How to edit an element in a streaming char array - c++

Question

I have records coming in from fgets (web data using popen(), not a file) that are in const char * array format.
const char * cstr = buff;
The 3rd item is a string and needs to either be removed or changed to a zero.
How do I access the 3rd element in a const char * array stream in increments of 5?
1
2
string
4
5
1
2
string
4
5
code:
while(fgets(buff, sizeof buff, fp) != NULL)
{
const char * cstr2 = buff;
for(int i = 0; i < 5; ++i){
if(i == 3){
if (!strcmp(cstr2, "Buy\n")) {
printf("Found One!\n");
cstr2[2] = 0;
}
if (!strcmp(cstr2, "Sell\n")) {
printf("Found Two!\n");
cstr2[2] = 0;
}
}
}
}
expected output:
1
2
0
4
5
1
2
0
4
5
error:
no match and :
error: assignment of read-only location '*(cstr2 + 2u)'
How do you correctly access the 3rd element in a char streaming char array?

Answer 1

This solution was previously posted by anonymous:
char* getmyData()
{
char buff[BUFSIZ];
FILE *fp = popen("php getMyorders.php 155", "r");
if (fp == NULL) perror ("Error opening file");
size_t size = 1000;
char* data = (char*)malloc(size);
char* ptr = data;
std::string::size_type sz;
int i=0;
while(fgets(buff, sizeof(buff), fp) != NULL)
{
const char * cstr2 = buff;
const char* test = ptr;
//for(int i = 0; i < 5; ++i)
{
if(i == 2){
if (!strcmp(cstr2, "Buy\n")) {
printf("Found One!\n");
strcpy(ptr,"0\n");
//ptr+=2;
}
else
if (!strcmp(cstr2, "Sell\n")) {
printf("Found Two!\n");
strcpy(ptr,"0\n");
//ptr+=2;
}
else
{
strcpy(ptr,cstr2);
ptr+=strlen(cstr2);
}
}
else
{
strcpy(ptr,cstr2);
ptr+=strlen(cstr2);
}
try
{
int nc = std::stod (test,&sz);
std::cout << "Test: " << 1+nc << "\n";
}
catch(...)
{
}
i++;
if (i==5)
i=0;
}
if (ptr-data+100>=size)
{
int ofs = ptr-data;
size*=2;
data = (char*)realloc(data,size);
ptr = data+ofs;
}
}
return data; // DONT FORGET TO call free() on it
}

Answer 2

From your sample code it is not clear what is the expected output. Is it an array of integer ? a formatted text string ? a byte array ? we can't know.
Assuming you have a text formatted input and want a text formatted output, a simple solution is to write a new string with the correct values and not try to modify your input buffer.
If you know the exact format of your input records you could use fscanf to do the parsing instead of doing it by hand. And you could use ssprintf to do the formatting of the output string.
As others pointed out, if you can use C++, you'd have safer/easier options. Please comment about your willingness to use C++.