Multiplying variables but all I get is <null>

[Josh Nako]

I'm trying to write a program to calculate meat and the prices and stuff but the program comes up with <null> embarrassingly enough I have spent over 8 hours trying to do this

#include <stdio.h>#include <stdlib.h>
#define beef     1 /* multiplication factor of beef*/
#define chicken  1.5 /*factor of chicken*/
#define pork     2 /* factor of pork*/
#define s1   0.5 /* state1 is close to expiry*/
#define s2   1 /*state2 is frozen storage*/
#define s3   1.5 /*state3 is fresh*/


int main(int argc, char *argv[])
{
  float amt = 0; /*amount of meat*/  
  char st = 0; /*state of meat*/
  char meat = 0; /*type of meat*/
  float price = 0; /*total price*/
  
  printf ("This program calculates price of meat. \n");
  printf ("\n Please enter type of meat ( beef, chicken, pork):");
  fflush (stdin);
  scanf ("%s",&meat);
  if ((meat != beef) || (meat != chicken) || (meat != pork))
  price, "NA";
  printf ("\n Please enter state of meat(s1, s2, s3):");
  fflush (stdin);
  scanf ("%s",&st);
  if ((st != s1) || (st != s2) || (st != s3))
  price, "NA";
  printf ("\n Please enter quantity of grams of meat (0 - 2000):");
  fflush (stdin);
  scanf ("%d",&amt);
  if ((amt < 0 ) || (amt > 2000))
  price, "NA";
  else if (amt >= 1500)
  price = (meat*st)*8;
  else if (amt >= 1000)
  price = (meat*st)*6;
  else if (amt >= 500)
  price = (meat*st)*4;
  else
  price = (meat*st)*2;
  
  printf( "\n the price is: %s", price);

  if (price [0] == 'N', price [1] == 'A')
  printf ("Variable is not recognized, please use provided variables");
  
  system("PAUSE");    
  return 0;
}

[migf1]

In your scanf()s you are reading your variables as if they were of different types from the ones you have defined them.

For example, you define meat to be a char and then you read it as if it was a cstring. Or you read amt as an int, but you have defined it to be a float.

[Josh Nako]

So how do I change it? Sorry I am really new to this.

[migf1]

So how do I change it? Sorry I am really new to this.

I'd suggest you have a look at the documentation of the function scanf(). Here's a random source I just got from google.

For example, you should read meat like this:

scanf( " %c", &meat );

Moreover, you should make sure that the comparisons you do in your conditional statements are between variables of the same type (or at least between compatible types).

[Josh Nako]

I tried changing it and it doesn't do anything different

[migf1]

Well, I'm not supposed to solve your exercise for you. I think I've hinted you enough to get you started fixing your code. If you have more specific questions about things you tried and didn't work, I'll be glad to help.

[Josh Nako]

oh okay, my bad. I'm just having heaps of trouble. Thanks for the help

[Josh Nako]

is the way I did the multiplication and everything correct though

[rags_to_riches]

Unfortunately for you there's next to nothing correct in that program. You need to go back to your book and notes and read them more carefully and give it another try.

[Josh Nako]

:O I tried reading my notes but I have trouble understanding and the teacher doesn't help much either all together now I have sat in this class for 16 hours trying to do this, thanks though, I'll give it another try

[anduril462]

Okay, let's start with the basics:
1. Indent/format your code properly (link). If your code is hard to read, it's easy to make mistakes, and hard to find and fix them.

2. Use sensible names. Don't abbreviate and be lazy. Also, it's conventional to use ALL_UPPER_CASE for constants. s1 should be STATE_EXPIRING, s2 STATE_FROZEN, s3 STATE_FRESH. Then you don't need silly comments by their name to describe it. Also, beef should be more descriptive, like BEEF_COST_FACTOR, and similarly CHICKEN_COST_FACTOR and PORK_COST_FACTOR. That helps makes your code easy to read, and has the same benefits as good indentation/formatting.

3. Compile at the maximum warning level. How to do this varies based on what compiler you're using, but the result is the same. It helps ensure you are writing clean, proper code. Below is what happens when I compile your code with maximum warnings:

$ make meat
gcc -Wall -Werror -std=c99 -o meat meat.c
meat.c: In function ‘main’:
meat.c:23:8: error: left-hand operand of comma expression has no effect [-Werror=unused-value]
meat.c:23:3: error: statement with no effect [-Werror=unused-value]
meat.c:28:8: error: left-hand operand of comma expression has no effect [-Werror=unused-value]
meat.c:28:3: error: statement with no effect [-Werror=unused-value]
meat.c:31:3: error: format ‘%d’ expects argument of type ‘int *’, but argument 2 has type ‘float *’ [-Werror=format]
meat.c:33:8: error: left-hand operand of comma expression has no effect [-Werror=unused-value]
meat.c:33:3: error: statement with no effect [-Werror=unused-value]
meat.c:43:3: error: format ‘%s’ expects argument of type ‘char *’, but argument 2 has type ‘double’ [-Werror=format]
meat.c:45:13: error: subscripted value is neither array nor pointer nor vector
meat.c:45:31: error: subscripted value is neither array nor pointer nor vector
meat.c:45:23: error: left-hand operand of comma expression has no effect [-Werror=unused-value]
cc1: all warnings being treated as errors
make: *** [meat] Error 1

• Those errors are because price, "NA"; does not do anything. It does not assign "NA" to price or anything of the sort. It's like saying x; Nothing happens to x, it's just a wasted statement.
• That error is because scanf is trying to read an int (%d), but you are telling it to store that in a float. Read the scanf docs (see migf1's post #4), use the right format specifier and variable type to store it.
• That error is because scanf is looking for a string (%s), but you are telling it to store the result in a double. Again, read the scanf docs.
• Those errors are because price is not an array, thus you can't use the array index operator [ ]. It's not even of char type, or any kind of integer type that would support comparing to 'N' or 'A'. It's a float.
• That error is because of the if condition. You do not use comma to combine conditionals. You need to use && or || for compound conditionals.

More things I noticed at quick glance:

fflush(stdin);

That is undefined behavior. Read this link and this link.

scanf ("%s",&meat);

That is wrong. scanf with %s will read a string until a white space or EOF occurs, then it will append the null character. That means that you will always need at least one extra char for the null terminator '\0', meaning 2+ chars in your string. meat is declared as a single char. You need to declare it as a string (i.e. char array) if you wish to read a string into it. You will need one more character than the longest string you plan to read. So if you want to store "chicken" (7 chars), you need 8 chars (7 for the word + 1 for the null).

if ((meat != beef) || (meat != chicken) || (meat != pork))

Look at your #defines. beef is 1, chicken is 1.5 and pork is 2. You intend meat to store a string (based on the scanf above it), yet you compare it to an int/double value. You can't do that in C, you need to compare things of equivalent types. You can't even compare two strings with the == sign (or != or <, <=, > or >=). String comparison uses the strcmp function. Google for examples of how to use it.

You have a similar problem with how you deal with the meat state.

That is a lot of stuff for you to work on and fix. It should keep you busy for a while. If your teacher alone is not enough, find some books and online tutorials. We have some here (link), and Google will turn up lots more.