Simple library or implementation for a mathematical expression evaluator [closed]

Answer 1

For C et al, here's a quick-n-dirty and highly unsafe cheat that requires Perl:

double eval(const char* expr) {
    char buf[1024];
    snprintf(buf, sizeof(buf), "perl -e 'print (%s)'", expr);
    FILE* p = popen(buf, "r");
    double d;
    fscanf(p, "%lf", &d);
    fclose(p);
    return d;
}