Thread: I could use a little help with this simple program

<rant>
Why is everyone posting pictures rather than code inside [code][/code] tags?
Maybe they see one picture in a post and decide that's the norm.
Perhaps I should go round deleting all "here is a picture of my code" posts to stop people thinking this is a good idea.
</rant>

The short answer is, it's complicated.
c expression Evaluator - Stack Overflow

C doesn't have an 'eval' like interpreted languages, so you can't just type in "x*x+7*x+3" and hand it over to some built-in language feature to do all the work for you.
Sure you can do it, but it's not your average newbie question.

Fancy input is certainly a nice feature, though not usually really necessary in most cases. A more simplified format might help get things up and running in the meantime.

Example using command line parameters as input:

Code:

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

#define MAX_TERMS 100

void reverse_polynomial(double* polynomial, size_t length)
{
 double* left = polynomial;
 double* right = left + length;
 while(left < --right)
 {
  double swap = *right;
  *right = *left;
  *left++ = swap;
 }
}

double evaluate_polynomial(double base, double* polynomial, size_t length)
{
 double sum = 0;
 for(size_t power = 0; power < length; ++power)
  sum += polynomial[power] * pow(base, power);
 return sum;
}

void print_polynomial(double* polynomial, size_t length)
{
 size_t degree = length - 1;
 for(size_t index = 0; index < length; ++index)
 {
  size_t power = degree - index;
  double value = polynomial[power];
  if(value == 0)
   continue;
  if(power == degree)
   printf("%g", value);
  else
  {
   if(value < 0)
    printf(" - %g", -value);
   else
    printf(" + %g", value);
  }
  if(power == 1)
   putchar('x');
  else if(power != 0)
   printf("x^%zu", power);
 }
 puts("");
}

#define die(message)\
 do\
 {\
  puts(message);\
  printf("Usage: %s [START] [END] [INCREMENT] [TERMS...]\n", argv[0]);\
  printf("Example: %s -1 1 0.1 2 0 -12 0.5 -1.5e3\n", argv[0]);\
  puts(" (From -1 to 1 in 0.1 increments evaluate 2x^4 - 12x^2 + 0.5x - 1500)");  \
  exit(EXIT_FAILURE);  \
 }\
 while(0)

#define usage() die("")

int main(int argc, char** argv)
{
 if(argc < 5)
  usage();
 size_t adx = 1;
 double current = atof(argv[adx++]);
 double end = atof(argv[adx++]);
 if(current > end)
  die("END range must be less than START range");
 double increment = atof(argv[adx++]);
 if(increment == 0)
  die("Increment must be nonzero");
 double terms[MAX_TERMS];
 size_t count = 0;
 for(;;)
 {
  char* text = argv[adx++];
  if(text == NULL)
   break;
  if(count >= MAX_TERMS)
   die("Too many terms!");
  terms[count++] = atof(text);
 }
 reverse_polynomial(terms, count);
 print_polynomial(terms, count);
 puts("*** Evaluate ***");
 while(current <= end)
 {
  double x = current;
  printf("x: %g\n", x);
  double y = evaluate_polynomial(x, terms, count);
  printf("y: %g\n", y);
  current += increment;
  puts("");
 }
 return EXIT_SUCCESS;
}