c++ - 返回指针无法正常工作

Question

当我尝试通过函数返回的指针显示数组的内容时，程序只显示零。我不确定缺少什么。我试过几次检查出了什么问题，但似乎没有任何线索。我正在使用 Dev-C++。代码如下。非常感谢您的帮助。

#include <iostream>
#include <math.h>
#include <cstdlib>
#include <cstring>

using namespace std;

bool vowel(char c)
{
int i, val;
char alphabet[52]={'a','A','b','B','c','C','d','D','e','E','f','F','g','G','h','H','i','I','j','J','k','K','l','L','m','M','n','N','o','O','p','P','q','Q','r','R','s','S','t','T','u','U','v','V','w','W','x','X','y','Y','z','Z'};
 int const is_vowel[52]={1,1,0,0,0,0,0,0,1,1,0,0,0,0,0,0,1,1,0,0,0,0,0,0,0,0,0,0,1,1,0,0,0,0,0,0,0,0,0,0,1,1,0,0,0,0,0,0,1,1,0,0};

    for (i=0;i<52;i++)
    if (c!=alphabet[i])
    {   
    val=is_vowel[i];
    return val;
    }       

}

bool consonant(char c)
{
    int i, val;
char alphabet[52]={'a','A','b','B','c','C','d','D','e','E','f','F','g','G','h','H','i','I','j','J','k','K','l','L','m','M','n','N','o','O','p','P','q','Q','r','R','s','S','t','T','u','U','v','V','w','W','x','X','y','Y','z','Z'};
     int const is_coson[52]={1,1,0,0,0,0,0,0,1,1,0,0,0,0,0,0,1,1,0,0,0,0,0,0,0,0,0,0,1,1,0,0,0,0,0,0,0,0,0,0,1,1,0,0,0,0,0,0,1,1,0,0};

    for (i=0;i<52;i++)
    if (c==alphabet[i])
    {   
    val=is_coson[i];
    return val;
    }       

}

int* scan(char* sentence, int len_sent)
{
    char c;
int count_cons=0, count_vow=0, count_dig=0, count_lc=0, count_uc=0, i,j;
int con_value, vow_value;
int* ptr; 
int array_all_counts[5];

for (i=0; i<len_sent; i++)
{
    c=sentence[i];

    //check if the c is a digit
    if (c>=48 && c<=57)
        count_dig++;

    else if (isalpha(c))
    {
        con_value=consonant(c);
        vow_value=vowel(c);
        if (con_value==0)
            count_cons++;

        else if (vow_value!=0)  //vow_value==1
            count_vow++;

        if (96<c && c<123)
             count_lc++;

        if (64<c && c<91)
             count_uc++;                   
    }

}
cout<<"\n-------------------"<<endl;

array_all_counts[0]=count_uc;
array_all_counts[1]=count_lc;
array_all_counts[2]=count_dig;
array_all_counts[3]=count_vow;
array_all_counts[4]=count_cons;

    ptr=array_all_counts;


    cout<<"\n\n\nTesting the output of pointer: "<<endl;
    for (i=0; i<5; i++)
      cout<<ptr[i]<<"  ";

return ptr;

}


int main()
{

    int j, length;

char sentence[256];
int* ptr_array;  

cout<<"Please, enter a sentence; ";
cin.getline(sentence,256);
length=strlen(sentence);
cout<<"the sentence: ";
cout<<sentence<<endl;

ptr_array=scan(sentence, length);  //Address of first element returned into ptr_array

cout<<endl;
/// cout<<"Upper case: "<<" Lower case: "<<" Digits: "<<" Vowels: "<<" Consonants: "<<endl;
for (j=0; j<5; j++)
    cout<<ptr_array[j]<<"  ";  //Where the problem is...

return 0;
} 

Answer 1

   ptr=array_all_counts;

ptr是一个局部的int*，它指向一个局部的静态数组array_all_counts，这个局部数组会在函数退出时销毁扫描。因此，由于 ptr 指向的内存已释放，您在 main 中什么也得不到。

您可以在 scan 函数中尝试以下操作以使其工作:

int* ptr = new int [5];  //allocate memory on heap

array_all_counts[0]=count_uc;
array_all_counts[1]=count_lc;
array_all_counts[2]=count_dig;
array_all_counts[3]=count_vow;
array_all_counts[4]=count_cons;

//add this block
for (int i = 0; i < 5 ; ++i)
{
   ptr[i] = array_all_counts[i];  
}

如果输入是 "abcdefghijk"，它将正常工作并打印如下内容:

Testing the output of pointer:
0  11  0  3  8
0  11  0  3  8