Thread: Need help breaking from a program

The code below works out the area under a curve using a series of trapezium's areas. This means that the x coordinates must be in increasing order (y coords dont have to be).

If an x value is entered that is less than the previous the code breaks and doesnt ask for the next x value, but DOES ask for all the y values. How can i rewrite this code so that it breaks completely, asking if the user wants to try again?

Code:

#include <stdio.h>
int main()
#define N 15
{
 int i, sets;
 float x[N], y[N], area;
 char again='Y';	
 while(again=='y'||again=='Y')
 {
  area=0;

  printf("This is a program to integrate using the trapezium rule\n");
  printf("Enter the number of coordinate sets you wish to use, up to %d\n\n", N);
  scanf("%d",&sets);

  if(sets<=15 && sets>0)
  {
   printf("\nType in %d values for the x coordinates\n", sets);
   for(i=0; i<=sets-1; i++)
   {
    scanf("%f", &x[i]);
    if(x[i] < x[i-1])
    {
     printf("The value you entered is less than the previous\n\n");
     break;
    }
   }
   printf("Type in %d values for the y coordinates\n", sets);
   for(i=0; i<=sets-1; i++) scanf("%f", &y[i]);

   for(i=0; i<sets-1; i++)
   {
   area = area + ((x[i+1]-x[i])*(y[i]+y[i+1]))/2;
   }
   printf("area is %f\n\n", area);

  }
  else printf("\nYou have entered an invalid integer\n\n");
  
  printf("Would you like to run the program again?\n");
  printf("Type Y or y for yes, anything else for no\n");
  again=getchar();
  if(again=='\n')again=getchar();
 }
 return 0;
}

many people might shoot me now but the best solution here is to use "goto"
just put instead of break:
goto hello;
then before the try again? stuff put
hello:

Great, thanks!

GOTO?!

Relax, I shall not shoot you. I often see starting programmers using things like break, exit and return on the wrong places, I sometimes even see them using goto. A well-designed program doesn't need to use such things to force exits and that kind of things.

You don't need break here. Introduce a flag. In the conditional part of the for-loop you can check the value of that flag. If the total condition evaluates to FALSE, the for-loop will stop. This flag can be used in other cases.

My suggestion: redesign your program and perhaps use a function. Take pencil and paper and use a flow-chart to visualize the algorithm.

I havnt learnt about flags or goto yet, but goto looks very easy. But if it is frowned upon then im uneasy about using it. Can anyone enlighten me on the "proper" methid of rewriting my code using the (all be it basic) standard of coding i have used please?

A flag is nothing but a variable which represents a certain state.

A flag could be:

int flag;

You could then decide that 0 is OK and 1 is ERROR. So when checking the value of flag, you check if it's value is 0 or 1. In case of 1, you know that an error occurred.

Rewriting your code starts with redesiging your algorithm.

yeah, yeah....
i arleady know this story:
1) a person (me) says "goto" and is not saying "evil" or "that shall not be used" after.
2) all the programers in the world are coming to explain why goto shall be destroyed and why you should build the "perfect" program!

now look here i just said that when you want to break from few loops goto will be much more usefull than this falgs that takes more memory ( goto label statment is only 1 line in assembly!)
and it uses an extra 2 bytes ( very "optimized" programming indeed...)

i suggest that sometimes you (all) should use goto's, breaks and continues, place a comment next to it and it all be clear!

i remember that in my school i wrote a program that used a goto (on computers lesson) when the teacher heared this "baaaaaaaaaaaaaaaaaaaaaaaaah" she screamed

so go ahead, take a pen and paper and start breaking your mind for the "optimized" program (that is far from being optimized as explained in my previos post)

i dont like to push people so you all do whatever you want.

Goto is the evilest thing I have come across. Except for bunnies.

>i suggest that sometimes you (all) should use goto's, breaks >and continues, place a comment next to it and it all be clear!

For very small programs it probably will be. But when you're doing this with large programs, you'll create a mess. A very important aspect of programming is to create software that is flexible and easy to maintain. So I would advise using functions to split up functionality. Modern computer systems are equipped with a lot of memory and have a relative high speed micoprocessor, so I wouldn't worry too much about optimisation. Unless you're working on time-critical applications. But in that case one or more flags are not the point, the most important point is: designing an efficient algorithm.

>If you're thinking of your program as a state machine (ie, >flowchart), then break; continue; and goto X; are most certainly >your friends. State machines are intuitive and good for >expressing algorithms that aren't too large.

At work I'm daily working on real-time systems and I'm using state machines a lot. But I never use things like continue and goto, ofcourse I use break in the switch-structures.

In my opinion, when you need continue or goto, you're software is hard to maintain and inflexible.

By the way, you don't always need flags. Ofcourse you could do this:

for (...; ... && x[i] < x[i-1]); ..)

>And I've never seen an commercial Application using goto.

This is not an argument for not using goto. But you could think about: why don't they use goto in commercial applications?

Well, at some time when you have some more programming experience you'll note that there are far better solutions than goto. And you'll see the advantages of not using goto and other such kind of constructions.