Forgive me for my noobishness as I have only just begun learning C++, but I came across something that's confusing me. The following code is me following along the examples from the C++ Primer 5th Edition PDF found here.
#include<iostream>
int main(){
int sum = 0, value = 0;
while (std::cin >> value)
sum += value;
std::cout << "Sum is: " << sum << std::endl; //this doesn't do anything
std::cin.ignore().get();
return 0;
}
Now for some reason, the last std::cout statement doesn't do anything at all, and it's perplexing me as aside from the std::cin.ignore().get() this is the exact same as the example in the pdf and yet this doesn't do anything. What confuses me the most is that if I write value from inside the while loop you can observe that it doesn't recur infinitely; it stops like it should after the last input value, so the error shouldn't be in the execution of the while loop.
Any help is appreciated, thanks!
EDIT: I'm using Visual Studio 2013 Ultimate
EDIT2: The input I'm using is 3 4 5 6
EDIT3: For comparison's sake, here is the code from C++ Primer 5th Edition:
#include <iostream>
int main()
{
int sum = 0, value = 0;
// read until end-of-file, calculating a running total of all values read
while (std::cin >> value)
sum += value; // equivalent to sum = sum + value
std::cout << "Sum is: " << sum << std::endl; return 0;
}
So the ONLY difference between mine and this is the std::cin.ignore().get().
while (std::cin >> value) means it will not stop until failed to read an integer, i.e encountering an EOF or a non-integer input.
On Windows, use ^Z (CtrlZ) to enter an EOF.
Try input:
1 2 3 4
^Z
It should hit the cout line. And it's totally a different question about the cin.ignore() stuff.
Related
The exercise: Write a program that reads several transactions and counts how many transactions occur for each ISBN.
The answer I found on Github:
#include <iostream>
#include "include/Sales_item.h"
int main()
{
Sales_item currItem, valItem;
if (std::cin >> currItem)
{
int cnt = 1;
while (std::cin >> valItem)
{
if (valItem.isbn() == currItem.isbn())
{
++cnt;
}
else
{
std::cout << currItem << " occurs " << cnt << " times " << std::endl;
currItem = valItem;
cnt = 1;
}
}
std::cout << currItem << " occurs "<< cnt << " times " << std::endl;
}
return 0;
}
For example if the input ISBN is like this:
A A B A B C
The output would be:
A occurs 2 times
B occurs 1 times
A occurs 1 times
B occurs 1 times
C occurs 1 times
Which isn't quiet right because they're not properly grouped. The excercise didn't specify that records for each ISBN should be grouped together. (Though it's specified in the next exercise.)
I haven't found a right solution online yet. I've been thinking about this excercise for a while and still don't have much clue on how to solve it.
I've only learned about while, for, if so far. This seemingly easy exercise is kind of a challenge for me. Hope you guys can give me, a beginner some clue about it. Thanks in advance!
use a std::map<char, int> chcounts; where the key is the character.
You can then do
chcounts[ch]++;
this takes advantage of the fact that if you reference a map element using [] and the entry does not exist it will get created and default initialized
or if you have strings do
std::map<std::string, int> counts;
....
counts[isbn]++;
I am learning C++ using the book C++ Primer.
In Section 1.4.3, the following example code about reading the unknown number of inputs is given.
#include <iostream>
int main()
{
int sum = 0, value = 0;
// read until end-of-file, calculating a running total of all values read
while (std::cin >> value)
sum += value; // equivalent to sum = sum + value
std::cout << "Sum is: " << sum << std::endl;
return 0;
}
According to the book, if we give an input of 3 4 5 6, the output will be Sum is: 18
But when I try this on my computer(Windows 10 with MinGW), The code does not end. It just keeps on asking for input even if I enter a newline. It works only when I enter a non-int input like f.
Is this expected? If yes, is there any code that stops after inputting a newline?
I am quite new to c++ and I have already learned python, so getting stuck so early on is quite frustrating.
Thanks and regards.
You need to terminate your input by an End-Of-File-character (i.e. CTRL-Z on Windows, CTRL-D on Mac/Unix), not just by an End-Of-Line (i.e. Enter).
A simple Enter is interpreted as white space, which will be simply ignored by operator>> when reading into an integral data type.
CTRL-Z / End-Of-File, in contrast, makes any operator>> fail with an error.
See also this SO answer.
Note: Entering f will also terminate your loop, since f is not considered a valid integral number; Hence, std::cin >> value with value being of type int and an input like f will fail as well. To be more accurate: operator>> actually returns a reference to the input stream, but if reading in a value fails, failbit is set on the stream, and then interpreting the stream object in a boolean expression (implicitly calling basic_istream::operator bool()) returns false; So maybe the author of the book did not want to explain these details at the respective section in the book :-)
Is this expected?
Yes, as operator>> ignores leading whitespace by default, which includes line breaks.
If yes, is there any code that stops after inputting a newline?
Use std::cin.getline() or std::getline() instead of operator>>. You can then use operator>> with a std::istringstream to parse values from each line, eg:
#include <iostream>
#include <string>
#include <sstream>
int main()
{
std::string line;
int sum, value;
do
{
std::cout << "Enter a set of numbers, or a blank line to exit: ";
if (!std::getline(std::cin, line) || line.empty())
break;
// read until end-of-line, calculating a running total of all values read
std::istringstream iss(line);
sum = 0;
while (iss >> value)
sum += value; // equivalent to sum = sum + value
std::cout << "Sum is: " << sum << std::endl;
}
while (true);
return 0;
}
Live Demo
Is this expected?
Yes, Thats what while (std::cin >> value) does. See this answer for more explanations: How is "std::cin>>value" evaluated in a while loop?
is there any code that stops after inputting a newline?
No, as >> simply ignore a white space(also #StephanLechner mentioned it)
What you can do instead is:
Just give a condition; if it satisfies just break the loop. you
can also provide a console out to make it more understandable to the
user. For example:
std::cout << "Enter value to sum or press -1 to exit" << std::endl;
while (std::cin >> value && value != -1) // if value == -1, loop stops.
{
sum += value;
}
You can simply terminate by the end of character:
CTRL-Z on Windows
CTRL-D on Mac/Unix
Im trying to write a code in c++ that takes an unknown number of numbers and adds them all together. I do not get any errors but when i input numbers it wont do anything.
#include <iostream>
int main()
{
int sum = 0, val;
while (std::cin >> val)
sum += val;
std::cout << "Sum is: " << sum << std::endl;
return 0;
}
My guess is you never ended your input. It will continue looping and adding numbers until you trigger end of file. Just pressing enter won't do that - you need to hit ctrl+d on Linux or ctrl+z on Windows to end the standard input file, allowing the while loop to exit.
(I apologize that this is so low level compared to most of the questions I have seen on this website, but I have run out of ideas and I do not know who else to ask.)
I am working on a school project that requires me to read basketball statistics from a file named in06.txt. The file in06.txt looks exactly as follows:
5
P 17 24 9 31 28
R 4 5 1 10 7
A 9 2 3 6 8
S 3 4 0 5 4
I am required to read and store the first number, 5, into a variable called "games." From there, I must read the numbers from the second line and determine the high, the low, and the average. I must do the same thing for lines 3, 4, and 5. (FYI, the letters P, R, A, and S are there to indicate "Points," "Rebounds," "Assists," and "Steals.")
Since I only have been learning about programming for a few weeks, I do not want to overwhelm myself by jumping right into dealing with every aspect of the project. So, I am first working on determining the average from each line. My plan is to keep a running total of each line and then divide the running total by the number of games, which is 5 in this case.
This is my code:
#include <iostream>
#include <fstream>
#include <cstdlib>
using namespace std;
int main()
{
int games;
int points_high, points_low, points_total;
int rebounds_high, rebounds_low, rebounds_total;
int assists_high, assists_low, assists_total;
int steals_high, steals_low, steals_total;
double points_average, rebounds_average, assists_average, steals_average;
ifstream fin;
ofstream fout;
fin.open("in06.txt");
if( fin.fail() ) {
cout << "\nInput file opening failed.\n";
exit(1);
}
else
cout << "\nInput file was read successfully.\n";
int tempint1, tempint2, tempint3, tempint4;
char tempchar;
fin >> games;
fin.get(tempchar); // Takes the endl; from the text file.
fin.get(tempchar); // Takes the character P from the text file.
while( fin >> tempint1 ) {
points_total += tempint1;
}
fin.get(tempchar); // Takes the endl; from the text file.
fin.get(tempchar); // Takes the character R from the text file.
while( fin >> tempint2 ) {
rebounds_total += tempint2;
}
fin.get(tempchar); // Takes the endl; from the text file.
fin.get(tempchar); // Takes the character A from the text file.
while( fin >> tempint3 ) {
assists_total += tempint3;
}
fin.get(tempchar); // Takes the endl; from the text file.
fin.get(tempchar); // Takes the character S from the text file.
while( fin >> tempint4 ) {
steals_total += tempint4;
}
cout << "The total number of games is " << games << endl;
cout << "The value of total points is " << points_total << endl;
cout << "The value of total rebounds is " << rebounds_total << endl;
cout << "The value of total assists is " << assists_total << endl;
cout << "The value of total steals is " << steals_total << endl;
return 0;
}
And this is the (incorrect) output:
Input file was read successfully.
The total number of games is 5
The value of total points is 111
The value of total rebounds is 134522076
The value of total assists is 134515888
The value of total steals is 673677934
I have been reading about file input in my textbook for hours, hoping that I will find something that will indicate why my program is outputting the incorrect values. However, I have found nothing. I have also researched similar problems on this forum as well as other forums, but the solutions use methods that I have not yet learned about and thus, my teacher would not allow them in my project code. Some of the methods I saw were arrays and the getline function. We have not yet learned about either.
Note: My teacher does not want us to store every integer from the input file. He wants us to open the file a single time and store the number of games, and then use loops and if statements for determining the high, average, and low numbers from each line.
If anyone could help me out, I would GREATLY appreciate it!
Thanks!
You have all these variables declared:
int games;
int points_high, points_low, points_total;
int rebounds_high, rebounds_low, rebounds_total;
int assists_high, assists_low, assists_total;
int steals_high, steals_low, steals_total;
double points_average, rebounds_average, assists_average, steals_average;
And then you increment them:
points_total += tempint1;
Those variables were never initialzed to a known value (0), so they have garbage in them. You need to initialize them.
Besides what OldProgrammer said, you've approached the reading of integers incorrectly. A loop like this
while( fin >> tempint2 ) {
rebounds_total += tempint2;
}
will stop when an error occurs. That is, either it reaches EOF or the extraction encounters data that cannot be formatted as an integer - or in other words, good() returns false. It does not, as you seem to think, stop reading at the end of a line. Once an error flag is set, all further extractions will fail until you clear the flags. In your case, a loop starts reading after P, extracts five intergers, but then it encounters the R from the next line and errors out.
Change this to a loop that reads a fixed number of integers or alternatively, read a whole line using std::getline into a std::string, put it into a std::stringstream and read from there.
In any case, learn to write robust code. Check for success of extractions and count how many elements you get.
An example of a loop that reads at most 5 integers:
int i;
int counter = 0;
while (counter < 5 && file >> i) {
++counter;
// do something with i
}
if (counter < 5) {
// hm, got less than 5 ints...
}
This question already has answers here:
Infinite loop with cin when typing string while a number is expected
(4 answers)
Closed 3 years ago.
I'm learning C++ and writing little programs as I go along. The following is one such program:
// This program is intended to take any integer and convert to the
// corresponding signed char.
#include <iostream>
int main()
{
signed char sch = 0;
int n = 0;
while(true){
std::cin >> n;
sch = n;
std::cout << n << " --> " << sch << std::endl;
}
}
When I run this program and keep inputs at reasonably small absolute values, it behaves as expected. But when I enter larger inputs, e.g., 10000000000, the program repetitively spits out the same output. Some combinations of input cause erratic behavior. For example:
#: ./int2ch
10
10 -->
10000000000
10 -->
10 -->
10 -->
10 -->
The program spits out "10 --> " until it's killed. (With this particular sequence of inputs, the program's output changes speed erratically.) I also noticed that the output of large values is determined by the previous legal input as well as the value of the current illegal input.
What's going on? (I don't care about fixing the program, that's easy. I want to understand it.)
Basically your cin stream is in a fail state and thus returns immediately when you try to read it. Rewrite your example like this:
#include <iostream>
int main()
{
signed char sch = 0;
int n = 0;
while(std::cin >> n){
sch = n;
std::cout << n << " --> " << sch << std::endl;
}
}
cin >> n will return a reference to cin, which you can test for "good-ness" in a conditional. So basically the the "while(std::cin >> n)" is saying "while i could still read from standard input successfully, do the following"
EDIT: the reason it repeatedly output the last good value entered is because that was the last value successfully read in n, the failed reads won't change the value of n
EDIT: as noted in a comment, you can clear the error state and try again something like this would probably work and just ignore bad numbers:
#include <iostream>
#include <climits>
int main() {
signed char sch = 0;
int n = 0;
while(true) {
if(std::cin >> n) {
sch = n;
std::cout << n << " --> " << sch << std::endl;
} else {
std::cin.clear(); // clear error state
std::cin.ignore(INT_MAX, '\n'); // ignore this line we couldn't read it
}
}
}
Yes, Evan Teran pointed out most things already. One thing i want to add (since i cannot comment his comment yet :)) is that you must put the call to istream::clear before the call to istream::ignore. The reason is that istream::ignore likewise will just refuse to do anything if the stream is still in the fail state.
Given that you are on a 32 bit machine, 10000000000 is too big a number to be represented by an int. Also converting an int to a char will only give you from 0..255 or -128..127 depending on the compiler.
One problem here is that a char has a size of one byte, and thus can only hold a number between -127 and 128. An int on the other hand, is typically 4 bytes, and can take on much larger values. Second problem is that you are inputting a value that is too large even for an int.