calc.c

Jonas Altrock ew20b126@technikum-wien.at

To overview.

The whole source file calc.c.

/* Compile with `clang -std=c99 -Wall -o calc calc.c` */

Include C-libraries for input/output, math, allocation of memory, and string manipulations.

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

Task

Implement a calculator. The calculator should be able to execute seven instructions in total and should use decimal numbers (double).

First read a decimal number, then read an arbitrary number of instructions (all separated by space or newline). These instructions are executed and the result is used for the next instruction. Finally, after = has been printed, your program should print Result: , followed by the final result and terminate. Your program should handle the following instructions:

• + (addition),
• - (subtraction),
• * (multiplication),
• / (division),
• neg (negate) and
• sqrt (square root).

The instructions +, -, * and / are followed by another number, the second argument. The result should be printed with three decimal numbers.

Solution

These are macros, they are replaced by the pre-processor. An instruction is at most 4 characters, which is the length of the longest instruction sqrt. For a string buffer that holds an instruction, we need one more character for the null-termination.

#define BUFFER_LENGTH 5

An instruction is a string, so read with format %s with length 4.

#define instruction(x) scanf("%4s", x)

A number is read with %lf to read a double (long float).

#define number(x) scanf("%lf", &(x))

The main part of the program.

int main() {

We only ever need two numbers - the result from a previous calculation and the next number we want to add, subtract etc.

    double x, y;

Our instruction buffer.

    char *op = calloc(BUFFER_LENGTH, sizeof(char));

Start by reading a number.

    number(x);

Next, read an instruction.

    instruction(op);

Continue reading and calculating until a = is read.

    while (op[0] != '=') {

The switch construct lets us decide on the first character of the instruction, which is thankfully enough to decide between all 7 instructions.

        switch (op[0]) {
        case '+':
            number(y);
            x = x + y;
            break;
        case '-':
            number(y);
            x = x - y;
            break;
        case '*':
            number(y);
            x = x * y;
            break;
        case '/':
            number(y);
            x = x / y;
            break;
        case 'n':
            if (strcmp(op, "neg") != 0) printf("Interpreted '%s' as neg\n", op);
            x = -x;
            break;
        case 's':
            if (strcmp(op, "sqrt") != 0) printf("Interpreted '%s' as sqrt\n", op);
            x = sqrt(x);
            break;
        default:
            printf("Unrecognized instruction: %s\n", op);
            break;
        }

Read in next instruction.

        instruction(op);
    }

Finally, print result.

    printf("Result: %.3lf\n", x);
    return 0;
}