Unhandled exception in working with cstrings [duplicate]

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Why do I get a segmentation fault when writing to a string?
What is the difference between char a[] = “string”; and char *p = “string”;

I have find the fault ,but I don't know why. so would you help me ?

If I define the char str[] instead the char * str in the main function , it can work normally , else the line of *pSlow = *pFast; will crash with "Unhandled exception at 0x012314f3 in shanchu.exe: 0xC0000005: Access violation writing location 0x0123583c."

Thanks

#include <stdio.h> #include <string.h> #include <iostream> char * delChar(char *s,int iLen) { if((s == NULL) || iLen <= 0) { return NULL; } int i; const int MAXLEN = 26; unsigned int min,hashTable[MAXLEN]; for(i = 0;i < MAXLEN;i ++) { hashTable[i] = 0; } for(i = 0;i < iLen;i ++) { hashTable[*(s + i) - 'a'] ++; } while(hashTable[i] == 0) { i ++; } min = hashTable[i]; for(i = 0;i < MAXLEN;i ++) { if(hashTable[i] != 0) { if(hashTable[i] < min) { min = hashTable[i]; } } } char *pSlow = s; char *pFast = s; while(*pFast != '\0') { if(hashTable[*pFast - 'a'] != min) { *pSlow = *pFast; pSlow ++; } pFast ++; } *pSlow = '\0'; return s; } int main() { char* str = "abadccdehigiktk"; int iLen = strlen(str)/sizeof(char); char *tmp = delChar(str,iLen); printf("%s\n",tmp); system("pause"); }

consider using std::string instead

nogard
– nogard

2012-09-11 05:41:00 +00:00
Commented Sep 11, 2012 at 5:41 — nogard
– nogard, Commented Sep 11, 2012 at 5:41

ForEveR · Accepted Answer · 2012-09-11 05:45:17Z

char* str = "abadccdehigiktk";

string-literal should not be modified. In your function delChar you are trying to modify string-literal. It is undefined behaviour.

You should use

char[] str = "abadccdehigiktk";

or mb std::string (since you write on C++).

Some programmer dude · Accepted Answer · 2012-09-11 06:26:28Z

This line

char* str = "abadccdehigiktk";

defines a pointer to a constant string, i.e. the string can not be modified. If you declare it as an array (char str[]) it's an array on the stack, and can therefore be modified.

As for the deletion of characters, why not use e.g. memmove instead?

// "Delete" the fourth character of a string memmove(str + 3, str + 4, strlen(str) - 3);

If you use std::string instead, it's suddenly much easier using std::string::erase:

std::string str = "abadccdehigiktk"; // Remove the fourth character str.erase(3, 1);

You also don't have to worry about pointers versus arrays.

You helped me ,Thanks. but if I have defined the const string like "char* str = "abadccdehigiktk";" and I want to delete the 4th character, what should i do ? Thanks.
@crazyer You simply can't as the string when using a pointer is constant. You have to use an array.
@crazyer Of course, if you use std::string you don't have to worry about pointers or arrays, and removing a character becomes much simpler, as I updated my answer to show.
Wow, so beautiful.Yes, the std::string is the best method when using the C++ language.

Paul R · Accepted Answer · 2012-09-11 05:45:56Z

This part is completely wrong:

while(hashTable[i] == 0) { i ++; } min = hashTable[i]; for(i = 0;i < MAXLEN;i ++) { if(hashTable[i] != 0) { if(hashTable[i] < min) { min = hashTable[i]; } } }

Firstly i has not been initialised so it will be out of range here (it will initially be equal to 'iLen' because of the previous 'for' loop). Secondly the logic is a mess and you can do this in just one loop. I think what you're probably trying to do here is:

min = UINT_MAX; for (i = 0; i < MAXLEN; i++) { if (hashTable[i] > 0 && hashTable[i] < min) { min = hashTable[i]; } }

i.e. find the minimum non-zero value in hashTable.

Yes, I should initialize i as 0, and my logic is a mess. Thanks for your method. it is perfect. Thanks . But my fault is not here, thanks for your answer and you help me on optimizing my program.

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Unhandled exception in working with cstrings [duplicate]

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