Reverse the order of user inputted string - mips

Question

goodafternoon, i made this code that takes in user input and reverses the order. the code works perfectly except when there is long user input such the alphabet. in this case the output excludes the last letter, i am unsure why this is and any insight would be greatly appreciated.
.data
.align 2
array: .space 80
size: .word 20
string: .space 20000
op: .asciiz "Enter n, followed by n lines of text:"
prompt: .asciiz ""
text: .asciiz "The values are:"
newline: .asciiz "\n"
.text
.globl main
main:
#prompt user for array length
li $v0,4
la $a0,op
syscall
jal new_line #output newline
#read in array count
li $v0,5
syscall
addi $s0,$v0,0 #$v0 contains the integer we read
add $t0,$zero,$zero #index of array
addi $t1,$zero,1 #counter=1
la $s2,string #load address of string storage area
read_string:
bgt $t1,$s0,L1 #if ($t1 > length) then array is done
# prompt the user for next string
li $v0,4
la $a0,prompt
syscall
#get the string
move $a0,$s2 #place to store string
li $a1,20
li $v0,8
syscall
#store pointer to string into array
sw $a0,array($t0)
addi $t0,$t0,4 #advance offset into pointer array
addi $t1,$t1,1 #advance iteration count
addi $s2,$s2,20 #advance to next string area
j read_string
#### here i want to print the array ####
L1:
addi $t1,$zero,1 #counter = 1
#output the title
la $a0,text
li $v0,4
syscall
jal new_line
while:
addi $t0,$t0,-4 #advance array index
bgt $t1,$s0,done #if no more strings to output then done
lw $t2,array($t0) #get pointer to string
#output the string
li $v0,4
move $a0,$t2
syscall
addi $t1,$t1,1 #advance count
j while
new_line:
la $a0,newline
li $v0,4
syscall
jr $ra
done:
li $v0,10
syscall
if the user enters a n value of 26 and following lines a b c d e f g h i j k l m n o p q r s t u v w x y z then the output should be z y x v u t s r q p o n m l k j i h g f e d c b a. However what is outputted is this: z y x v u t s r q p o n m l k j i h g f e d c b. so its missing the letter a

Answer 1

From what I can tell, the problem is that you're ending the loop one pass too early.
while:
addi $t0,$t0,-4 #advance array index
bgt $t1,$s0,done #if no more strings to output then done
Here, you're effectively comparing $s0 to 26, since that's the whole alphabet size.
But since you started $t1 as equaling 1, you're encountering an "off-by-one" error most likely.
When compilers implement a loop like while(count < 26), often they don't do this literally like you have done here. It's usually easier to do the following:
#read in array count
li $v0,5
syscall
addi $s0,$v0,1 #$v0 contains the integer we read plus 1.
# do your other stuff here
while:
addi $t0,$t0,-4 #advance array index
lw $t2,array($t0) #get pointer to string
#output the string
li $v0,4
move $a0,$t2
syscall
addi $s0,$s0,-1
bnez $s0,while
Since you're only adding 1 to your count variable it's easier just to make it a downward for-loop (as I said eariler, a C compiler typically does this even if you declare the loop as a while.)
Sometimes you just need to adjust your loop counter by 1 and that may be enough to fix things.