再次执行 while 循环的标准习惯用法

Question

C 中是否有一种模式可以再执行一次 while 循环。 目前我正在使用

while(condition) {
    condition = process();
    // process() could be multiple lines instead of a function call
    // so while(process());process(); is not an option
}
process();

如果进程是多行而不是单个函数调用，那就太可怕了。

另一种选择是

bool run_once_more = 1;
while(condition || run_once_more) {
    if (!condition) {
        run_once_more = 0;
    }
    condition = process();
    condition = condition && run_once_more;
}

有没有更好的办法？

---

注意：do while 循环不是解决方案，因为它等同于

process();
while(condition){condition=process();}

我想要

while(condition){condition=process();}
process();

---

根据请求，更具体的代码。 我想从 another_buffer 填充缓冲区并获取 (indexof(next_set_bit) + 1) 到 MSB，同时保持掩码和指针。

uint16t buffer;
...
while((buffer & (1 << (8*sizeof(buffer) - 1))) == 0) { // get msb as 1
    buffer <<= 1;
    // fill LSB from another buffer
    buffer |= (uint16_t) (other_buffer[i] & other_buffer_mask);
    // maintain other_buffer pointers and masks
    other_buffer_mask >>= 1;
    if(!(other_buffer_mask)) { 
        other_buffer_mask = (1 << 8*sizeof(other_buffer[0]) -1)
        ++i;
    }
}
// Throw away the set MSB
buffer <<= 1;
buffer |= (uint16_t) (other_buffer[i] & other_buffer_mask);
other_buffer_mask >>= 1;
if(!(other_buffer_mask)) { 
    other_buffer_mask = (1 << 8*sizeof(other_buffer[0]) -1)
    ++i;
}
use_this_buffer(buffer);

Answer 1

因为这不是一件非常典型的事情，所以不太可能有一个标准的习惯用法来做这件事。但是，我会这样编写代码：

for (bool last = 0; condition || last; last = !(condition || last)) {
    condition = process();
}

只要condition 为真，循环就会执行，然后再执行一次，或者如果condition 在循环开始时为假，则执行零次。我将您的问题解释为这意味着这是所需的行为。如果不是，并且您总是希望循环至少执行一次，那么do...while 就是您要寻找的成语。

Answer 2

这个呢：

int done, condition = 1;
for (;;) {
    ...
    done = !condition;
    condition = process();
    if (done) break;
    ...
}

我并不是说这是一个标准的成语，只是一个临时的 hack。

Answer 3

我会用这个：

bool done = false;
do {
  if (!condition) done = true;
  condition = process();
} while (!done);

假设condition 未预定义，则可读性要差得多：

for (bool condition=true, done=false;
     !done && (condition || done = true);
    ) {
  condition = process();
}

Answer 4

我也遇到了同样的问题，我的解决方案是：

仅当您确定要运行 process() 至少一次时：

while(1){
    process();
    if(!condition()) break;
    /*The loop breaks before bad things happen*/
    things_you_want_to_happen_only_while_condition_is_true();
}

如果您不确定：

if(condition()){ /*Same as before, but it checks first*/
    while(1){
        process();
        if(!condition()) break;
        /*The loop breaks before bad things happen*/
        things_you_want_to_happen_only_while_condition_is_true();
    }
}

例如：

/*A pointer that goes over the string T from right to left.
It prints "Yes" when pointing at the character 'O' and "No" when not*/
char T[] = {'O', 'V', 'E', 'R', 'F', 'L', 'O', 'W', 0};
char *P = &T[strlen(T)-1]; /*We want the pointer to start at the last character of T*/
while(1){
    if(*P=='O') printf("Yes\n");
    else printf("No\n");
    if(P==&T[0]) break; /*When it reaches the beginning of T, it stops*/
    /*We only want the pointer to go further back if it is not in the beginning of T.
    That's because we don't know what's before in the memory.*/
    P--;
}