Why is my function endlessly looping - c++

Question

#include <string>
#include <iostream>
#include <iomanip>
using namespace std;
void add(string names[], int section[], int grade[]);
void printRecords(const string names[], const int sections[], const int grades[], int size);
void insertionSort(int numbers[], int size);
void display(const int numbers[], int size);
int main(){
//command
string command;
//add
string names[20] = { "" };
int sections[20];
int grades[20];
cout << "Enter a command (add, print, drop, sort, average, quit):\n";
cin >> command;
while (command != "quit"){
if (command == "add"){
add(names,sections, grades);
}
else if (command == "print"){
printRecords(names, sections, grades, 20);
}
else if (command == "drop"){
cout << command;
}
else if (command == "sort"){
cout << command;
}
else if (command == "average"){
cout << command;
}
else if (command == "smallest"){
cout << command;
}
else{
cout << "Invalid command.\n";
}
cin >> command;
}
cout << "Quitting the program.\n";
return 0;
}
void printRecords(const string names[], const int sections[], const int grades[], int size) {
int i;
for (i = 0; i < size; i = i + 1) {
cout << "[" << i << "]:\t";
cout << setw(20) << left << names[i];
cout << setw(5) << sections[i];
cout << setw(10) << grades[i];
cout << "\n";
}
return;
}
void insertionSort(int numbers[], int size) {
int i;
int j;
int toInsert;
// outer loop i is the element we are to insert in to the sorted portion.
// [0, i-1] is the sorted range.
for (i = 1; i < size; i = i + 1) {
toInsert = numbers[i];
j = i - 1;
while (j >= 0 && numbers[j] > toInsert) {
numbers[j + 1] = numbers[j];
j = j - 1;
}
// either j >= 0 and numbers[j] <= toInsert
// or j = -1 here
// we want to store toInsert at numbers[j+1]
numbers[j + 1] = toInsert;
}
return;
}
void add(string names[], int sections[], int grades[]){
int i = 0;
while (names[i] != "" && i < 20){
i = i + 1;
}
cin >> names[i];
cin >> sections[i];
cin >> grades[i];
return;
}
I only need the user to enter a new set of information only once when the function is called.
The add function is what i care about the most. Why is it endlessly lopping?
it is suppose add one student’s record (name, section and grade) after the last record in the 
current spreadsheet. Your spreadsheet should be able to hold up to 20 student records. If 
there are already 20 student records, your code should prompt “Spreadsheet is full. Can 
not add.”

Answer 1

Change cin >> command into getline(cin, command).

Answer 2

Here, this should fix it. What's going on is exactly what Chnossos suspects. When you cin >> the input buffer is not cleared of the newline character unless you call cin.ignore() to ignore it. However if there is no newline character you will end up waiting for it, making you enter extra input to get there.
Now, getline(cin, var) consumes the newline character, so you don't need to ignore after using this function or else you would also be waiting for more data.
I didn't look over it too terribly hard, but I coded this and it is working, thus I believe that you were just getting too many newlines and not consuming enough, thus it seemed like an infinite loop.
There are more eloquent ways to do this, but the basic gist is for strings to use getline() and for other types you can continue to use cin >> if you use cin.ignore() afterwards.
I would suggest you look this up more, also. Here is the code.
#include <string>
#include <iostream>
#include <iomanip>
using namespace std;
void add(string names[], int section[], int grade[]);
void printRecords(const string names[], const int sections[], const int grades[], int size);
void insertionSort(int numbers[], int size);
void display(const int numbers[], int size);
int main(){
//command
string command;
//add
string names[20] = { "" };
int sections[20];
int grades[20];
cout << "Enter a command (add, print, drop, sort, average, quit):\n";
getline(cin, command);
while (command != "quit"){
if (command == "add"){
add(names,sections, grades);
}
else if (command == "print"){
printRecords(names, sections, grades, 20);
}
else if (command == "drop"){
cout << command;
}
else if (command == "sort"){
cout << command;
}
else if (command == "average"){
cout << command;
}
else if (command == "smallest"){
cout << command;
}
else{
cout << "Invalid command.\n";
}
getline(cin, command);
}
cout << "Quitting the program.\n";
return 0;
}
void printRecords(const string names[], const int sections[], const int grades[], int size) {
int i;
for (i = 0; i < size; i = i + 1) {
cout << "[" << i << "]:\t";
cout << setw(20) << left << names[i];
cout << setw(5) << sections[i];
cout << setw(10) << grades[i];
cout << "\n";
}
return;
}
void insertionSort(int numbers[], int size) {
int i;
int j;
int toInsert;
// outer loop i is the element we are to insert in to the sorted portion.
// [0, i-1] is the sorted range.
for (i = 1; i < size; i = i + 1) {
toInsert = numbers[i];
j = i - 1;
while (j >= 0 && numbers[j] > toInsert) {
numbers[j + 1] = numbers[j];
j = j - 1;
}
// either j >= 0 and numbers[j] <= toInsert
// or j = -1 here
// we want to store toInsert at numbers[j+1]
numbers[j + 1] = toInsert;
}
return;
}
void add(string names[], int sections[], int grades[]){
int i = 0;
while (names[i] != "" && i < 20){
i = i + 1;
}
getline(cin, names[i]);
cin >> sections[i];
cin.ignore();
cin >> grades[i];
cin.ignore();
return;
}

Answer 3

The problem is that if you attempt to read into an integer, e.g. cin >> sections[i];, but you typed characters that aren't digits, then this operation fails, and cin is put into a fail state.
After that, cin remains in a fail state until you cancel that state by doing cin.clear(). This operation does not remove the problem characters from the stream either, so you usually want to combine the clear() with another read operation to read and discard the problem characters.
If you only ever type integers when your program expects integers, then your code will not "endlessly loop". Obviously you have typed in words when it was expecting integers though.
To fix this there are two things to do. First of all, always test your input functions for failure. After doing cin >> command;, also do:
if ( !cin )
{
break; // exit instead of infinitely looping
}
But it is good to detect the failure state as soon as it occurred. Inside the add function, after the last cin, you should do:
if ( !cin )
{
cout << "Invalid input entered - discarding entry\n";
names[i] = "";
cin.clear();
string junk;
getline(cin, junk);
}
(There are other ways to ignore junk but I think this one is easy to understand).
Finally, the add function uses formatted extraction (i.e. >>), so you cannot tell if the person presses Enter or not. If you want to make it so that them pressing Enter always means they've finished inputting the current entry (and invalid entry is to be discarded), then you will need to use std::getline to store the whole line in a string, and then use a stringstream to do formatted extraction from that line.
For example, the add function could be:
void add(string names[], int sections[], int grades[])
{
int i = 0;
while (names[i] != "" && i < 20){
i = i + 1;
if ( i == 20 )
return;
string line;
getline(cin, line);
istringstream iss(line);
iss >> names[i] >> sections[i] >> grades[i];
if ( !iss )
{
names[i] = "";
cerr << "Invalid entry ignored.\n";
}
}
You may need #include <sstream> for this. Note that in this version you don't need to discard the "junk", as the junk is in the stringstream which is being destroyed anyway.
As a rule of thumb, if you are mixing string and integer input, then using getline plus a stringstream will make your program more user-friendly.