Calculator with Decimal, Octal and Hexadecimal Numbers

I want to make a calculator that is capable of calculation with decimal numbers and is able to return the decimal values in their respective binary, octal or hexadecimal representation.

So far in the main method the program reads the command line and I can invoke the program by two ways.

The first way would be with 3 values:

"number1" "operator" "number2".

And the second way would be with 4 values:

"wished numeral system for the output" "number1" "operator" "number2".

Where for the wished numeral system output b would stand for for binary, o for octal and h for hexadecimal. In both ways the user should be able to input decimal, octal and hexadecimal numbers for the inputs number1 and number2.

#include "zahlen.h"
#include <stdio.h>
#include "stringTOint.h"   

int main(int argc, char *argv[]) {
    char o,op,sx[DIGITS+1],sy[DIGITS+1],sz[DIGITS+1];
    int x,y,z;
    char flag_x,flag_y;

    /* 1) Read Commandline */
    if (argc != 4 && argc != 5) {
        printf("Aufruf: %s -o <x> <op> <y> \n",argv[0]);
        return 1;
    } else if(argc == 4) {
        x = stringTOint(argv[1]);
        op = argv[2][0];
        y = stringTOint(argv[3]);
    } else if(argc == 5) {
        o = argv[1][0];
        x = stringTOint(argv[2]);
        op = argv[3][0];
        y = stringTOint(argv[4]);
        if(o != 'b' && o != 'o' && o != 'h') {
            printf("Wrong Operation\n");
            return 1;
        }
    }

    /* 2) Solve the equation */
    if(argc==4) {
        printf("solve: %s %c %s \n", argv[1], op, argv[3]);
        z = solve(x, op, y);
    } else if(argc==5) {
        printf("solve: %s %c %s \n", argv[2], op, argv[4]);
        z = solve(x, op, y);
    }

    /* 3) Calculate the Representation of the wished Numeral System */
    switch(o) {
        case 'b':
            intTObinaer(x, sx);
            intTObinaer(y, sy);
            intTObinaer(z, sz);
            break;

        case 'o':
            intTOoctal(x,sx);
            intTOoctal(y,sy);
            intTOoctal(z,sz);
            break;

        case 'h':
            intTOhexal(x,sx);
            intTOhexal(y,sy);
            intTOhexal(z,sz);
            break;

        default:
            intTObinaer(x, sx);
            intTObinaer(y, sy);
            intTObinaer(z, sz);
            break;
    }

    /* 4) Return the results */
    printf("\n  %s %d\n%c %s %d\n= %s %d\n", sx,x,op,sy,y,sz,z);

    return 0;
}

The methods intTObinaer, intTOoctal and intTOhexal only differ by the base with which the decimal number gets divided.

intTObinaer(int i, char str[]) {
    unsigned int zahl = i;
    int j;

    /* Fill Array with zeros */
    int x = 0;
    for (x; x < DIGITS+1; x++) {
        str[x] = '0';
    }

    /*Calculate the Binary representation of the given Decimal integer */
    for (j = DIGITS-1; j > 0; j--) {
        /* This base gets changed to 8 or 16 for octal and hexal representation */
        str[j] = (char) (zahl % 2) + '0';
        zahl = zahl / 2;
        if (zahl == 0) {
            break;
        }
    }

    /* Set the end of the Array */
    str[DIGITS] = '\0';
}

The actual equation gets solved in the solve method, where the right operation for number1 and number2 gets chosen by an switchcase where the different cases can be selected by the char op that the user had input between the two numbers.

#include <stdio.h>

int solve(int x, char op, int y) {
    int ergebnis = 0;
    switch(op) {
        case '+':
            ergebnis = x + y;
            break;

        case '-':
            ergebnis = x - y;
            break;

        case '*':
            ergebnis = x * y;
            break;

        case '/':
            ergebnis = x / y;
            break;

        case '&':
            ergebnis = x & y;
            break;

        case '|':
            ergebnis = x | y;
            break;

        default:
            printf("Wrong input\n");
    }

    return ergebnis;
}

My question now is due to the fact the the user should be able to input different numeral systems(e.g. decimal, octal or hexadecimal) how can I identify the different numeral systems and then transfer them into decimal so that I can calculate the result. After that these decimal Numbers have to be converted back into the desired numeral system that the user wanted.

Looks like you only need to add two lines to do that:

#include "stdlib.h"

#define stringTOint(arg) ((int)strtol(arg,NULL,0))

Or better yet, replace those invocations of stringTOint() with corresponding strtol() invocations (and add the #include, of course).

strtol() uses the same prefixes as for C literals: 0 for octal, 0x for hex, no prefix is decimal.