reversing an array of characters without creating a new array

Question

write a program that enters an array of characters from the user and reverses the sequence without creating a new array

Answer 1

Swap the ends constantly, using a single variable as a temporary buffer. In pseudo-code:

temp = a[0]
a[0] = a[size - 1]
a[size - 1] = temp

and so on.

Answer 2

Dont bother reversing the array in memory, just iterate over it backwards!

Answer 3

Homework means pseudo-code only from me, which you've made relatively easy by not specifying what language you want anyway :-)

Turn this into your language of choice:

Set i1 to index of first element in array
Set i2 to index of last element in array
while i1 < i2:
    Set temporary variable to element number i1
    Set element number i1 to element number i2
    Set element number i2 to temporary value
    Add 1 to i1
    Subtract 1 from i2

An ideal thing to do is to actually run that algorithm in your head, using a piece of paper to keep track of variables:

• each the elements in the array.
• i1 and i2.
• temporary variable.

I tend to do that for simpler algorithms. Harder ones I insert debug statement into so that the computer can do that grunt work for me. Start with a piece of paper thus:

i1 | i2 | tempvar | el[0] | el[1] | el[2] | el[3] | el[4] | el[5]
---+----+---------+-------+-------+-------+-------+-------+------
                      H       e       l       l       o       !

and just run through the steps one by one, checking and/or changing each column as you go. That will result in you understanding how it works far better than just being given some code.