Monday 02/13/2006, CISC181, the day after the big snow




If we compile readNums.cc with

  CC readNums.cc

we get a file called: a.out

we can do the same thing with

  g++ readNums.cc

However, a better way to compile is:


  make readNums



When we return a value through 
   return 0; // in the main
   return 1; // in the main
   exit (13); // anywhere

the values we can return are 0 through 255.

(If we return a larger value, the upper
bits get through away... only 8 bits of unsigned
integer are retained.

So, for example, 500 gets returned as 244,
because 500 = 256 + 244.

1029 gets returned as 5, because
1029 = 1024 + 5.

You can see what gets return by doing
echo $?



Running the program with a nums.dat file




































We started with three versions of the readNums.cc program 
written by students:

(1) a variation by bwadswor (Burke) to compute the average

(2) a variation by Nevin (nev) to compute min

(3) a variation by Brian (bflad) to compute max


In each case we saw the importance of doing:


read 
while (not end of file)
{  
  process
  read
}

e.g.

infile >> x;
while (!infile.eof())
{
   // do something with x
   sum += x;
   infile >> x;
}

We'll show what happens if you DON'T do that today or Wednesday.

Next, we'll show a different way to compile:

instead of 

CC readNums.cc
  which gives us an a.out file

or

g++ readNums.cc
  which also gives us an a.out file

We can just type:

make readNums

 Note: we leave off the .cc

That echos a longer version of the compiler command, like so:

> make readNums
CC    -o readNums readNums.cc 
> 

IF the readNums.cc file is "newer" than the most recent
version of readNums, OR if readNums (the executable) does
not exist, it will compile the program.

If I've already compiled readNums.cc into readNums,
and if I have NOT made any changes, I just get something like this:

> make readNums
`readNums' is up to date.
> 

Now, to run the program, instead of ./a.out

I do ./readNums

> ./readNums
Could not open nums.dat, sorry!
> 

Here, we get the error message because we don't have a nums.dat file:


> ls
02.13.1.25pm.txt  02.13.1.25pm.txt~  readNums  readNums.cc  readNums.cc~
> 

Now, one more thing.. remember I told you that exit(1) vs. return 0
is something we could see at the command line.  Here's how:

echo $?

Next time:
   Show what happens with an empty file

   Look at the code where infile >> x is not at the top and bottom

   Look at how to write the code so that if there are characters in the
   file it still works.