);

if (lexemes_per_line – 4 == 0 || lexemes_per_line – 7 == 0)

{

if (lexemes_per_line == 7)

{

Variables.RemoveAt(Variables.Count – 1);

}

for (; ST_index < KeyWords.Count; ST_index++)

{

Symboltable.Add(new List<string>()); Symboltable[ST_index].Add(ST_index + 1 + “”); Symboltable[ST_index].Add(Variables[ST_index] + “”); Symboltable[ST_index].Add(KeyWords[ST_index] + “”); Symboltable[ST_index].Add(Constants[ST_index] + “”); Symboltable[ST_index].Add(LineNumber[ST_index] + “”);

}

if (lexemes_per_line – 6 == 0)

{

Variables.RemoveAt(Variables.Count – 1);

Variables.RemoveAt(Variables.Count – 1); Variables.RemoveAt(Variables.Count – 1);

}

#endregion

Home Activities:

Activity 1:

Implement symbol table using hash function

Assignment:

Submit the home activity before next lab.

Related Links

#Compiler Construction complete course # Compiler Construction past paper # Compiler Construction project #Computer Science all courses #University Past Paper #Programming language #Introduction to C# #Lexical Analyzer Recognition of operators/variables #Recognition of keywords/constants #Lexical Analyzer Input Buffering scheme #Symbol Table in Compiler Construction #First set of a given grammar using Array #Follow set of a given grammar using Array #Bottom-up Parser-I DFA Implementation #Bottom-up Parser-II Stack parser using SLR #Semantic Analyzer

Symbol Table in Compiler Construction

Symbol Table in Compiler Construction

Activity Outcomes:

Introduction

Activity 1:

Solution:

Home Activities:

Activity 1:

Assignment:

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