exercise1-24.md

Exercise1-24

Write a program to check a C program for rudimentary syntax errors like unbalanced parentheses, brackets and braces. Don't forget about quotes, both single and double, escape sequences and comments. (This program is hard if you do it in full generality.)

Process

Wrote this one over a day and kept a small scope.

It checks for:

1. matching parentheses, brackets and braces
2. matching single and double quotes
3. matching comments
4. escape sequences within single or double quotes

I set a hierarchy so that it doesn't check for parentheses, brackets or braces if there is a char, string or comment being set, which could result in strange error messages further down the line

This code is quite verbose and I'm wondering if theres a simple way to cut down on a lot of repetitive conditional statements, but overall I'm happy it works. I can see you could really go down a rabbit hole creating all the ins and outs of syntax validation

Final Code

#include <stdio.h>

int main() {
    int c;
    int foundError = 0;
    
    int singleQuote = 0;
    int doubleQuote = 0;
    int escapeSeq = 0;
    int comment = 0;
    
    int braceBegin = 0;
    int braceEnd = 0;
    int bracketBegin = 0;
    int bracketEnd = 0;
    int parenthesesBegin = 0;
    int parenthesesEnd = 0;
    
    /* 
        COMMENT STATUS
        0 no comment
        1 found '/'
        2 single line comment
        3 multiline comment
        4 found '*'
    */
    
    while((c = getchar()) != EOF) {
        
        /* single quote mode */
        if(c=='\'' && doubleQuote==0 && escapeSeq==0 && comment==0){
            if(singleQuote==1)
                singleQuote=0;
            else
                singleQuote=1;
        }
        
        /* double quote mode */
        if(c=='\"' && singleQuote==0 && escapeSeq==0 && comment==0){
            if(doubleQuote==1)
                doubleQuote=0;
            else
                doubleQuote=1;
        }
        
        
        /* comment begin mode */
        if(c=='/' && singleQuote==0 && doubleQuote==0 && escapeSeq==0 && comment!=2){

            if(comment==1)
                comment=2; /* single line comment begin */
            else {
                if(comment==4)
                    comment=0; /* found end of multiline comment*/
                else
                    comment=1;
            }
        }
        
        /* comment single line mode */
        if(c=='\n' && doubleQuote==0 && comment==2)
            comment=0; /* turn off single line comment */
        
        /* comment start mode */
        if(c=='*' && singleQuote==0 && doubleQuote==0 && escapeSeq==0 && comment!=0 && comment!=2){
            if(comment==1){
                comment = 3; /* multiline comment begin */
            }
            else {
                if(comment==3){
                    comment = 4; /* maybe multiline comment end */
                }
            }
        }
        
        
        /* bracket begin */
        if(c=='[' && singleQuote==0 && doubleQuote==0 && escapeSeq==0 && comment==0)
            bracketBegin++;
        
        /* bracket end */
        if(c==']' && singleQuote==0 && doubleQuote==0 && escapeSeq==0 && comment==0)
            bracketEnd++;
        
        /* brace begin */
        if(c=='{' && singleQuote==0 && doubleQuote==0 && escapeSeq==0 && comment==0)
            braceBegin++;
        
        /* brace end */
        if(c=='}' && singleQuote==0 && doubleQuote==0 && escapeSeq==0 && comment==0)
            braceEnd++;
        
        /* parentheses begin */
        if(c=='(' && singleQuote==0 && doubleQuote==0 && escapeSeq==0 && comment==0)
            parenthesesBegin++;
        
        /* parentheses end */
        if(c==')' && singleQuote==0 && doubleQuote==0 && escapeSeq==0 && comment==0)
            parenthesesEnd++;
        
        /* any character with escape sequence mode on*/
        if(escapeSeq==1){
            if(c=='n' || c=='\\' || c=='t'){
                escapeSeq = 0;
            } else {
                printf("Error with escape sequence\n");
                return 0;
            }
        }
        
        /* escape sequence mode */
        if(c=='\\' && (singleQuote==1 || doubleQuote==1) &&escapeSeq == 0){
                escapeSeq = 1;
        }
    }
    
    if(singleQuote==1) {
        foundError++;
        printf("Error! ");
        printf("Single quote not closed\n");
    }
  
    if(doubleQuote==1){
        foundError++;
        printf("Error! ");
        printf("Double quote not closed\n");
    }
        
    if(escapeSeq==1){
        foundError++;
        printf("Error! ");
        printf("Escape sequence not closed\n");
    }

    if(comment==1 || comment ==3){
        foundError++;
        printf("Error! ");
        printf("Comment not closed\n");
    }
        
    if(comment==2){
        foundError++;
        printf("Error! ");
        printf("Singleline comment not closed\n");
    }
        
    
    if(bracketBegin!=bracketEnd){
        foundError++;
        printf("Error! ");
        printf("Found bracket not closed\n");
    }
        
    if(braceBegin!=braceEnd){
        foundError++;
        printf("Error! ");
        printf("Found brace not closed\n");
    }
        
    if(parenthesesBegin!=parenthesesEnd){
        foundError++;
        printf("Error! ");
        printf("Found parentheses not closed\n");
    }
    
    if(foundError==0)
        printf("No Errors Found");
    else     
        printf("Found %d error types", foundError);
    
    return 0;
}

Output

Without syntax errors

With syntax errors

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