MIPS 中的回文生成

Question

我需要帮助来编写一个生成回文的程序。我设法使字符串反转，但我无法将原始字符串和反转字符串组合在一起。当我写 abc 时，我需要得到 abccba，或者当我写 hello 时，我需要得到 helloolleh。现在我只得到 cba 或 olleh。有人可以帮我解决这个问题吗？

.data
msg1: .asciiz "Enter the length of your input: "
msg2: .asciiz "Enter your input: "
msg3: .asciiz "Output of the program is: "

output: .space 256 # will store the output palindrome

.text
la $a0,msg1
li $v0,4
syscall

li $v0,5
syscall
move $s1,$v0 # $s1 has the length of the string



la $a0,msg2
li $v0,4
syscall


li $a1,1024
li $v0,8
syscall
move $s0,$a0 # $s0 has the starting address of the string

#
# YOUR CODE GOES HERE(You can use additional labels at the end)
#


move $a3,$s0
move $a1,$s1




add $a3,$a3,$a1
la $a2, output($zero)
jal reverse 



la $a0,msg3
li $v0,4
syscall




la $a0,output($zero)
li $v0,4
syscall

li $v0,10
syscall


reverse:
    # $a0 - address of string to reverse
    # a1 - length of the string
    # a2 - address of string where to store the reverse
    addi $sp, $sp, -4
    sw $ra, 0($sp)
    bltz $a1, reverse_end
    lb $t0, 0($a3)
    subi $a1, $a1, 1
    subi $a3, $a3, 1
    sb $t0, 0($a2)
    addi $a2, $a2, 1
    jal reverse
reverse_end:
    lw $ra, 0($sp)
    addi $sp, $sp, 4
    jr $ra

编辑：这也是回文生成的 C++ 实现。

回文生成递归算法的C++实现

#include <iostream>
#include <string>

using namespace std;

string palindrom(string input, int rem_length)
{

  if(rem_length!=0)
  {
    input=input.substr(0,1)+palindrom(input.substr(1,input.length()-1), \\
                                      rem_length-1)+input.substr(0,1);
  }
  return input;
}

int main()
{
  string input;
  cin >> input;

  input = palindrom(input, input.length());

  cout << input<< endl;

  system("pause");

  return 0;
}

Answer 1

看来您只需要先将输入字符串复制到output 字符串，当您将output 数组传递给reverse 函数时，您需要将指针前进输入字符串长度（传递@987654324 @) 所以反转的字符串将写在它旁边。

我还认为您应该使用缓冲区进行输入。还要注意输入字符串末尾的 '\n' 字符，因为它会在那里。

考虑到这些，我将您的代码更改为此

.data
msg1: .asciiz "Enter the length of your input: "
msg2: .asciiz "Enter your input: "
msg3: .asciiz "Output of the program is: "

input: .space 128
output: .space 256 # will store the output palindrome

.text
la $a0,msg1
li $v0,4
syscall

li $v0,5
syscall
move $s1,$v0 # $s1 has the length of the string

la $a0,msg2
li $v0,4
syscall

move $a1, $1
la $a0, input
li $v0,8
syscall
move $s0,$a0 # $s0 has the starting address of the string

#
# YOUR CODE GOES HERE(You can use additional labels at the end)
#
move $a3,$s0 # a3 = (char *)input
move $a1,$s1 # a1 = strlen(input)


# replacing the read in new line char with 0
la $t4, input
add $t4, $t4, $a1
sb $zero, ($t4)

# copy the string to the resulting string
move $t0, $s1 # i = strlen(input)
move $t1, $s0
la $t3, output

copy_loop:
beq $t0, $zero, copy_end # i != 0
lb $t2, ($t1) 
sb $t2, ($t3) # *output = *input
addi $t1, $t1, 1 # advancing the input ptr
addi $t3, $t3, 1 # advancing the output ptr
subi $t0, $t0, 1 # i--
b copy_loop

copy_end:

add $a3,$a3,$a1 # a3 = &input[strlen(input)]
subi $a3,$a3,1 # subtracting one since the last input char is at input[strlen(input)-1]
la $a2, output
add $a2, $a2, $a1 # passing the address of output advanced by the input length
jal reverse 

la $a0,msg3
li $v0,4
syscall

la $a0,output($zero)
li $v0,4
syscall

li $v0,10
syscall

reverse:
    # $a0 - address of string to reverse
    # a1 - length of the string
    # a2 - address of string where to store the reverse
    addi $sp, $sp, -4
    sw $ra, 0($sp)
    bltz $a1, reverse_end
    lb $t0, 0($a3)
    subi $a1, $a1, 1
    subi $a3, $a3, 1
    sb $t0, 0($a2)
    addi $a2, $a2, 1
    jal reverse
reverse_end:
    lw $ra, 0($sp)
    addi $sp, $sp, 4
    jr $ra

这似乎适用于您在 MARS 4.5 MIPS 模拟器中的示例。

这段代码很可能以更优化的方式完成，但似乎仍然可以完成工作。

编辑想法：将输入保存到输出字符串，这样就不需要复制了，因此代码可以减少到这个（仅相关部分）

move $a1, $1
la $a0, output # !! using the output buffer for the input
li $v0,8
syscall
move $s0,$a0 # $s0 has the starting address of the string

#
# YOUR CODE GOES HERE(You can use additional labels at the end)
#
move $a3,$s0 # a3 = (char *)input
move $a1,$s1 # a1 = strlen(input)

# the whole copy thing deleted and don't need to be bothered by the new
# lines, it will be overwritten with the reversed string anyway

add $a3,$a3,$a1 # a3 = &input[strlen(input)]
subi $a3,$a3,1 # subtracting one since the last input char is at input[strlen(input)-1]
la $a2, output
add $a2, $a2, $a1 # passing the address of output advanced by the input length
jal reverse