Question

在c中，我知道 unsigned char 的大小 1个字节（= 1个八位位组= 8位），我知道unsigned char实际上是一个0到255之间的整数值，现在如果我有以下unsigned char变量：

unsigned char c = 255; //(this value can be any value from 0 to 255)

如何获得一个包含8个元素的表，表明该变量的每个位值（0或1）？（像这样：{1,1,1,1,1,1,1,1}}在c中有没有简单的方法呢？

Answer 1

在C中提取位的常用方法是使用位移操作：

int bits[8];
for (int i = 0 ; i != 8 ; i++) {
    bits[i] = (c & (1 << i)) != 0;
}

1 << i生成1设置在i位置的二进制数，即1 ₂，10 ₂，100 ₂，1000 ₂，依此类推。

运营商&使用1 << i作为“掩码”，在掩码值中选择c标记为1的单个位。比较!= 0完成作业，产生零或一，取决于该位的值。

另一种方法是将c转移到右侧，并使用1进行屏蔽，如下所示：

int bits[8];
for (int i = 0 ; i != 8 ; i++) {
    bits[i] = (c >> i) & 1;
}

这与第一种方法类似，但屏蔽是在最低有效位位置完成的。

注意：在CHAR_BIT设置为8以外的数字的计算机上，这只会提取低8位。

Answer 2

除了按位运算（更好，我会说标准的方法），你可以使用联合来访问单个位：

 func showPickerInActionSheet(sentBy: String) {
    var title = "picker"
    var message = "Picker controller";
    var alert = UIAlertController(title: title, message: message, preferredStyle: UIAlertControllerStyle.ActionSheet);
    alert.modalInPopover = true;


    //Create a frame (placeholder/wrapper) for the picker and then create the picker
    var pickerFrame: CGRect = CGRectMake(17, 52, 270, 100); // CGRectMake(left), top, width, height) - left and top are like margins
    var picker: UIPickerView = UIPickerView(frame: pickerFrame);

    /* If there will be 2 or 3 pickers on this view, I am going to use the tag as a way
    to identify them in the delegate and datasource. /* This part with the tags is not required.
    I am doing it this way, because I have a variable, witch knows where the Alert has been invoked from.*/
    if(sentBy == "profile"){
        picker.tag = 1;
    } else if (sentBy == "user"){
        picker.tag = 2;
    } else {
        picker.tag = 0;
    }

    //set the pickers datasource and delegate
    picker.delegate = self;
    picker.dataSource = self;

    //Add the picker to the alert controller
    alert.view.addSubview(picker);

    //Create the toolbar view - the view witch will hold our 2 buttons 
    var toolFrame = CGRectMake(17, 5, 270, 45);
    var toolView: UIView = UIView(frame: toolFrame);

    //add buttons to the view
    var buttonCancelFrame: CGRect = CGRectMake(0, 7, 100, 30); //size & position of the button as placed on the toolView

    //Create the cancel button & set its title
    var buttonCancel: UIButton = UIButton(frame: buttonCancelFrame);
    buttonCancel.setTitle("Cancel", forState: UIControlState.Normal);
    buttonCancel.setTitleColor(UIColor.blueColor(), forState: UIControlState.Normal);
    toolView.addSubview(buttonCancel); //add it to the toolView

    //Add the target - target, function to call, the event witch will trigger the function call
    buttonCancel.addTarget(self, action: "cancelSelection:", forControlEvents: UIControlEvents.TouchDown);


    //add buttons to the view
    var buttonOkFrame: CGRect = CGRectMake(170, 7, 100, 30); //size & position of the button as placed on the toolView

    //Create the Select button & set the title
    var buttonOk: UIButton = UIButton(frame: buttonOkFrame);
    buttonOk.setTitle("Select", forState: UIControlState.Normal);
    buttonOk.setTitleColor(UIColor.blueColor(), forState: UIControlState.Normal);
    toolView.addSubview(buttonOk); //add to the subview

    //Add the tartget. In my case I dynamicly set the target of the select button
    if(sentBy == "profile"){
        buttonOk.addTarget(self, action: "saveProfile:", forControlEvents: UIControlEvents.TouchDown);
    } else if (sentBy == "user"){
        buttonOk.addTarget(self, action: "saveUser:", forControlEvents: UIControlEvents.TouchDown);
    }

    //add the toolbar to the alert controller
    alert.view.addSubview(toolView);

    self.presentViewController(alert, animated: true, completion: nil);
}

func saveProfile(sender: UIButton){
    // Your code when select button is tapped

}

func saveUser(sender: UIButton){
    // Your code when select button is tapped
}

func cancelSelection(sender: UIButton){
    println("Cancel");
    self.dismissViewControllerAnimated(true, completion: nil);
    // We dismiss the alert. Here you can add your additional code to execute when cancel is pressed
}

// returns number of rows in each component..
func pickerView(pickerView: UIPickerView, numberOfRowsInComponent component: Int) -> Int{
    if(pickerView.tag == 1){
        return self.profilesList.count;
    } else if(pickerView.tag == 2){
        return self.usersList.count;
    } else  {
        return 0;
    }
}

// Return the title of each row in your picker ... In my case that will be the profile name or the username string
func pickerView(pickerView: UIPickerView, titleForRow row: Int, forComponent component: Int) -> String! {
    if(pickerView.tag == 1){

            var selectedProfile: Profiles = self.profilesList[row] as Profiles;
            return selectedProfile.profileName;

    } else if(pickerView.tag == 2){

            var selectedUser: Users = self.usersList[row] as Users;
            return selectedUser.username;

    } else  {

        return "";

    }

}

func pickerView(pickerView: UIPickerView, didSelectRow row: Int, inComponent component: Int) {
    if(pickerView.tag == 1){
            var choosenProfile: Profiles = profilesList[row] as Profiles;
            self.selectedProfile = choosenProfile.profileName;
    } else if (pickerView.tag == 2){
            var choosenUser: Profiles = usersList[row] as Users;
            self.selectedUsername = choosenUser.username;
    }

}

注意：单个位变量的顺序是实现定义的，所以你应该小心它们并使用特定于编译器的标志来强制命令

如何获取unsigned char的每一位值？

2 个答案: