This is the first C++ program I have ever written and I'm having trouble understanding the order in which operands must be put in. This is for a class, but it looks like I'm not supposed to use the homework tag. Sorry if I'm doing this wrong. This is my input // Get DNA string string st; cout << "Enter the DNA sequence to be analysed: "; cin >> st; This seems to work ok, but I thought I would include it incase this is what I'm doing wrong. This is what I have so far to check that the input is exclusively C,T,A, or G. It runs through the program and simply prints "Please enter a valid sequnce1, please enter a valid sequence2, ... ect. I'm sure I'm doing something very stupid, I just can't figure it out. // Check that the sequence is all C, T, A, G while (i <= st.size()){ if (st[i] != 'c' && st[i] != 'C' && st[i] != 'g' && st[i] != 'G' && st[i] != 't' && st[i] != 'T' && st[i] != 'a' && st[i] != 'A'); cout << "Please enter a valid sequence" << i++; else if (st[i] == c,C,G,t,T,a,A) i++; The second half of my program is to count the number of Cs and Gs in the sequence for (i < st.size() ; i++ ;); for (loop <= st.size() ; loop++;) if (st[loop] == 'c') { count_c++; } else if (st[loop] == C) { count_c++; } else if (st[loop] == g) { count_g++; } else if (st[loop] == G); { count_g++; } cout << "Number of instances of C = " << count_c; cout << "Number of instances of G = " << count_g; It seems like it's not looping, it will count 1 of one of the letters. How do I make it loop? I can't seem to put in endl; anywhere without getting an error back, although I know I'll need it somewhere. Any help or tips to point me in the right direction would be greatly appreciated - I've been working on this code for two days (this is embarrassing to admit).