While loop in MIPS (Beginner in MIPS)

Question

I am trying to write a program to calculate nCr where n and r are input by the user. My approach is following:

I ask to user to enter value of n.
Then I ask the user to enter value of r.
Then I calculate value of n-r.
Then using three while loops I calculate the value of n! r! and (n-r)!
Then I divide by n! by r! and (n-r)!
Then I display it to the user.

Following is my code:

.data 
     prompt1: .asciiz "Enter the value of n:\n"
     prompt2: .asciiz "Enter the value of r:\n"
     message: .asciiz "The value of nCr is: "

 .text
      #Prompt the user to enter value of n
      li $v0,4
      la $a0,prompt1
      syscall

      #Get the value of n
      li $v0,5
      syscall

      #Store the value of n in $t0
      move $t0,$v0

      #Prompt the user to enter value of r
      li $v0,4
      la $a0,prompt1
      syscall

      #Get the value of r
      li $v0,5
      syscall

      #Store the value of r in $t2


      move $t2,$v0

      #Getting the value of (n-r)
      sub $t4,$t0,$t2

      #Calculating value of n!
      addi $t1,$zero,1

      while:
            blt $t0,1,exit
            mul $t1,$t1,$t0
            sub $t0,$t0,1
            j while
      exit:
            li $v0,10
            syscall

     #Calculating the value of r!
     addi $t3,$zero,1

     while:
           blt $t2,1,exit
           mul $t3,$t3,$t2
           sub $t2,$t2,1
           j while

      exit:
           li $v0,10
           syscall

     #Calculating the value of (n-r)!
     addi $t5,$zero,1

     while:
           blt $t4,1,exit
           mul $t5,$t5,$t4
           sub $t4,$t4,1
           j while

    exit:
          li $v0,10
          syscall

    #Getting the final value
    div $s0,$t1,$t3
    div $s1,$s0,$t5

    #Displaying the message
    li $v0,4
    la $a0,message
    syscall

    #Display the answer
    addi $a0,$s1,0
    syscall

The Error which I am getting is this:

line 46 column 6: label "while" already defined.
line 46 column 6: label "exit" already defined.

What I am doing wrong here? Is it not the correct way to use three while loops in a mips

You can't have three global symbols with the same name. How is the assembler going to know which one you're referring to? Aside from that, once you leave the first loop, you're executing system call 10 which will terminate the program. Nothing else in your program will be executed after that. — Michael
– Michael, Commented Apr 28, 2021 at 10:56

ziggystar · Accepted Answer · 2021-04-28 10:56:06Z

4

If you write while: in your program, you are creating a label named while, which is an alias for the line number where you place it. Within one file, you can only create one label with a certain name.

A solution for your problem would be to name the labels while1, while2 and while3. Same holds for your exit labels.

answered Apr 28, 2021 at 10:56

ziggystar

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3 Comments

Anurag Over a year ago

Thanks for the answer. It is working now:)

Peter Cordes Over a year ago

Another strategy is to try to come up with meaningful names for your labels, like n_fac: or r_loop:, instead of just a mess of numbers. @anurag1905: Also note that x! / y! is prod(i=y..x); instead of calculating two huge numbers and then dividing them, just skip the multiplies that cancel out.

Anurag Over a year ago

Great Suggestion. I will keep them in mind. Thankyou:)

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