问题[io]

有必要将有关机器的信息写入文件，并能够补充该文件并读取一定数量的记录。从二进制文件读取时，输出为：

    Automobile 1�犎�
                    Brand: dasf0�t�UMileage: asdf芎�
                Price: asdf 芎�
                               Automobile 2�犎�Brand: afdsfdas0�t�U`�t�UMileage: dafs芎�
                Price: fdas

（对于字段值中无意义的一组字母，我深表歉意）。据我了解，新行的转义序列在某种程度上被错误地解释了。对于录制，我使用以下功能：

void binary_write(std::ofstream& ofs, std::string& s){                                                             
     ofs.write((char*)&s, sizeof(s));                                                                               
 }

这是我写的几行：

first_line = "Automobile " + std::to_string(count+1);                                                      
         brand = "Brand: " + brand;                                                                                 
         manufacture = "Manufacture: " + manufacture;                                                               
         release = "Year of release: " + release;                                                                   
         mileage = "Mileage: " + mileage;                                                                           
         price = "Price: " + price;

阅读代码：

while(ifs.read(&temp, sizeof(char)))                                                               
                       std::cout << temp;

谁能建议如何使用二进制文件正确组织 I/O？提前致谢。

播放这段代码时：

half x = if even x
           then Just (x `div` 2)
           else Nothing

printMaybe m = case m of
  Nothing -> putStrLn "List was empty!"
  Just x -> print x

main = do
  let halfN = Just 4 >>= half
  halfN >>= printMaybe

发生错误：

main.hs:9:1: error:
    * Couldn't match expected type `IO t0' with actual type `Maybe ()'
    * In the expression: main
      When checking the type of the IO action `main'
  |
9 | main = do
  | ^
main.hs:11:13: error:
    * Couldn't match type `IO' with `Maybe'
      Expected type: Maybe a0 -> Maybe ()
        Actual type: Maybe a0 -> IO ()
    * In the second argument of `(>>=)', namely `printMaybe'
      In a stmt of a 'do' block: halfN >>= printMaybe
      In the expression:
        do let halfN = Just 4 >>= half
           halfN >>= printMaybe
   |
11 |   halfN >>= printMaybe

这是解决此问题的一种方法：

half x = if even x
           then Just (x `div` 2)
           else Nothing

printMaybe m = case m of
  Nothing -> putStrLn "List was empty!"
  Just x -> print x

main = do
  let halfN = Just 4 >>= half
  printMaybe $ halfN

我无法弄清楚为什么原始程序不起作用。首先，我们得到函数中数字的一半half（以 monad 的形式Maybe），然后我们将这个值“推”到输出函数 -printMaybe中。

为什么原始版本的代码不起作用？

有一个功能，我需要逐行输出

x y результат

所有没有更多的字符，如何做到这一点，以便 this ... 通常输出到每一行而没有额外的字符到控制台。

唯一的选择，但我发现只有一个论点

main = print([f1 x1 0 | x1 <- [0.0,0.1..10.0]])

再会。编写程序并运行它。该程序本身没有给出任何错误，它似乎可以工作。创建文件但不向文件写入任何内容。两个程序的行为已经相同。这是一个

program ioposneg
implicit none
integer::i,a,x
do i=1,10
  open(10,file='posneg')
  read(10,*) a
  if (a>0) then
    open(15,file='pos')
    x=a
    write(15,*) x
  else 
    open(20,file='neg')
    x=a
    write(20,*) x
  end if
end do
print*,'finished'
end program ioposneg

这是第二个

program ioformat
implicit none
real::x=0,ex
integer::i
character(len=5)::c='x',ec='ex'
open(10,file='xex')
write(10,2) c,ec
2 format(2a10)
do i=1,11
  ex=x**2
  open(10,file='xex')
  write(10,5)x,ex
  5 format(2f10.5)
  x=x+0.1
end do
print*,'finished'
end program ioformat

第一个程序的可读文件如下所示：

2
3
-9
15
-25
-47
85
68
-10
-5