Question

我正在尝试创建一个mips汇编程序来递归计算nCr。

我编写了整个程序，包括驱动程序，但它运行不正常。我的所有输入和输出都有效但我的递归算法正在返回疯狂的数字。例如，nCr == 268501120而不是10。

这只是我算法的一小部分：

nCk:
sub $sp, $sp, 16 #allocate the needed space in stack.
sw $ra, 0($sp) #save return address in first position
sw $t3, 4($sp) #save n in the stack
sw $t4, 8($sp) #save k in the stack

sub $t3, $t3, 1 #Subtract one from n
sub $t4, $t4, 1 #Subtract one from k

jal checkBounds #Check for end of recursion.
sw $v0, 12($sp) #copy returned 1 or 0 into stack.

lw $t3, 4($sp) #Load original n back into t3.
lw $t4, 8($sp) #Load original k back into t4.

sub $t3, $t3, 1 #Subtract one from n again. (n-1 step of recursive algorithm)
jal checkBounds #Check for end of recursion with n 1 number lower.

lw $t2, 12($sp) #Load the value held in the previously returned v0.
add $v0, $v0, $t2 #Add old returned value to new returned value.

lw $ra, 0($sp) #Load the original return address.
addi $sp, $sp, 16 #Add 16 more bytes to the stack.
jr $ra


checkBounds: #Check if program should still recurse
beq $t3, $t4, return1 #If n==k
beq $t4, $0, return1  #if k==0
li $v0, 0 #If (j!=k || k!=0){ return 0};
jal nCk
jr $ra 


return1: #Returns 1
li $v0, 1
jr $ra

Answer 1

此代码有很多错误，很难列出。对于初学者来说，它有语法错误和缺少标签，所以它甚至没有组装。然后，输入值永远不会写入$t3和$t4，因为您反转了操作数顺序。此外，您的CheckBounds使用JAL而不保存$ra。同样适用于main。至于打印，结果是$v0，因此您需要在打印前面的内容之前保存$v0。

修复所有这些似乎使它工作。

你应该学会使用调试器/模拟器来修复你的错误。例如，发现寄存器没有预期值很容易。

Answer 2

我冒昧地重构你的代码并跳过错误检查部分来向你展示最重要的部分。基本上我已经实现了迭代factorial过程，它不对输入值进行任何错误检查。然后在主程序中，我从用户那里获得输入，计算因子并应用公式。希望有所帮助。

.data
    enterN: .asciiz "Please enter the n value: \n"
    enterK: .asciiz "Please enter the k value: \n"
    output: .asciiz "Result is: "
.text

j main

factorial:
    # iterative factorial procedure
    # $a0 - number, no error checking is performed on input
    # $v0 - factorial of the number
    addi $sp, $sp, -4
    sw $ra, 0($sp)

    li $v0, 1
    li $s0, 1
factorial_begin:
    beq $s0, $a0, factorial_end # n == 1?
    mul $v0, $v0, $a0 # $v0 = $v0 * n
    subi $a0, $a0, 1  # n = n - 1
    j factorial_begin
factorial_end:
    lw $ra, 0($sp)
    addi $sp, $sp, 4
    jr $ra


main:
    # compute cobination (n choose k) = n! / k!(n-k)!   
    # ----------------------------------------------
    la $a0, enterN #Ask for the first param, n.
    li $v0, 4 #String syscall
    syscall #Prints out string.
    li $v0, 5 
    syscall #Places inputted value in v0.
    la $t0, ($v0) # $t0 = n

    # computer factorial of n
    move $a0, $t0
    jal factorial
    move $t1, $v0 # $t1 = n!

    la $a0, enterK #Asks for the second param, k.
    li $v0, 4 #String syscall
    syscall #Prints out string
    li $v0, 5 
    syscall #Places inputted value in v0.
    la $t2, ($v0) # $t2 = k

    # computer factorial of k
    move $a0, $t2
    jal factorial
    move $t3, $v0 # $t3 = k!

    sub $a0, $t0, $t2 # $a0 = n - k
    jal factorial
    move $t4, $v0 # $t4 = (n-k)!

    mul $t3, $t3, $t4 # $t3 = k! * (n-k)!
    div $t1, $t1, $t3 # $t1 = n! / (k! * (n-k)!)

    # print out the result
    la $a0, output
    li $v0, 4
    syscall 

    move $a0, $t1
    li $v0, 1
    syscall

MIPS汇编 - 尝试编写递归程序来计算nCr（组合）

2 个答案: