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Recursively transfer strings from istream into array - C++

I am trying to copy in strings from a .txt file (every word is on a new line) into an array. The main routine would look like this.

.txt

const int MAXDICTWORDS = 5000;

int main()
{
string dict[MAXDICTWORDS];
ifstream dictfile; // file containing the list of words

dictfile.open("words.txt");
if (!dictfile) {
cout << "File not found!" << endl;
return (1);
}

int nwords = dicReader(dictfile, dict);
// dict should now hold array of words from words.txt
// nwords will be the amount of words in the array
}


This is my current implementation of dicReader. dict will always be empty when passed to this function. I am practicing with recursion, so no while or for loops can be used. Any ideas on what I am missing?

dicReader

dict

while

for

int dicReader(istream &dictfile, string dict)
{
int count = 0; // know this is wrong, recursive call will always reset count to 0
string item;

if (!dictfile.eof())
{
dictfile >> item;
dict[0] = item;
dicReader(dictfile, dict + 1); // is this the correct recursive call?
count++;
}

return count;
}


dicReader

dictionaryReader

2 Answers
2

You are not using count correctly.

count

You need to keep accumulating the value in the recursive calls.

Also, if (!dictfile.eof()) is wrong. See Why is iostream::eof inside a loop condition considered wrong? for details.

if (!dictfile.eof())

The function can be simplified to:

int dictionaryReader(istream &dictfile, string dict)
{
if (dictfile >> dict[0])
{
return (1 + dictionaryReader(dictfile, dict + 1));
}

return 0;
}


true

R Sahu has given you a complete answer, but there is an alternative way to formulate the recursive function. This alternative has the count variable as a parameter to the function. Like this

count

int dictionaryReader(istream &dictfile, string dict, int count)
{
if (dictfile >> dict[0])
{
return dictionaryReader(dictfile, dict + 1, count + 1);
}
return count;
}


When the function is initially called you supply a value of zero for count.

count

int nwords = dictionaryReader(dictfile, dict, 0);


In this implementation dictionaryReader calls itself and returns the resulting value directly. This is called tail recursion. In the alternative implementation dictionaryReader calls itself and then adds one to the result, so it isn't tail recursive.

The advantage of tail recursion is that it is trivially converted into a while loop (the while loop you were told to avoid). Some compilers perform this conversion for you as an optimisation. So you write a tail recursive function but end up with the same code as if you had written a while loop.

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