Fast C++ I/O to read a string (char array) into a vector, plus resolving error

Question

Right now I am trying to use a vector of charpointers to hold a bunch of individual strings

vector<char*> strings(10);

The individual string is of form char indString[100]

And I am doing this to assign each one

for (int i=0; i < iLimit; ++i){
    gets(indString);
    strings[i] = indString;
}

But when I output everything

for (auto & stri: strings) cout << stri << endl;

I just get a bunch of repeats of the last string read.

Silly question - but are you incrementing `i` in between read operations? Need to show a bit more code... — Floris, Aug 03 '13 at 16:02
http://stackoverflow.com/questions/1694036/why-is-the-gets-function-is-dangerous-why-should-not-be-used — Mat, Aug 03 '13 at 16:06
Show enough code that we can reproduce your problem. Right now this doesn't run. — Floris, Aug 03 '13 at 16:07

score 1 · Answer 1 · answered Aug 03 '13 at 16:03

1

You are just storing the pointer to the buffer used to read... all the pointers will point to the same buffer and this is why you see the last string repeated.

Just use an std::vector<std::string> instead.

answered Aug 03 '13 at 16:03

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I'd like to do this without strings if possible, I get a speed advantage using char arrays – MyNameIsKhan Aug 03 '13 at 16:08

score 1 · Answer 2 · answered Aug 03 '13 at 16:30

You are adding the same pointer to the vector at every iteration which is causing you to see duplicates in the output. You can do this efficiently and cleanly using std::getline, std::string and std::move.

Below is a working example that reads strings from cin and outputs the contents of the vector.

#include <vector>
#include <string>

void readtext(
    std::istream& input,
    std::vector<std::string>& text,
    const size_t limit)
{
    std::string line;
    for (size_t count = 0; count < limit && std::getline(input, line); ++count)
        text.push_back(std::move(line));
}

int main()
{
    std::vector<std::string> text;
    readtext(std::cin, text, 3);
    for (auto& line : text)
        std::cout << line << std::endl;
}

Nicholas Pipitone · Answer 3 · 2013-08-03T16:14:04.850

They all are pointing to the same pointer. I'd suggest doing this:

char** PointerArray = new char*[NumOfStrings];

//input strings

for( int i = 0; i < NumOfStrings; i++ )
     cout << PointerArray[i];

for( int i = 0; i < NumOfStrings; i++ )
     delete[] PointerArray[i];

EDIT: If you require vectors, make an array of character arrays, then pass each character into it.

OR you could try this:

char** PointerArray = new char*[NumOfStrings];

vector<char*> pointerArray(NumOfStrings);
for( int i = 0; i < NumOfStrings; i++ )
     pointerArray[i] = PointerArray[i];
//stuff
for( int i = 0; i < NumOfStrings; i++ )
     delete[] pointerArray[i];

Floris · Accepted Answer · 2013-08-03T16:22:10.033

0

You are pointing all your strings[i] to the same location... you need to allocate different memory locations for each string, and read them into that location. With char arrays, it would look like this:

char* strings[10];

for (int i=0; i < iLimit; ++i){
    strings[i] = malloc(sizeOfLargestString + 1);
    gets(strings[i]);
}

Or full code (tested):

#include <stdio.h>
#include <stdlib.h>
#define sizeOfLargestString 100
#define N 3

int main(void) {
  int ii;
  char* strings[N];
  for (ii=0; ii < N; ii++) {
    strings[ii] = (char*)malloc(sizeOfLargestString + 1);
    gets(strings[ii]);
  }
  for(ii=0 ;ii< N; ii++) {
    printf("%s\n", strings[ii]);
  }
  return 0;
}

edited Aug 03 '13 at 16:22

answered Aug 03 '13 at 16:13

Floris

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This doesn't technically work. I am trying to hold an array of strings basically – MyNameIsKhan Aug 03 '13 at 16:14
"error: invalid conversion from 'void*' to 'char*' [-fpermissive]" on the strings[i] = malloc line – MyNameIsKhan Aug 03 '13 at 16:17
I have edited so your compiler ought not to complain (tested with `gcc - Wall`) – Floris Aug 03 '13 at 16:22
outputs a blank line at the front, then outputs the rest fine, ignores the last line of input – MyNameIsKhan Aug 03 '13 at 16:24
just used a workaround (read string once arbitrarily before the loop) and I think it works now... thanks! – MyNameIsKhan Aug 03 '13 at 16:25
I did strings[0] = (char*)malloc(sizeOfLargestString+ 1); gets(strings[0]); before the input loop – MyNameIsKhan Aug 03 '13 at 16:26

Fast C++ I/O to read a string (char array) into a vector, plus resolving error

4 Answers4