Detecting Simple Syntax Errors

Syntax errors are very common in source programs. Suppose, a given C source program has been scanned, filtered, lexically analyzed and tokenized as that were done in earlier sessions. In addition, line numbers have been assigned to the source code lines for generating proper error messages. As the first step to Syntax Analysis, we now perform detection of simple syntax errors like duplication of tokens except parentheses or braces, Unbalanced braces or parentheses problem, unmatched ‘else’ problem, etc

Session 4 by Nafis Islam on Scribd

	#include<bits/stdc++.h>
	using namespace std;
	string int_to_string(int a){
	stringstream ss;
	ss << a;
	string str = ss.str();
	return str;
	}
	vector<string> number_lines(vector<string>sp){
	int flag = 0;
	string s;
	int flag3 = -1;
	for(int i=0;i<sp.size();i++){
	s = "";
	int sz = sp[i].size();
	flag3 = -1;
	for(int j=0;j<sz;j++) if(sp[i][j]=='\t') sp[i][j] = ' ';
	for(int j=0;j<sz;j++){
	if(j!=sz-1 && sp[i][j]!=' ' && sp[i][j+1]==' ') s = s + sp[i][j] + ' ';
	else if(sp[i][j]!=' ') s += sp[i][j];
	}
	for(int j=0;j<sz;j++){
	if(sp[i][j]=='"'){
	flag3 = j;
	break;
	}
	}
	if(flag3!=-1){
	string p = "";
	for(int j=0;s[j]!='"';j++) p += s[j];
	p += "\"";
	for(int j=flag3+1,r=0;sp[i][j]!='"';j++) p += sp[i][j];
	for(int j=0,r=0;j<s.size();j++){
	if(s[j]=='"') r++;
	if(r==2) p +=s[j];
	}
	swap(s,p);
	}
	swap(sp[i],s);
	}
	vector<string>sp1;
	int flag1 = 0,flag2=0;
	for(int i=0;i<sp.size();i++){
	string str = int_to_string(i+1);
	int sz = sp[i].size();
	if(sz==0){
	sp1.push_back(str);
	continue;
	}
	for(int j=0;j<sz;j++){
	if(j!=sz-1 && sp[i][j]=='/' && sp[i][j+1]=='/'){
	flag1 = 1;
	for(int k=0;k<j;k++){
	cout<<sp[i][k];
	cerr<<sp[i][k];
	}
	break;
	}
	if(j!=sz-1 && sp[i][j]=='/' && sp[i][j+1]=='*'){
	flag2 = 1;
	for(int k=0;k<j;k++){
	cout<<sp[i][k];
	cerr<<sp[i][k];
	}
	}
	if(j!=sz-1 && sp[i][j]=='*' && sp[i][j+1]=='/'){
	flag2 = 0;
	flag1 = 1;
	break;
	}
	}
	if(flag1){
	flag1 = 0;
	sp1.push_back(str);
	continue;
	}
	if(flag2){
	sp1.push_back(str);
	continue;
	}
	str = str + " " + sp[i];
	sp1.push_back(str);
	}
	return sp1;
	}
	vector<string> paranthesis_error(vector<string> sp){
	stack<int>st;
	vector<string>err;
	for(int i=0;i<sp.size();i++){
	for(int j=0;j<sp[i].size();j++){
	if(sp[i][j]=='{') st.push(i+1);
	else if(sp[i][j]=='}'){
	if( !st.empty() ) st.pop();
	else err.push_back("Error: Misplaced '}' at line "+int_to_string(i+1));
	}
	}
	}
	if( !st.empty() ) err.push_back("Error: Not Balanced Parentheses at line "+int_to_string(sp.size()));
	return err;
	}
	vector<string> if_else_error(vector<string> sp){
	bool ok = false;
	vector<string>err;
	int sz = sp.size();
	for(int i=0;i<sz;i++){
	if(sz<4)continue;
	int x = sp[i].size();
	for(int j=0;j<x;j++){
	if(j+1<x && sp[i][j]=='i' && sp[i][j+1]=='f') ok = true;
	if(j+3<x && sp[i][j]=='e' && sp[i][j+1]=='l' && sp[i][j+2]=='s' && sp[i][j+3]=='e'){
	if( ok ){
	ok = false;
	continue;
	}
	else err.push_back("Error: Not Matched else at line "+int_to_string(i+1));
	}
	}
	}
	return err;
	}
	bool comp(char a){
	if(a=='=' || a=='>' || a=='<' ) return false;
	return true;
	}
	bool col(char a){
	if(a==',' || a==';' || a=='+' || a=='-' || a=='*' || a=='/' || a=='(' || a==')' || a=='\'') return true;
	return false;
	}
	vector<string> dup_token_error(vector<string> sp){
	vector<string>err;
	int sz = sp.size();
	for(int j=0;j<sz;j++){
	string p = "",s=sp[j];
	for(int i=0;i<s.size();i++){
	if(col(s[i]) && col(s[i+1])==false) p = p+" "+s[i]+" ";
	else if(col(s[i]) && col(s[i+1])) p = p+" "+s[i];
	else p += s[i];
	}
	s = p[0];
	for(int i=1;i<p.size()-1;i++){
	if(p[i]=='=' && comp(p[i-1]) && comp(p[i+1])) s = s+" "+p[i]+" ";
	else s +=p[i];
	}
	p = "";
	for(int i=0;i<s.size();i++){
	if(i!=s.size()-1 && s[i]!=' ' && s[i+1]==' ') p = p + s[i] + ' ';
	else if(s[i]!=' ') p += s[i];
	}
	s = p[0];
	for(int i=1;i<p.size()-1;i++){
	if(comp(p[i])==false && comp(p[i+1])==false){
	s = s + " "+ p[i]+p[i+1] + " ";
	i++;
	}
	else s += p[i];
	}
	s+= p[p.size()-1];
	istringstream ss(s);
	string last = "";
	while(ss>>s){
	if(s==last) err.push_back("Error: Duplicate token at line "+int_to_string(j+1));
	last = s;
	}
	}
	return err;
	}
	int main(){
	freopen("input.txt","r",stdin);
	freopen("out.txt","w",stdout);
	string s;
	vector<string>sp,paran_error,if_else_err,dup_token_err,error;
	cerr<<"input\n";
	while(getline(cin,s)){
	sp.push_back(s);
	cerr<<s<<"\n";
	}
	cerr<<"\n";
	sp = number_lines(sp);
	cerr<<"\noutput:\n";
	cerr<<"Recognized tokens in the lines of code:\n";
	for(int i=0;i<sp.size();i++){
	cout<<sp[i]<<"\n";
	cerr<<sp[i]<<"\n";
	}
	paran_error = paranthesis_error(sp);
	if_else_err = if_else_error(sp);
	dup_token_err = dup_token_error(sp);
	paran_error.erase( unique( paran_error.begin(), paran_error.end() ), paran_error.end() );
	if_else_err.erase( unique( if_else_err.begin(), if_else_err.end() ), if_else_err.end() );
	dup_token_err.erase( unique( dup_token_err.begin(), dup_token_err.end() ), dup_token_err.end() );
	cout<<"\n\nERROR: \n";
	cerr<<"\n\nERROR: \n";
	for(int i=0;i<paran_error.size();i++){
	cout<<paran_error[i]<<"\n";
	cerr<<paran_error[i]<<"\n";
	}
	for(int i=0;i<if_else_err.size();i++){
	cout<<if_else_err[i]<<"\n";
	cerr<<if_else_err[i]<<"\n";
	}
	for(int i=0;i<dup_token_err.size();i++){
	cout<<dup_token_err[i]<<"\n";
	cerr<<dup_token_err[i]<<"\n";
	}
	return 0;
	}

view raw Detecting Simple Syntax Errors.cpp hosted with ❤ by GitHub

	/* A program fragment*/
	float x1 = 3.125;;
	/* Definition of function f1 */
	double f1(int int x)
	{if(x<x1)
	double z;
	else z = 0.01+x*5.5;}}
	else return z;
	}
	/* Beginning of 'main' */
	int main(void)
	{
	int n1; double z;
	{{ n1=25; z=f1(n1);}

view raw input.txt hosted with ❤ by GitHub

	1
	2
	3 float x1 = 3.125;;
	4
	5 double f1(int int x)
	6 {if(x<x1)
	7 double z;
	8 else z = 0.01+x*5.5;}}
	9 else return z;
	10 }
	11
	12 int main(void)
	13 {
	14 int n1; double z;
	15 {{ n1=25; z=f1(n1);}
	ERROR:
	Error: Misplaced '}' at line 8
	Error: Misplaced '}' at line 10
	Error: Not Balanced Parentheses at line 15
	Error: Not Matched else at line 9
	Error: Duplicate token at line 3
	Error: Duplicate token at line 5

view raw output.txt hosted with ❤ by GitHub