How to read two consecutive characters from a file in C?

Question

I have the following code, where I want to create a simply scanner for a simple calculator language. I am using fgetc to get the character from the file. Though, at some places I also need to check the next character that is followed. For this reason I have been using the ++ operator, but it seems like it is not working properly. Can someone help me fix my problem.

For example, when I have := in my text file, it prints colon, and then shows an error message saying: "; cannot follow : (colon).", whereas it should print "assign".

Here is my full code:

#include <regex.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>

int main(int argc, char** argv)
{
    FILE *fp;
    int c, check, reti_d, reti_l;

    regex_t digit, letter;
    reti_d = regcomp(&digit, "[0-9]", 0);
    reti_l = regcomp(&letter, "[a-zA-Z]", 0);

    if (reti_d || reti_l)
    {
        fprintf(stderr, "Couldn't compile the regular expression(s).\n");
        exit(1);
    }

    if (argc != 2)
    {
        fprintf(stderr, "Usage: %s filename.txt\n", argv[0]);
        exit(1);
    }

    if (!(fp = fopen(argv[1], "r")))
    {
        perror("Error opening file!\n");
        exit(1);
    }

    while ((c = fgetc(fp)) != EOF)
    {
        char regdtest[1];
        char regltest[1];
        regdtest[0] = (char)c;
        reti_d = regexec(&digit, regdtest, 0, NULL, 0);
        reti_l = regexec(&letter, regltest, 0, NULL, 0);

        switch (c)
        {
            case '(': printf("lperen "); break;
            case ')': printf("rparen "); break;
            case '+': printf("plus "); break;
            case '-': printf("minus "); break;
            case '*': printf("times "); break;
        }

        if (c == ':')
        {
            if (c++ == '=')
                printf("assign ");
            else printf("\n%c cannot follow : (colon).\n", (char)c);
        }

        if (c == '/')
        {
            if (++c == '/' || ++c == '*')
            {
                while (c != '\n' || (c == '*' && ++c == '/'))
                    c++;
            }
            else printf("div ");
        }

        if (c == '.')
        {
            regdtest[0] = (char)++c;
            reti_d = regexec(&digit, regdtest, 0, NULL, 0);

            if (!reti_d)
            {
                printf("number ");
            }
            else printf("\n. (dot) should be followed by digits.\n");
        }

        if (!reti_d)
        {
            while (!reti_d)
            {
                regdtest[0] = (char)c;
                reti_d = regexec(&digit, regdtest, 0, NULL, 0);
            }

            printf("number ");
        }

        if (!reti_l)
        {
            int i, j = 0;
            char read[5], write[6];
            read[i] = write[j] = (char)c;

            while(!reti_d || !reti_l)
            {
                c++;
                i++;
                j++;

                if (i >= 5)
                    i = 0;
                if (j >= 6)
                    j = 0;

                read[i] = write[j] = (char)c;
                if (strcmp(read, "read") == 0)
                    printf("read ");
                else printf("id ");

                if (strcmp(write, "write") == 0)
                    printf("write ");
                else printf("id ");

                regdtest[0] = (char)c;
                regltest[0] = (char)c;
                reti_d = regexec(&digit, regdtest, 0, NULL, 0);
                reti_l = regexec(&letter, regltest, 0, NULL, 0);
            }

            printf("letter ");
        }

    }

    fclose(fp);
    return 0;
}

Answer 1

You need to get the 2nd character by calling fgetc() once more.