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    <p>It seems you want to create sequences of length 4 choosing 2 out of 4 letters, contained twice each ?</p> <p>My (quite rude) approach would be like this: It adds front to end, char by char to a sequence and narrows down possible solutions according to your rule in every step.</p> <p>This will create something like :</p> <pre><code>aabb, abba, baba, abab an so on </code></pre> <p>You can uncomment some lines to only allow for atomic pairs, this is done after the firs two letters, to add the second pair accordingly. (see comments in code)</p> <pre><code>aabb, abab, baba, bbaa .... </code></pre> <p>Here goes some code:</p> <pre><code>#include &lt;vector&gt; using namespace std, // ..... string letters = "abcd", tmpStr; string wrkString; // string to hold sequence during creation &lt;vector&gt;string sequences; // vector to hold all valid sequences for(int first=0;i&lt;letters.length();first++){ // every char as first char wrkString = letters.at(first); for(int second=0;i&lt;letters.length();second++){ // every char as second char wrkString += letters.at(second); // got first 2 now differentiate if(first == second) // having aa letters.erase(first,1); // if same twice, erase from possible elements else{ // having ab tmpStr = letters; // if different only this two remain possible solutions letters = tmpStr.at(first) + tmpStr.at(second); // uncomment here to add this pair again in same order (ab ab) // wrkString += wrkString; // sequences.push_back(wrkString); // letters = ''; } for(int third=0;i&lt;letters.length();third++){ // having aa or ab or ba or bb or ac ... wrkString += letters.at(third); if(wrkString.find(letters.at(third))==1) // having aac letters = letters.at(third); // if included it has to be added once more else // having acc letters -= letters.at(third); // if included twice, remove from possible elements for(int fourth=0;i&lt;letters.length();fourth++){ // here allways 1 letter left wrkString += letters.at(fourth); sequences.push_back(wrkString); // adding copy of sequence to vector } } } letters = "abcd"; // reset possible elements for next iteration } </code></pre>
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    1. POHow do I print all the ways 4 different letters can combine in 2-letter pairs to fill four spots with each individual letter appearing exactly twice?
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