for i in 0...A.count-1 { <== this way will not work with strings
 if A[i] > 0 {ct += 1}
 if A[i] < 0 {ct -= 1}
}

for i in 0..<A.count { <== this way will not work with strings
 if A[i] > 0 {ct += 1}
 if A[i] < 0 {ct -= 1}
}

for i in A.indices { <== this way totally works with strings
 if A[i] > 0 {ct += 1}
 if A[i] < 0 {ct -= 1}
}