Some programming help? - Printable Version

Code:

// creation of an array with some initial values.

// The i'th value is accessed using num[i] starting at 0, so num[2] is currently 10

int [] num= {20,50,10,70,40,80,30,90,60};

int t=0;

// place the thumb on each position in the array, 'from left to right'

for (int i=0; i<num.length; i++) {

    // go with the finger through the rest of the array

    for (int j=i+1; j<num.length; j++) {

        // whenever the value at the finger is less than the value at the thumb, do something

        if (num[i]>num[j]) {

            t=num[i];

            num[i]=num[j];

            num[j]=t;

        }

    }

}

determine what 'do something' does

I could just tell you what the 'do something' is, but it may be more helpful to try and to this with some paper;
- write a couple of numbers (or any other type of value of course) next to each other in boxes (a horizontal ladder, as it were); this is the array
- for convenience, you could write the index of each element in the array above/below it. Be sure to start with 0.
- draw an empty box somewhere else on the paper. This represents the value of 't'. (you could fill it with a 0, as it's t's initial value)
- pick any value for i and j, for which i < j and both i and j are valid indices of the array (the array should have a number at both i and j).
- the perform the 'do something' on the paper;

analogue version of 'do something' sequence

-- erase the value at t, and replace it with the value of the i'th element in the array
-- erase the value at the i'th element in the array, and replace it with the value of the j'th element in the array
-- erase the value at the j'th element in the array, and replace it with the value of t

- If you remember the original set of values of the array (or if you wrote them down / re-used the example array from the exercise), you should be able to see what has just happened. (you can ignore the value of t for that; it's the t of Temporary, and is only used to enable the operation that is being performed in the 'do something' part)

what 'do something' does

-- It swaps the values at location i and j in the array

hint 0

- the value at the thumb is swapped with the value at the finger whenever the value at the finger is less than the value at the thumb

hint 1

- Will the value at the thumb ever decrease?

answer

No, it is only swapped when the thumb is higher than the finger.

hint 2

- Will there be a value 'to the right of' the thumb that is smaller than the thumb?

answer

No. The thumb is always the minimum value between the thumb and the finger (inclusive).

hint 3

- Since the thumb is moved from left to right, this, means that for every element in the array, all values to the right will be ...

hint 4

- ergo, the values are not decreasing when read from left to right.

hint 5'/answer

- ergo, the array is sorted in ascending order

Spoiler

Given the following pseudo-code;

Code:
for(DEF; COND; INCR) {

 CODE

}

This could be rewritten as: (there are probably some obscure cases where this will not work, but it should suffice for now)

Code:
{

  DEF;

  while (COND) {

    CODE;

    INCR;

  }

}

In other words;
- Before anything else, the DEF-code is executed. This will usually be definition or assignment of some counter.
- The COND (condition) is checked.
- If COND evaluates to true, the CODE is executed, after which INCR is executed. INCR usually increments the counter defined in DEF. After this, the COND is checked again.
- Once the COND evaluates to false, the loop stops.
(- if a variable was defined in DEF, it will not be visible anywhere else. hence the surrounding brackets)

With example code:

Code:
for(int i = 0; i < 3; i++) {

  System.out.print(i + " ");

}

System.out.println(".");

step-by-step evaluation scheme

DEF Wrote:- define variable i
- initialize i to 0

COND Wrote:- evaluate i < 3.
- 0 is less than 3 -> 'true'

CODE Wrote:- print i (which is 0) followed by a whitespace

INCR Wrote:- increment i by 1.

COND Wrote:- evaluate i < 3.
- 1 is less than 3 -> 'true'

CODE Wrote:- print i (which is 1) followed by a whitespace

INCR Wrote:- increment i by 1.

COND Wrote:- evaluate i < 3.
- 2 is less than 3 -> 'true'

CODE Wrote:- print i (which is 2) followed by a whitespace

INCR Wrote:- increment i by 1.

COND Wrote:- evaluate i < 3
- 3 is not less than 3 -> 'false'

The loop ends, and a '.' and newline-character is printed.