C program to check syntax of 'for' loop

As defined by C standards, the for loop syntax is:

for (initialisation; condition; increment/decrement)
        ...

Syntactically, there should be two semicolons, one opening parenthesis, one closing parenthesis, and correct spelling of “for”. Hence, to check only the syntax of for loop, what a compiler does is check the following conditions:

Only “for” is written, and not “For”, “FOR”, “foR” or any of its variants.
Total statement consists of two semicolons “;” before the closing parenthesis “)” ends.
Presence of an opening parenthesis “(” after the “for” keyword, and presence of a closing parenthesis “)” at the end of statement.

Examples:

Input : for (i = 10; i < 20 i++) 
Output : Semicolon Error

Input : for(i = 10; i < 20; i++
Output : Closing parenthesis absent at end

Code –

#include <stdio.h> 
#include <stdlib.h> 
#include <string.h> 
  
//array to copy first three characters of string str 
char arr[3];  
  
void isCorrect(char *str) 
{ 
      
    //semicolon, bracket1, bracket2 are used  
        //to count frequencies of 
    //';', '(', and ')' respectively 
    //flag is set to 1 when an error is found, else no error 
    int semicolon = 0, bracket1 = 0, bracket2 = 0, flag = 0; 
      
    int i; 
    for (i = 0; i < 3; i++) 
        arr[i] = str[i]; 
          
    //first 3 characters of the for loop statement is copied 
    if(strcmp(arr, "for") != 0) 
    { 
        printf("Error in for keyword usage"); 
        return; 
    } 
      
    //Proper usage of "for" keyword checked 
    while(i != strlen(str)) 
    { 
        char ch = str[i++]; 
        if(ch == '(') 
        { 
            //opening parenthesis count 
            bracket1 ++; 
        } 
        else if(ch == ')') 
        { 
            //closing parenthesis count 
            bracket2 ++; 
              
        } 
        else if(ch == ';') 
        { 
            //semicolon count 
            semicolon ++; 
        } 
        else continue; 
          
    } 
      
    //check number of semicolons 
    if(semicolon != 2) 
    {  
        printf("\nSemicolon Error"); 
        flag++; 
    } 
      
    //check closing Parenthesis 
    else if(str[strlen(str) - 1] != ')') 
    {  
        printf("\nClosing parenthesis absent at end"); 
        flag++; 
    } 
      
    //check opening parenthesis 
    else if(str[3] == ' ' && str[4] != '(' ) 
    {  
        printf("\nOpening parenthesis absent after for keyword"); 
        flag++; 
    } 
      
    //check parentheses count 
    else if(bracket1 != 1 || bracket2 != 1 || bracket1 != bracket2) 
    {  
        printf("\nParentheses Count Error"); 
        flag++; 
    } 
      
    //no error  
    if(flag == 0) 
        printf("\nNo error"); 
              
} 
  
int main(void) { 
      
    char str1[100] = "for (i = 10; i < 20; i++)"; 
    isCorrect(str1); 
      
    char str2[100] = "for i = 10; i < 20; i++)"; 
    isCorrect(str2); 
      
    return 0; 
} 

Output :

No error
Opening parenthesis absent after for keyword

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C program to check syntax of ‘for’ loop

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