Lexical Analyzer Syntax Tokenization

#include<iostream>
#include<fstream>
#include<stdlib.h>
#include<string.h>
#include<ctype.h>

using namespace std;

int isKeyword(char buffer[]){
    char keywords[32][10] = {"auto","break","case","char","const","continue","default",
                            "do","double","else","enum","extern","float","for","goto",
                            "if","int","long","register","return","short","signed",
                            "sizeof","static","struct","switch","typedef","union",
                            "unsigned","void","volatile","while"};
    int i, flag = 0;

    for(i = 0; i < 32; ++i){
        if(strcmp(keywords[i], buffer) == 0){
            flag = 1;
            break;
        }
    }

    return flag;
}
bool isInteger(char* str)
{
    int i, length = strlen(str);

    if (length == 0)
        return false;
    for (i = 0; i < length; i++) {
        if (str[i] != '0' && str[i] != '1' && str[i] != '2'
            && str[i] != '3' && str[i] != '4' && str[i] != '5'
            && str[i] != '6' && str[i] != '7' && str[i] != '8'
            && str[i] != '9' || (str[i] == '-' && i > 0))

        return false;
    }
    return true;
}
bool isRealNumber(char* str)
{
    int i, length = strlen(str);
    bool hasDecimal = false;


    if (length == 0)
        return false;
    for (i = 0; i < length; i++) {
        if (str[i] != '0' && str[i] != '1' && str[i] != '2'
            && str[i] != '3' && str[i] != '4' && str[i] != '5'
            && str[i] != '6' && str[i] != '7' && str[i] != '8'
            && str[i] != '9' && str[i] != '.' ||
            (str[i] == '-' && i > 0))
            return false;
        if (str[i] == '.')
            hasDecimal = true;
    }
    return hasDecimal;
}

int main(){
    char character, buffer[15], operators[] = "+-*/%=";
    ifstream file("compiler.txt");
    int i,j=0;

    if(!file.is_open()){
        cout<<"Can not open the file\n";
        exit(0);
    }

    while(!file.eof()){ // .eof return true when there is no data in the file and return false otherwise
        character = file.get();

        for(i = 0; i < 6; ++i){
            if(character == operators[i])
                cout<<character<<" is operator\n";
        }

        if(isalnum(character)){
            buffer[j++] = character;
        }
        else if((character == ' ' || character == '\n') && (j != 0)){
                buffer[j] = '\0';
                j = 0;

                if(isKeyword(buffer) == 1)
                    cout<<buffer<<" is keyword\n";
                else if(isInteger(buffer))
                    cout<<buffer<<" is an Integer\n";
                else if(isRealNumber(buffer))
                    cout<<buffer<<" is a Real Number\n";
                else
                    cout<<buffer<<" is Identifier\n";
        }

    }

    file.close();

    return 0;
}