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我正在尝试创建一个 mips 汇编程序来递归计算nCr 。

我已经编写了整个程序，包括一个驱动程序，但它运行不正常。我所有的输入和输出都有效，但我的递归算法返回了疯狂的数字。例如，nCr ==268501120 而不是 10。

更新代码： http: //pastebin.com/52ueQu99

这只是我的算法的一个片段：

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
4

2 回答 2

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这段代码有很多错误，很难一一列举。对于初学者来说，它有语法错误和缺少标签，所以它甚至没有汇编。然后，输入值永远不会被写入$t3，$t4因为您将操作数顺序颠倒了。此外，您无需保存即可CheckBounds使用. 也一样。至于打印，结果就在里面，所以你需要先保存它，然后再打印前面的东西。JAL$ramain$v0$v0

修复所有这些似乎使它工作。

你真的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。

于 2013-12-04T18:33:31.683 回答
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我冒昧地稍微重构了您的代码并跳过错误检查部分来向您展示最重要的部分。本质上，我已经实现factorial了不对输入值进行任何错误检查的迭代过程。然后在主程序中，我从用户那里获取输入，计算阶乘并应用公式。希望有帮助。

.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
于 2013-12-05T07:38:47.793 回答