声明未设置但是变量计算结果为真？ [英] Declared but unset variable evaluates as true?

问题描述

我在做有以下code一个简单的计算器。现在它完美地执行。当我试图改变周围的事物，但是，这是行不通的。我用 BOOL程序来检查是否继续要求输入由人或完成计划。

如果我改变前$ P $ ，而 pssion声明只是（程序）和变更是 / NO 在程序语句，为什么用code不能做的是里面的，而？

  //一个简单的印刷计算器
{
    NSAutoreleasePool *池= [[NSAutoreleasePool的alloc]初始化]
    计算器* deskCalc = [[计算器的alloc]初始化];
    双值1;
    焦炭运营商
    BOOL程序;    [deskCalc setAccumulator：0];    而（！程序）{
    的NSLog（@请输入你的前pression）;
    scanf函数（％LF％C，＆安培;值1，和放大器;运营商）;
    程序= NO;        如果（==操作符'+'）{
            [deskCalc添加：值1];
        }
        否则，如果（==操作符' - '）{
            [deskCalc减：值1];
        }
        否则，如果（==操作符'*'||运算符=='X'）{
            [deskCalc乘法：值1];
        }
        否则，如果（操作符=='/'）{
            如果（值== 0）
                的NSLog（@除以零！）;
            其他
                [deskCalc鸿沟：值1];
        }
        否则，如果（==操作符'S'）{
            [deskCalc设置：值1];
        }
        否则，如果（操作员=='E'）{
            [deskCalc蓄电池]
            程序= YES;
        }
        其他{
            的NSLog（@未知运营商）;
        }
    }    的NSLog（@结果是％F，[deskCalc蓄电池]）;    [deskCalc发布]    [池排水]
    返回0;
}
 

解决方案

您还没有设置的程序，所以它默认为一个垃圾值初始值只发生的是非零

设置的初始值程序，当你把它声明：

  BOOL程序= NO; //或YES，较合适
 

I was doing a simple calculator with the following code. Right now it executes perfectly. When I tried to change things around, however, it doesn't work. I used BOOL program to check whether to continue asking for input from the person or finish the program.

If I change the expression of while statement to just (program) and change YES/NO in the program statements, why does the code fail to do what is inside the while?

// A simple printing calculator
{
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init]
    Calculator *deskCalc = [[Calculator alloc] init];
    double value1;
    char operator        
    BOOL program;

    [deskCalc setAccumulator: 0];

    while (!program) {
    NSLog (@"Please type in your expression");
    scanf (" %lf %c", &value1, &operator);
    program = NO;

        if (operator == '+') {
            [deskCalc add: value1];
        }
        else if (operator == '-') {
            [deskCalc subtract: value1];
        }
        else if (operator == '*' || operator == 'x') {
            [deskCalc multiply: value1];
        }
        else if (operator == '/') {
            if (value1 == 0)
                NSLog (@"Division by zero!");
            else
                [deskCalc divide: value1];
        }
        else if (operator == 'S') {
            [deskCalc set: value1];
        }
        else if (operator == 'E') {
            [deskCalc accumulator];
            program = YES;
        }
        else {
            NSLog (@"Unknown operator");
        }
    }

    NSLog (@"The result is %f", [deskCalc accumulator]);

    [deskCalc release];

    [pool drain];
    return 0;
}

解决方案

You haven't set the initial value of program, so it defaults to a garbage value which just happens to be non-zero.

Set the initial value of program when you declare it:

BOOL program = NO; // or YES, whichever is appropriate

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