Styjun

# replacing all digits in a string with their complement in 9.
.data
 string: .asciiz "123471863"

.text
main:
 # load string's 1st address into the memory
 la $a0, string

 # initialize the loop-counter
 li $t0, 0
 li $t1, 9 # complement envelope for later use

 # start the loop 
start_loop: 
 lb $t2, ($a0) # Take one character from string

 # loop termination condition
 beq $t2, $zero, end_loop # terminate if null-value found

 subi $t2, $t2, 48 # convert it to a digit
 sub $t2, $t1, $t2 # apply complement to $t2
 sw $t2,($a0) # restore the string-byte content

 addi $a0, $a0, 1 # go to next string-byte
 addi $t0, $t0, 1 # increment loop-counter

 j start_loop
end_loop: 

 # print string 
 la $a0, string # load 1st address of the string
 li $v0, 4 # syscall for string print 
 syscall

 move $a0, $t0 # load 1st address of the string
 li $v0, 1 # syscall for string print 
 syscall

 # exit program
 li $v0, 10
 syscall

The program isn't working as expected. After 1st iteration, the $a0 register isn't giving the correct value. Apparently, sw $t2,($a0) is destroying the original address.

How can I get over this issue?

There is no problem to differentiate a null and a '0'. null is 0, while '' is 48.

Your test

 beq $t2, $zero, end_loop # terminate if null-value found

is perfectly correct and will detect the end of string.

What is incorrect is you algorithm.

A way to complement a number in C, would be :

while(c=*str)
 c=c-'0' ; // transform the number to integer
 c=9-c; // complement it
 c += '0'; // add 48 to turn it back to a character
 str++;

You are missing the last conversion to character.

If you change

 sub $t2, $t1, $t2 # apply complement to $t2

to

 sub $t2, $t1, $t2 # apply complement to $t2
 addi $t2, $t2, 48

all should work.

Alternatively, ou can simplify your algorithm and remark that the computation c=9-(c-48)+48 is equivalent to c=105-c. Add before start_loop

 li $t4 105 ## 

and replace the three lines

 subi $t2, $t2, 48 # convert it to a digit
 sub $t2, $t1, $t2 # apply complement to $t2
 addi $t2, $t2, 48

by

 sub $t2,$t4,$t2 # complement to 9 directly on char representing the digit