why does my program skip steps using getline()? - c++

Question

I think my program skips steps because I use getline() inside While and For loops without using cin.clear() and cin.ignore(). If I'm right, where will I have to insert them?
I tried to write the code with cin.clear() and cin.ignore(10000, '\n') after each getline() (really I don't know how these functions work properly, I found them surfing on Google) and the code doesn't work correctly.
The code is :
main.cpp
#include <iostream>
#include <list>
#include <string>
#include <vector>
#include <sstream>
#include <iostream>
#include <utility>
using namespace std;
int main(){
string S;
cout<<"insert test....input exaple: test 1"<<endl;
getline(cin, S);
while(S!="-1"){
cout<<"START WHILE"<<endl;
int nB = 0; //number of Lego Bricks;
cout<<"insert number of Lego bricks"<<endl;
cin>>nB;
for(int i=0; i<nB; i++){
cout<<"START FOR"<<endl;
cout<<"insert Lego brick (the number of faces must be even)....input example: NameBrick -> face1 face2 face3 face4...."<<endl;
getline(cin, S);
}
getline(cin, S);
}
return 0;
}

Answer 1

cout<<"insert number of Lego bricks"<<endl;
cin>>nB;
This is the problem. getLine() keeps reading until the next newline character. Let us say that '|' is our newline symbol.
When you write something into the terminal or when you are reading from file you have many lines to work through.
Example input(Greeting, name, age and hobby):
Hello.|
Magnus Elden|
24|
Tennis|
GetLine() will give you the entire line. The first call to getLine() will return the string "Hello." and stop since it reached a newline character, '|'.
The important part is that it stops AFTER the newline character.
Thus, the next time the first character it reads is the 'M' in "Magnus Elden".
When you get to the age part of the input, you use cin which only read the first item, be it a string or a number. As such it will stop BEFORE the newline character. The next time you call getLine() it reads until a newline character comes, but since the newline character still remains in the buffer getLine() returns immediately with "" as its return value.
That is why it seems to skip a step.
Step by step breakdown:
You input your greeting "Hello." and the buffer will then look like this:
Buffer:Hello.|
Let us say that the start of the reading is denoted by a ^. These are the steps.
Step 1.
Input:Hello.
Buffer:Hello.|
^
Function call:getLine()
Return value:"Hello."
Buffer:
Step 2.
Input:Magnus Elden
Buffer:Magnus Elden.|
^
Function call: getLine()
Return value:"Magnus Elden"
Buffer:
Step 3.
Input:24
Buffer:24|
^
Function call: cin
Return value:24
Buffer:|
Step 4.
Input:Tennis.
Buffer:|Tennis.|
^
Function call: getLine() //Remember that it runs until the first newline '|'.
Return value:""
Buffer:Tennis|
Just change the code bit to:
cout<<"insert number of Lego bricks"<<endl;
getLine(cin, s);
nB = atoi(s.c_str());
I hope this helps.

Answer 2

getline(cin,s) will do what you expect. The rest of the line (except the newline itself) is stored in s. And, crucially, the newline itself will be extracted and discarded.
So, if you call getline(cin,s) multiple times, each line will be read once, as you expect.
Similarly, assuming x in an int, cin >> x will read an integer. Multiple consecutive calls to cin >> x will read numbers from the input into x. Each time you call cin>>x it will skip over any whitespace before the number, then read the number. So, if you have a set of numbers, perhaps on the same line separated by spaces, or perhaps on different lines separated by newlines, then cin>>x will read them for you.
But it will not read-and-discard any whitespace after the number. Whitespace is discarded at the start of each call to cin>>x (i.e. before the number itself is read), but not after the number is read.
The problem occurs if you have a cin>>x followed by a getline. Imagine you type a number and then press enter. cin>>x will consume the number. but cin>>x will not consume the newline. Then, the getline will attempt to read the rest of the line. It will not try to read the next line. We're are still stuck on the same line as the number. You probably hit Enter immediately after entering the number, therefore the "rest of the line" is just an empty string.
There's a difference between "typing a number" and "typing a number followed by hitting the Enter key"

Answer 3

Thanks to all.
I solved replacing cin>> with getline how many of you suggested.
precisely:
cout<<"insert number of Lego bricks"<<endl;
cin>>nB;
with
cout<<"insert number of Lego bricks"<<endl;
getLine(cin, s);
nB = atoi(s.c_str());