Simple interpreter in C

Question

I made simple interpreter in C and now, I want to make my code better.

Here is my code:

#include <stdio.h>
#include <stdlib.h>
#include <stddef.h>
#include <string.h>

#include "def.h"

int main(int argc, char *argv[])
{
    int i;
    char ch[100];
    char input[100];
    char sinput[100];
    char output[100];
    int while_int;
    if (argv[1] != NULL)
    {
        char *file_extension = strrchr(argv[1], '.');
        if (file_extension == NULL)
        {
            printf("Error\nNo File Extension\n");
        }
        else if (strcmp(file_extension, ".pri") == FALSE)
        {
            FILE *fp = fopen(argv[1], "r");
            fgets(ch, 100, fp);
            fclose(fp);
        }
    }
    do
    {
        if (argv[1] == NULL)
        {
            printf(">>> ");
            scanf("%[^\n]%*c", ch);
        }
        if (ch[0] != '@' & argv[1] == NULL)
        {
            while_int = TRUE;
        }
        else
        {
            while_int = FALSE;
        }
        if (ch[0] == '"' & ch[strlen(ch) - 1] == '"')
        {
            if (strlen(ch) == 1 | strlen(ch) == 2)
            {
                printf("Error\nError Code: PRINTLANG.1\n");
                ch[0] = '@';
            }
            else
            {
                chh(ch, output, 0, strlen(ch) - 1);
                printf("%s\n", output);
                ch[0] = '@';
            }
        }
        if (ch[0] == 'p' & ch[1] == 'r' & ch[2] == 'i' & ch[3] == 'n' & ch[4] == 't')
        {
            if (ch[5] == '(' & ch[strlen(ch) - 1] == ')')
            {
                if (ch[6] == '"' & ch[strlen(ch) - 2] == '"')
                {
                    if (strlen(ch) == 8 | strlen(ch) == 9)
                    {
                        printf("Error\nError Code: PRINTLANG.2\n");
                        ch[0] = '@';
                    }
                    else
                    {
                        chh(ch, output, 6, strlen(ch) - 2);
                        printf("%s\n", output);
                        ch[0] = '@';
                    }
                }
                else if (ch[6] == 'i' & ch[7] == 'n' & ch[8] == 'p' & ch[9] == 'u' & ch[10] == 't')
                {
                    if (ch[11] == '(' & ch[strlen(ch) - 2] == ')')
                    {
                        if (strlen(ch) == 14)
                        {
                            scanf("%[^\n]%*c", input);
                            printf("%s\n", input);
                            ch[0] = '@';
                        }
                        else
                        {
                            if (ch[12] == '"' & ch[strlen(ch) - 3] == '"')
                            {
                                if (strlen(ch) == 16)
                                {
                                    printf("Error\nError Code: PRINTLANG.3\n");
                                    ch[0] = '@';
                                }
                                else
                                {
                                    chh(ch, output, 12, strlen(ch) - 3);
                                    printf("%s", output);
                                    scanf("%[^\n]%*c", input);
                                    printf("%s\n", input);
                                    ch[0] = '@';
                                }
                            }
                            else
                            {
                                printf("Error\nError Code: PRINTLANG.4\n");
                            }
                        }
                    }
                    else
                    {
                        printf("Error\nError Code: PRINTLANG.5\n");
                        ch[0] = '@';
                    }
                }
                else
                {
                    printf("Error\nError Code: PRINTLANG.6\n");
                    ch[0] = '@';
                }
            }
            else
            {
                printf("Error\nError Code: PRINTLANG.7\n");
                ch[0] = '@';
            }
        }
        else if (ch[0] == 'e' & ch[1] == 'c' & ch[2] == 'h' & ch[3] == 'o')
        {
            if (ch[4] == ' ')
            {
                if (ch[5] == '"' & ch[strlen(ch) - 1] == '"')
                {
                    chh(ch, output, 5, strlen(ch) - 1);
                    printf("%s\n", output);
                    ch[0] = '@';
                }
                else
                {
                    printf("Error\nError Code: PRINTLANG.8\n");
                    ch[0] = '@';
                }
            }
            else
            {
                printf("Error\nError Code: PRINTLANG.9\n");
                ch[0] = '@';
            }
        }
        else
        {
            ch[0] = '@';
        }
    }
    while (while_int);
}

def.h:

#ifndef _DEF_H
# define _DEF_H
#endif

#define TRUE (1 == 1)
#define FALSE (1 == 0)

char *chh(char *ch, char *output, int x, int y)
{
    int i;
    for (i = x + 1; i < y; i++)
    {
        output[i - (x + 1)] = ch[i];
    }
    return output;
}

GitHub Link

Recommended question (My question)

Answer 1

Use a parser generator

Since it seems like you intend to extend the language you are currently supporting, I cannot recommend that you continue writing the parsing code yourself. It is already not pretty, and even though it could be improved, there are common tools available that, given a description of the tokens and the grammar of your language, generate the C code necessary to parse that language. Most well known are GNU Flex and GNU Bison.

Split up your program into multiple functions

Apart from the one function in def.h, which should actually be in its own .c file, you have all the functionality inside main(). You should try to split up your program into individual functions that each do a simple thing. This will make your code easier to read and easier to maintain.

Print useful error messages

When your interpreter encounters an error, it just prints:

Error
Error Code: PRINTLANG.n

Where n is some number. This is very unhelpful. Error messages are there to inform the user what has gone wrong. So write something that tells the user what was wrong, and if possible also a suggestion of how they can do to fix the problem. For example, instead of PRINTLANG.1, write:

Error: empty or unterminated quoted string

Also make it a habit to write error messages to stderr.

Answer 2

I assume that you are learning how to write code. So, to keep it simple, I think you should focus on dividing your code into layers of abstraction. If a function becomes too long, break it into subfunctions. Try to keep code in a function at a uniform abstraction level, avoid mixing high level and low level work.

This is an art that takes a lot of practice to get right. Instead of thinking of your code as something that the computer will execute, you should think of it as something another human will read. That will give you the right mindset.

Introducing layers of abstraction to your code, might look like the code below. Notice how redundant variables have been eliminated. This code makes more sense if you start at the bottom. Try to improve it further by making it readable from the top as well. To do this you could introduce some comments that explains the context, or you could rethink the functions to be generic, instead of specific to a parent calling them at one exact spot - like they are now.

You should spend as much time making your code readable, as you have spent time making it work correctly.

Happy coding.

#include <stdio.h>
#include <stdlib.h>
#include <stddef.h>
#include <string.h>

#include "def.h"

void do_quotes(char* ch)
{
    char output[100];

    if (strlen(ch) == 1 | strlen(ch) == 2)
    {
        printf("Error\nError Code: PRINTLANG.1\n");
        ch[0] = '@';
    }
    else
    {
        chh(ch, output, 0, strlen(ch) - 1);
        printf("%s\n", output);
        ch[0] = '@';
    }
}

void do_print_quote(char* ch)
{
    char output[100];

    if (strlen(ch) == 8 | strlen(ch) == 9)
    {
        printf("Error\nError Code: PRINTLANG.2\n");
        ch[0] = '@';
    }
    else
    {
        chh(ch, output, 6, strlen(ch) - 2);
        printf("%s\n", output);
        ch[0] = '@';
    }
}

void do_print_input(char* ch)
{
    char output[100];
    char input[100];

    if (ch[11] == '(' & ch[strlen(ch) - 2] == ')')
    {
        if (strlen(ch) == 14)
        {
            scanf("%[^\n]%*c", input);
            printf("%s\n", input);
            ch[0] = '@';
        }
        else
        {
            if (ch[12] == '"' & ch[strlen(ch) - 3] == '"')
            {
                if (strlen(ch) == 16)
                {
                    printf("Error\nError Code: PRINTLANG.3\n");
                    ch[0] = '@';
                }
                else
                {
                    chh(ch, output, 12, strlen(ch) - 3);
                    printf("%s", output);
                    scanf("%[^\n]%*c", input);
                    printf("%s\n", input);
                    ch[0] = '@';
                }
            }
            else
            {
                printf("Error\nError Code: PRINTLANG.4\n");
            }
        }
    }
    else
    {
        printf("Error\nError Code: PRINTLANG.5\n");
        ch[0] = '@';
    }
}

void do_print(char* ch)
{
    if (ch[5] == '(' & ch[strlen(ch) - 1] == ')')
    {
        if (ch[6] == '"' & ch[strlen(ch) - 2] == '"')
        {
            do_print_quote(ch);
        }
        else if (ch[6] == 'i' & ch[7] == 'n' & ch[8] == 'p' & ch[9] == 'u' & ch[10] == 't')
        {
            do_print_input(ch);
        }
        else
        {
            printf("Error\nError Code: PRINTLANG.6\n");
            ch[0] = '@';
        }
    }
    else
    {
        printf("Error\nError Code: PRINTLANG.7\n");
        ch[0] = '@';
    }
}

void do_echo(char* ch)
{
    char output[100];

    if (ch[4] == ' ')
    {
        if (ch[5] == '"' & ch[strlen(ch) - 1] == '"')
        {
            chh(ch, output, 5, strlen(ch) - 1);
            printf("%s\n", output);
            ch[0] = '@';
        }
        else
        {
            printf("Error\nError Code: PRINTLANG.8\n");
            ch[0] = '@';
        }
    }
    else
    {
        printf("Error\nError Code: PRINTLANG.9\n");
        ch[0] = '@';
    }
}

int main(int argc, char *argv[])
{
    char ch[100];
    int while_int;
    if (argv[1] != NULL)
    {
        char *file_extension = strrchr(argv[1], '.');
        if (file_extension == NULL)
        {
            printf("Error\nNo File Extension\n");
        }
        else if (strcmp(file_extension, ".pri") == FALSE)
        {
            FILE *fp = fopen(argv[1], "r");
            fgets(ch, 100, fp);
            fclose(fp);
        }
    }
    do
    {
        if (argv[1] == NULL)
        {
            printf(">>> ");
            scanf("%[^\n]%*c", ch);
        }
        while_int = (ch[0] != '@' & argv[1] == NULL);
        if (ch[0] == '"' & ch[strlen(ch) - 1] == '"')
        {
            do_quotes(ch);
        }
        if (ch[0] == 'p' & ch[1] == 'r' & ch[2] == 'i' & ch[3] == 'n' & ch[4] == 't')
        {
            do_print(ch);
        }
        else if (ch[0] == 'e' & ch[1] == 'c' & ch[2] == 'h' & ch[3] == 'o')
        {
            do_echo(ch);
        }
        else
        {
            ch[0] = '@';
        }
    }
    while (while_int);
}