CODEWITHSUNDEEP
assemblymips
Kevin
Kevin

Reputation: 111

The middle elements in my array are not being swapped

I have been trying to resolve this issue for the last five hours. I have to compare the first index with the last index. If the first < last, they are to be swapped. This works fine if my array has five elements or less. Anything larger and the swapping stops after two iterations. What am I doing wrong?

My subroutine that reverses the array:

ReverseArray:
    add $s0, $a0, $0

    li  $t2, 1
    la  $s0, array
    li  $s1, 4
    sub $s2, $t0, 1         #lines 177 - 180 finds the last index of the array
    mult    $s2, $s1
    mflo    $s2
    add $t3, $s2, $s0

    swap:
        lw  $t5, 0($s0)
        lw  $t4, 0($t3)     #stores the element of last index into $t4

        sle $t1, $t5, $t4
        beq $t1, $0, finish

        li  $v0, 1          #only in the loop for debug purposes
        add $a0, $t1, $0
        syscall

        add $t6, $t5, $0
        sw  $t4, 0($s0)
        sw  $t6, 0($t3)

        sub $s2, $s2, 4
        add $t3, $s2, $s0
        add $s0, $s0, 4
        j   swap
    finish:

        add $v1, $s0, $0
        jr  $ra

Let's say I enter these numbers into my array: 

3 6 9 12 15 18

What I get is: 

18 15 9 12 6 3

Upvotes: 0

Views: 233

Answers (1)

Michael
Michael

Reputation: 58497

The exit condition for your loop seems to be that the first element is greater than the second, which isn't right if you're trying to reverse the array (you should be comparing addresses or inidices). Your way of updating $t3 also looks odd.

Here's a working loop:

swap:
    lw  $t5, 0($s0)
    lw  $t4, 0($t3)     #stores the element of last index into $t4

    sw  $t4, 0($s0)
    sw  $t5, 0($t3)

    sub $t3, $t3, 4
    add $s0, $s0, 4
    sle $t1,$t3,$s0
    beq $t1,$0,swap    # repeat until $t3 <= $s0

Upvotes: 1

Related Questions