C++ 比较字符数组和字符串

C++ Compare char array with string

提问by Chris Klepeis

I'm trying to compare a character array against a string like so:

我正在尝试将字符数组与字符串进行比较，如下所示：

const char *var1 = " ";
var1 = getenv("myEnvVar");

if(var1 == "dev")
{
   // do stuff
}

This if statement never validates as true... when I output var1 it is "dev", I was thinking maybe it has something to do with a null terminated string, but the strlen of "dev" and var1 are equal... I also thought maybe var1 == "dev" was comparing "dev" against the memory location of var1 instead of the value. *var1 == "dev" results in an error.... tried many things, probably a simple solution for the saavy c++ developer (I havent coded c++ in a looong time).

这个if语句永远不会验证为真......当我输出var1时它是“dev”，我在想它可能与空终止字符串有关，但是“dev”和var1的strlen是相等的......我还认为 var1 == "dev" 正在将 "dev" 与 var1 的内存位置而不是值进行比较。*var1 == "dev" 导致错误.... 尝试了很多东西，可能是一个简单的解决方案，适用于 saavy c++ 开发人员（我很久没有编写 c++ 了）。

edit: we've tried

编辑：我们试过了

if(strcmp(var1, "dev") == 0)

and

和

if(strncmp(var1, "dev", 3) == 0)

Thanks

谢谢

edit: After testing at home I'm just going to suggest my co-worker changes the datatype to a string. I believe he was comparing a char array of a large size against a string. I put together a program that outputs sizeof, strlen, etc to help us work through it. Thanks to everyone for the help.

编辑：在家测试后，我只是建议我的同事将数据类型更改为字符串。我相信他正在将一个大尺寸的 char 数组与一个字符串进行比较。我整理了一个输出 sizeof、strlen 等的程序来帮助我们解决它。感谢大家的帮助。

回答by John Millikin

Use strcmp()to compare the contents of strings:

使用strcmp()比较字符串的内容：

if (strcmp(var1, "dev") == 0) {
}

Explanation: in C, a string is a pointer to a memory location which contains bytes. Comparing a char*to a char*using the equality operator won't work as expected, because you are comparing the memory locationsof the strings rather than their byte contents. A function such as strcmp()will iterate through both strings, checking their bytes to see if they are equal. strcmp()will return 0 if they are equal, and a non-zero value if they differ. For more details, see the manpage.

解释：在 C 中，字符串是指向包含字节的内存位置的指针。使用相等运算符将achar*与 achar*进行比较不会按预期工作，因为您正在比较字符串的内存位置而不是它们的字节内容。诸如此类的函数strcmp()将遍历两个字符串，检查它们的字节以查看它们是否相等。strcmp()如果相等则返回 0，如果不同则返回非零值。有关更多详细信息，请参阅联机帮助页。

回答by jalf

You're not working with strings. You're working with pointers. var1is a char pointer (const char*). It is not a string. If it is null-terminated, then certain C functions will treatit as a string, but it is fundamentally just a pointer.

你不是在处理字符串。您正在使用指针。 var1是一个字符指针 ( const char*)。它不是一个字符串。如果是空值终止，那么一定C功能将对待它作为一个字符串，但它是从根本上只是一个指针。

So when you compare it to a char array, the array decays to a pointer as well, and the compiler then tries to find an operator == (const char*, const char*).

因此，当您将它与 char 数组进行比较时，该数组也会衰减为一个指针，然后编译器会尝试找到一个operator == (const char*, const char*).

Such an operator does exist. It takes two pointers and returns trueif they point to the same address. So the compiler invokes that, and your code breaks.

这样的运营商确实存在。它需要两个指针，true如果它们指向同一个地址就返回。所以编译器调用它，你的代码就会中断。

IF you want to do string comparisons, you have to tell the compiler that you want to deal with strings, not pointers.

如果要进行字符串比较，则必须告诉编译器您要处理字符串，而不是指针。

The C way of doing this is to use the strcmpfunction:

这样做的 C 方法是使用strcmp函数：

strcmp(var1, "dev");

This will return zero if the two strings are equal. (It will return a value greater than zero if the left-hand side is lexicographically greater than the right hand side, and a value less than zero otherwise.)

如果两个字符串相等，这将返回零。（如果左侧按字典顺序大于右侧，它将返回一个大于零的值，否则返回一个小于零的值。）

So to compare for equality you need to do one of these:

因此，要比较相等性，您需要执行以下操作之一：

if (!strcmp(var1, "dev")){...}
if (strcmp(var1, "dev") == 0) {...}

However, C++ has a very useful stringclass. If we use that your code becomes a fair bit simpler. Of course we could create strings from both arguments, but we only need to do it with one of them:

但是，C++ 有一个非常有用的string类。如果我们使用它，您的代码会变得更简单一些。当然，我们可以从两个参数创建字符串，但我们只需要使用其中一个来创建字符串：

std::string var1 = getenv("myEnvVar");

if(var1 == "dev")
{
   // do stuff
}

Now the compiler encounters a comparison between string and char pointer. It can handle that, because a char pointer can be implicitly converted to a string, yielding a string/string comparison. And those behave exactly as you'd expect.

现在编译器遇到字符串和字符指针之间的比较。它可以处理这种情况，因为字符指针可以隐式转换为字符串，从而产生字符串/字符串比较。这些行为完全符合您的预期。

回答by JaredPar

In this code you are not comparing string values, you are comparing pointer values. If you want to compare string values you need to use a string comparison function such as strcmp.

在此代码中，您不是在比较字符串值，而是在比较指针值。如果要比较字符串值，则需要使用字符串比较函数，例如 strcmp。

if ( 0 == strcmp(var1, "dev")) {
  ..
}

回答by sepp2k

"dev" is not a stringit is a const char *like var1. Thus you are indeed comparing the memory adresses. Being that var1is a char pointer, *var1is a single char (the first character of the pointed to character sequence to be precise). You can't compare a char against a char pointer, which is why that did not work.

“dev”不是 astring它是一个const char *like var1。因此，您确实在比较内存地址。因为它var1是一个字符指针，所以*var1是一个单独的字符（准确地说是指向字符序列的第一个字符）。您不能将 char 与 char 指针进行比较，这就是为什么它不起作用。

Being that this is tagged as c++, it would be sensible to use std::stringinstead of char pointers, which would make == work as expected. (You would just need to do const std::string var1instead of const char *var1.

因为这被标记为 c++，所以使用std::string而不是 char 指针是明智的，这将使 == 按预期工作。（您只需要执行const std::string var1而不是const char *var1.

回答by AraneaSerket6848

There is more stable function, also gets rid of string folding.

有更稳定的功能，也摆脱了字符串折叠。

// Add to C++ source
bool string_equal (const char* arg0, const char* arg1)
{
    /*
     * This function wraps string comparison with string pointers
     * (and also works around 'string folding', as I said).
     * Converts pointers to std::string
     * for make use of string equality operator (==).
     * Parameters use 'const' for prevent possible object corruption.
     */
    std::string var0 = (std::string) arg0;
    std::string var1 = (std::string) arg1;
    if (var0 == var1)
    {
        return true;
    }
    else
    {
        return false;
    }
}

And add declaration to header

并在标题中添加声明

// Parameters use 'const' for prevent possible object corruption.
bool string_equal (const char* arg0, const char* arg1);

For usage, just place an 'string_equal' call as condition of if (or ternary) statement/block.

对于用法，只需放置一个 'string_equal' 调用作为 if（或三元）语句/块的条件。

if (string_equal (var1, "dev"))
{
    // It is equal, do what needed here.
}
else
{
    // It is not equal, do what needed here (optional).
}

Source: sinatramultimedia/fl32 codec (it's written by myself)

来源：sinatramultimedia/fl32 codec（我自己写的）

回答by 101dolmations

your thinking about this program below

你对下面这个程序的想法

#include <stdio.h>
#include <string.h>

int main ()
{
char str[][5] = { "R2D2" , "C3PO" , "R2A6" };
int n;
puts ("Looking for R2 astromech droids...");
for (n=0 ; n<3 ; n++)
if (strncmp (str[n],"R2xx",2) == 0)
{
  printf ("found %s\n",str[n]);
}
return 0;
}
//outputs:
//
//Looking for R2 astromech droids...
//found R2D2
//found R2A6

when you should be thinking about inputting something into an array & then use strcmp functions like the program above ... check out a modified program below

当您应该考虑将某些内容输入数组然后使用 strcmp 函数（如上面的程序）时……查看下面的修改后的程序

#include <iostream>
#include<cctype>
#include <string.h>
#include <string>
using namespace std;

int main()
{
int Students=2;
int Projects=3, Avg2=0, Sum2=0, SumT2=0, AvgT2=0, i=0, j=0;
int Grades[Students][Projects];

for(int j=0; j<=Projects-1; j++){
  for(int i=0; i<=Students; i++) {
 cout <<"Please give grade of student "<< j <<"in project "<< i  <<  ":";
  cin >> Grades[j][i];

  }
  Sum2 = Sum2 + Grades[i][j];
     Avg2 = Sum2/Students;
}
SumT2 = SumT2 + Avg2;
AvgT2 = SumT2/Projects;
cout << "avg is  : " << AvgT2 << " and sum : " << SumT2 << ":";
return 0;
}

change to string except it only reads 1 input and throws the rest out maybe need two for loops and two pointers

更改为字符串，除了它只读取 1 个输入并抛出其余的可能需要两个 for 循环和两个指针

#include <cstring>
#include <iostream>
#include <string>
#include <stdio.h>
using namespace std;
int main()
{
char name[100];
//string userInput[26];
int i=0, n=0, m=0;
cout<<"your name? ";
cin>>name;
cout<<"Hello "<<name<< endl;

char *ptr=name;
for (i = 0; i < 20; i++)
{
cout<<i<<" "<<ptr[i]<<" "<<(int)ptr[i]<<endl;
}   
int length = 0;
while(name[length] != '##代码##')
{
length++;
}
                    for(n=0; n<4; n++)
                {
                            if (strncmp(ptr, "snit", 4) == 0)
                            {
            cout << "you found the snitch "    <<        ptr[i];
                            }
                }
cout<<name <<"is"<<length<<"chars long";
}