Simple math parser - Louis Wilke

/*\
Script by Louis Wilke
26.11.2015
Simple math parser
\*/

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

void error(int code);
int isOperator(char character);
int precedence(char character);

int main ( void ) { //MAIN
    char input[255], output[255], stack[255];
    int i = 0, o = 0, s = 0, var = 0, a;

    system("clear");
    fgets(input, 255, stdin); //read the math stuff
    do {var++;} while (input[var] != '\n');
    input[var] = 0; //remove \n

    printf("SHUNTING-YARD-ALGORITHM:\n");
    while (input[i] != 0) {
        if (input[i] > 47 && input[i] < 58) { //is int
            printf("%c is int => on output\n", input[i]);
            output[o] = input[i]; //put on output
            o++;
        } else if (input[i] == '(') { //'(' to stack
            printf("( => on stack\n");
            stack[s] = input[i];
            s++;
        } else if (input[i] == ')') { //empty stack till '(' is found
            printf(") => put stack on output till '(' is found\n");
            while (stack[s - 1] != '(') {
                if (s == 0) {error(1);} //stack is empty / no '(' found
                output[o] = stack[s - 1]; //put stack on output
                o++;
                s--;
            }
            s--; //remove '(' from stack
        } else if (isOperator(input[i])) { //operator
            while (s != 0 && isOperator(stack[s-1]) && precedence(input[i]) <= precedence(stack[s-1])) {
                output[o] = stack[s-1]; //put stack on output
                o++;
                s--;
            }
            stack[s] = input[i]; //put token on stack
            s++;
            printf("%c is operator => on stack, stack has %d element(s):", input[i], s);
            for(var=0; var < s; var++) {printf("%c", stack[var]);}
            printf("\n");
        } else { //unknown token
            if (input[i] != ' ') { //ignore spaces
                printf("%c", input[i]);
                error(4);
            }
        }
        i++;
    }
    printf("put whole stack on output (%d element(s)):", s);
    for(var=0; var < s; var++) {printf("%c", stack[var]);}
    while (s != 0) { //put stack on output
        if (stack[s-1] == '(') {error(3);}
        output[o] = stack[s-1];
        o++;
        s--;
    }
    output[o] = 0; //end string
    printf("\n%s\n\nREVERSE POLISH NOTATION:\n", output);

    i = 0;
    o = 0;
    s = 0;

    while (output[o] != 0) {
        if (output[o] > 47 && output[o] < 58) { //is int
            printf("%c is int => on stack\n", output[o]);
            stack[s] = output[o] - 48; //put as int on int_stack
            s++;
        } else { //is operator
            printf("%c is operator => pop 2 stack items with %c\n", output[o], output[o]);
            switch (output[o]) {
                case '+': stack[s - 2] = stack[s - 2] + stack[s - 1]; break;
                case '-': stack[s - 2] = stack[s - 2] - stack[s - 1]; break;
                case '*': stack[s - 2] = stack[s - 2] * stack[s - 1]; break;
                case '/': stack[s - 2] = stack[s - 2] / stack[s - 1]; break;
            }
            s--; //popped 2 items into 1
        }
        o++; //next char
    }
    printf("%s = %d\n", input, stack[0]);

    return 0;
} //END

int isOperator(char character) { //checks a character for opartors
    if (character > 41 && character < 48 && character != ',' && character != '.') {
        return 1;
    } else {
        return 0;
    }
} //END

int precedence(char character) { //returns the precedence (+ - is 0, * / is 1)
    if (character == '+' || character  == '-') {
        return 0;
    } else if (character == '*' || character  == '/') {
        return 1;
    } else {
        error(2);
    }
} //END

void error(int code) { //exit with error message
    switch (code) {
        case 1: printf("Fehlende oeffnende Klammer\n"); break;
        case 2: printf("Ungueltiges Zeichen\n"); break;
        case 3: printf("Zu viele oeffnende Klammern\n"); break;
        case 4: printf(" unbekanntes Zeichen\n"); break;
    }
    exit(code);
} //END