[prog] Re: [Courses] need a little help with extremly basic python programming

[Elizabeth Barham]

Mary writes:

> I've Cc-ed the programming list, since this post isn't part of a course
> it would probably be better off there.
> 
> On Thu, Dec 05, 2002, Guru - wrote:
> > "Exercise 1
> > Write a program that continually reads in numbers from the user and adds 
> > them together until the sum reaches 100. Write another program that reads 
> > 100 numbers from the user and prints out the sum. "

> > I know that it's to do with loops but I wasn't sure how to do it...

What do you need to know in order to complete task one:

    Write a program that continually reads in numbers from the user
    and adds them together until the sum reaches 100.

By this I mean, what variables are necessary to accomplish the task?

1) A number that is read in from the user (input_num)
2) A number that accumulates a running total (accumulator)

The only other datum needed is the number 100, which is a constant, and
is not a variable because it does not vary:

    Write a program that continually reads in numbers from the user
    [input_num] and adds them together until the sum [accumulator]
    reaches 100.

Next, we define the condition in which we loop. In this case, it is
simple: ( accumulator < 100 ). There also must need be an
initialization step if we use a variable prior to an assignment (x =
y), as prior to the assignment it is undefined and could contain
anything.

Steps:
  1) Define Variables
  2) Initialization
  3) Loop

There are two, general types of loops, the "check condition before"
and the "check condition after" and in C (I do not know python) these
can be respectively translated as:

while(CONDITION IS TRUE) {
  ACTIONs
}

and

do {
  ACTIONs
} while(CONDITION IS TRUE);

For the first assignment,

    Write a program that continually reads in numbers from the user
    [input_num] and adds them together until the sum [accumulator]
    reaches 100.

note that there needs to be an action prior to the condition being
checked, so I would suggest the latter (check condition after) loop.
Putting them all together:

Define_Variables:
    int input_num;
    int accumulator;

Initialization:
    accumulator = 0;

Loop:
    do {
        std::cout << "Please enter a number: ";
        std::cin >> input_num ; // place user's number in input_num
        accumlator = accumlator + input_num;
     }
  Loops_Condition:
        while(accumulator < 100);

The other while loop would work as well because of the initialization:

   int input_num;
   int accumulator;

   accumulator = 0;

   while(accumulator < 100) {
       // etc.
   }

But this wastes a test (the "if(accumulator < 100)" implied by
"while(accumulator < 100)") because we know that accumulator will
alway be less than 100 due to its initialization.

As for the second task:

    Write another program that reads 100 numbers from the user and
    prints out the sum.

We can use a similar series of steps as well as variables but the
*big* difference is that the condition does not depend on the user's
input at all, but rather the steps should occur a fixed number of
times (100).

In order to accomplish this, what steps do we do?

1) Define Variables
2) Initialization
3) Loop

These are the *same* steps as prior but there needs to be another
variable, a counter (in computer parlance, an "iterator", or "i" for
short). Here is step 1:

Define_Variables:

  int input_num;
  int accumulator;
  int i; // iterator

Next comes the Initialization:

Initialization:

  accumulator = 0;
  i = 0;

And then loop. Again, we can use the second form of the loops
mentioned above because we know that our iterator, i, shall be less
than 100.

Loop:
  {
     std::cout << "Please enter a number: ";
     std::cin >> input_num;
     accumulator = accumulator + input_num;
     // increase the iterator (running count)
     i = i + 1;
   }

  Loops_Condition:

     while(i < 100);

This is a set-up for a "for loop" that places the steps:

1) Initialization
2) Increase Iterator
3) Check Condition for loop

together:

for( INITIALIZATION ; CHECK CONDITION; ADJUST ITERATOR )

but the "ADJUST ITERATOR" step can really be anything, just something
that should change with each step. The for loop, however, is unlike
the:

    do {
    } while() 

loop but is like the 

    while() {
    }

loop as the condition is checked prior to the instructions are
performed.

Define_Variables:

  int input_num;
  int accumulator;
  int i;

Initialization:
Loop:
Adjust Condition:

  for(accumulator = 0, i = 0; i < 100; i = i + 1) {
       std::cout << "Please enter a number: ";
       std::cin >> input_num;
       accumulator = accumulator + input_num;
  }


Here are the steps of the "for loop":

1) Initialization (accumulator = 0, i = 0)
2) Check Condition (i < 100)

   If condition is true

3) Do Instructions in body
4) Adjust iterator (i = i + 1)
5) Goto 2 (check condition *again*)

I hope this helps some and isn't as clear as mud. If it is just ignore
me! :-)

Elizabeth