56表达式：（无符号）（c + 1）<= 256 [英] 56 Expression: (unsigned)(c + 1) &lt;= 256

问题描述

我正在做一个哈希程序，它计算一个单词在文本文件中的每个实例的数量。这是我的计数函数：当尝试运行它时，我得到一个错误。

I am making a hashing program that is counting the number of each instance of a word in a text file. This is my count function: I am getting an error when trying to run it.

56表达式：（unsigned）（c + 1） < = 256

56 Expression: (unsigned)(c + 1) <= 256

看起来当isalpha函数读取第一个nonalpha垃圾字符文本文件。

It appears as though it is crashing on the isalpha function when it is reading in the very first nonalpha garbage characters in the text file.

int
count(ifstream & fs,int size)
{
int find(const char *,int, int);

int f,i,l,y;
char ch,*p,s[maxs+1];

for(y = l = i = 0; i < size; i++)
{
    table[i].k = 0;
    table[i].p = nill;
}

p = s;

while(fs.get(ch))
{
    if(isalpha(ch))
    {
        if(l < maxs)
        {
            l++;
            *p++ = (char)(ch | 0x20);
        }
    }
    else
    {
        if(l)
        {
            *p = '\0';

            if(f = find(s,size,l) < 0)
            {
                return(f);
            }
            y += f;
            p = s;
            l = 0;
        }
    }
}
}

推荐答案

在我看来， isalpha 是失败的断言。很可能（unsigned）（c + 1）<= 256 是被断言的表达式。看起来这个断言试图确保 c 的值落在[0,255]内。

It looks to me like isalpha is failing an assertion. Most likely (unsigned)(c + 1) <= 256 is the expression that is being asserted. It looks like this assertion is trying to ensure the value of c falls within [0, 255].

假设 ch 是一个 signed char ，您尝试将值128存储在其中，然后传递给 isalpha ，断言的左侧将计算一个非常大的数字，导致它失败。

Assuming ch is a signed char and you try to store the value 128 in it, then pass it to isalpha, the left hand side of the assertion is going to evaluate to a very large number, causing it to fail.

128不能存储在 signed char 中，因此 ch 实际上变为-128，它是无符号128（二进制为1000 0000）的有符号表示。 isalpha 将 ch 作为 int code>（c + 1）实际上是（ - 128 + 1），它变为-127。

128 can't be stored in a signed char, so the value of ch actually becomes -128, which is the signed representation of unsigned 128 (1000 0000 in binary). isalpha is taking ch as an int, so the (c + 1) is actually (-128 + 1), which becomes -127. This value is then cast to an unsigned integer, which turns into a very large value.

一个解决方案是更改 ch 在代码中添加到 unsigned char ，如果它的值可能大于127.

A solution is to change ch in your code to an unsigned char, if it's possible that it's value can be greater than 127.

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