A simple brainfuck interpreter in C

bf.c

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

// initialize the tape with 30,000 zeroes
unsigned char tape[30000] = {0};
 
// set the pointer to point at the left-most cell of the tape
unsigned char* ptr = tape;


void interpret(char* input) {
    char current_char;
    size_t i;
    size_t loop;

    for (i = 0; input[i] != 0; i++) {
        current_char = input[i];
        if (current_char == '>') {
            ++ptr;
        } else if (current_char == '<') {
            --ptr;
        } else if (current_char == '+') {
            ++*ptr;
        } else if (current_char == '-') {
            --*ptr;
        } else if (current_char == '.' ) {
            putchar(*ptr);
        } else if (current_char == ',') {
            *ptr = getchar();
        } else if (current_char == '[') {
            continue;
        } else if (current_char == ']' && *ptr) {
            loop = 1;
            while (loop > 0) {
                current_char = input[--i];
                if (current_char == '[') {
                    loop--;
                } else if (current_char == ']') {
                    loop++;
                }
            }
        }
    }
}


int main() {
    interpret(",[.[-],]");  // outputs input
    return 0;
}

I don't think this is a proper bf interpreter because it doesn't handle nested loops. You need to use a stack to store loop locations instead of just a variable.

Probably true, I haven't tried. Feel free to make edits and I'll be happy to update. :)

Updated version should now supported nested loops.

Awesome!

Also wrote one with dynamic memory allocation and trivial garbage collection: https://gist.github.com/1714336

I don't think that the '[' is right because the spec says [ Jump forward past the matching ] if the byte at the pointer is zero. but you don't check ptr, you just continue, which turns [] into a do..while instead of a while. You can duplicate the body of the ']' case but preincrement i instead of predecrement it: current_char = input[++i]; and flip the square brackets in the conditionals. Tested with Hello World with a [[[]]] thrown in immediately after the first loop.

Why not turn the chain of if else if's into a single switch statement. would be much more readable.

Why not turn the chain of if else if's into a single switch statement. would be much more readable.

@TheSoftwareGuy that's debatable

@nickdesaulniers you are right, the implementation of [ is not correct

else if (current_char == '[' && *ptr == 0) { loop = 1; while (loop > 0) { current_char = input[++i]; if (current_char == '[') { loop++; } else if (current_char == ']') { loop--; } } }

this is a edit to make it actually work with the [

Hi @maxcountryman, just letting you know I used your code for personal work and did a little update of it, here is my gist.
Thanks for this :)

[ should branch
http://www.muppetlabs.com/~breadbox/bf/

Nice!

I'll borrow your code to create my own repository (I didn't figure out how to convert the gist => github repository, I'm still new here). Here is the link.

[ should branch http://www.muppetlabs.com/~breadbox/bf/

Indeed.

Did you use the tests on http://www.brainfuck.org/tests.b ?