I'm new to C++ and I'm trying some exercises. The first one I wanted to do already got me in trouble. The goal of the exercise is to reverse the string you input. This is easy. But when in try to limit the amount of characters with cin.get(array,int) it removes the first character.
Code:
char voornaam[7];
cin >> voornaam;
cin.get(voornaam,7);
cout << voornaam[6] << voornaam[5] << voornaam[4] << voornaam[3] << voornaam[2] << voornaam[1] << voornaam[0] << endl;
This is the code. So this should normally work but when I try it for example with Sander, it outputs 'redna' and then terminates.
Any thoughts on how to fix this. I would like a solution with cin.get(array,int) and not with an other function of cin.
Thanks.
Well, this is funny. At first instance everything is fine. But with one minor thing. Notice
cin >> voornaam
followed by
cin.get(voornaam,7)
What happens here is, when first prompt comes, you enter Sander.
Hence, voornaam[0] = 'S' , voornaam[1] = 'a' and so on. And you press enter. cin.get(voornaam, 7) takes this as \0. So, your voornaam looks something like
voornaam[0] = '\0'
voornaam[1] = 'a'
voornaam[2] = 'n'
and so on.
And when you reverse it, you get "redna". So, do not use, cin twice, or enter full string twice to get the correct result.
Related
Been working on a simple school project but I've run into a small problem in regards to validation of user input via the use of a while loop. The following code is used to do so:
cout << "How much woud you like to bet?[100, 300, 500]" << endl;
cin >> int_input;
if (cin.fail()){
int_input = 0;
}
//Validate check
while(int_input!=100 && int_input!=300 && int_input!=500){
cout << "Please enter a valid bet [100, 300, 500]" << endl;
cin >> int_input;
if (cin.fail()){
int_input = 0;
}
}
The issue arises when a non-integer is entered and the loop ends up spitting out the cout msg over and over again, skipping the request for new input. I assume that a non-integer always returns true for the while loop, but the program should still ask for user input, right?
The cin.fail is a recent addition and might not be necessary as it doesn't solve the problem, although I leave the code as it as it is.
If you have an error on the stream, you set your value to 0. So the loop is run again... and the error's still on the stream. Repeat.
You need to clear the error flag.
cin.clear();
Then extract enough "bad" data to get to the good part, or you'll just be right back where you started.
I am trying to get the user to input some data and then storing it in a structure, however I am having troubles knowing which function I should use and what's the difference? cin or getline()? Either function I use, it seems like it takes in the '\n' key and makes my program crash, but I am not 100% if that's the problem... Since it keeps crashing.
I've played around with both of them and here is what I have.
string temp;
int id;
cout << endl << "Full name (last, first): ";
cin >> temp;
cin.ignore(1, '\n');
myData[fileSize] = parseName(temp);
cout << endl << "ID: ";
cin >> id;
myData[fileSize].id = id;
cout << endl << "Address: ";
cin >> temp;
temp.copy(myData[fileSize].address, temp.length(), 0);
The variable fileSize is just which element the array is currently at and the function parseName splits the name into last and first.
I been reading on a couple of functions like, cin.ignore() and noskipws, but not sure how to use them. By the way, the way the user should input the data is "last, first", with a comma and white space after (this is what the parsing function is looking for).
Also I am not sure if the address section is the best way to do this, I have the structure myData.address to be a character array, because I don't know how to work with strings. I am still not confident with C++. Thanks for any help.
EDIT: If I comment out the ID and Address parts, the program loops itself 6 times saying I have an invalid entry (which is part of main), so it reads 6 or 7 keys after I press enter.
If I leave everything the way it is, this is what I get.
Full name (last, first): terminate called after throwing an instance of 'std::ou
t_of_range'
what(): basic_string::copy
This application has requested the Runtime to terminate it in an unusual way.
Please contact the application's support team for more information.
Process returned 3 (0x3) execution time : 4.328 s
Press any key to continue.
You should use cin.getline() instead for this case, and the cin.ignore is not necessary.
Here is an examination of the two methods - std::cin.getline( ) vs. std::cin
Also, check your parseName function and try testing it in isolation without any user I/O.
cout << "Would you like to make another transaction? (y/n)" << endl;
cin >> repeat_transaction;
static_cast<char>(repeat_transaction);
while (repeat_transaction != 'y' && repeat_transaction != 'n')
{
cout << "Invalid selection: Please enter y or n";
cin >> repeat_transaction;
static_cast<char>(repeat_transaction);
}
During the Invalid selection loop, I once accidentally pressed "mn". I noticed the console read out Invalid selection..., So, it did in fact finish and re-enter the while loop. However, after this the console terminated the program. If you enter a single character 'a' or 'y' or 'n' it acts just as it should. Ending or not ending. This was before I attempted to use static_cast to force the truncation of the user input.
Since you managed to get this program to compile I can only assume that repeat_transaction was specified as a char and not a std::string.
When you use cin to get a character it only gets one character but it doesn't flush the buffer. I believe you understand this issue since you wrote This was before I attempted to use static_cast to force the truncation of the user input. . You can attempt to use cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); instead of static_cast<char>(repeat_transaction); after each call to cin >> repeat_transaction; . There are downsides to this. If you enter 'mn' it will work as expected. It reads the m which is not y or n and then flushes the extra characters until it finds end of line \n. If you do nm, n will match and the m will be thrown away. So in that case it will accept nm as valid and exit the loop.
There are other ways that may be easier and give you the effect closer to what you are looking for. Instead of reading a character at a time you can read an entire line into a string using getline (See the C++ documentation for more information). You can then check if the length of the string is not equal to 1 character. If it's not length 1 then it is invalid input. If it is 1 then you want to check for y and n. Although basic (and not overly complex) this code would do a reasonable job:
#include <iostream>
#include <string>
using namespace std;
int main()
{
string repeat_transaction;
cout << "Would you like to make another transaction? (y/n)" << endl;
getline(cin, repeat_transaction);
while (repeat_transaction.length() != 1 || (repeat_transaction != "y" && repeat_transaction != "n"))
{
cout << "Invalid selection: Please enter y or n";
getline(cin, repeat_transaction);
}
return 0;
}
I said reasonable job since one deficiency you might see is that you want to trim white spaces from the beginning and end. If someone enters n or y with a space or tab in front it will be seen as invalid (whitespace at the end would be similar). This may not be an issue for you, but I thought I would mention it.
On a final note, you may have noticed I used using namespace std;. I did so to match what was in the original question. However, this is normally considered bad practice and should be avoided. These StackOverflow answers try to explain the issues. It is better to not do it and prepend all standard library references with std::. For example string would be std::string, cin would be std::cin etc.
I have a simple little script I am coding, and I am trying to not allow people to enter a string, or if they do make it revert to the beginning of the function again. Here's the input code I have:
int main()
{
cout << "Input your first number" << endl;
cin >> a;
cout << "Input your second number" << endl;
cin >> b;
}
The rest of the code beyond this part works just fine for what's going on, although if a string is entered here it obviously doesn't work.
Any help would be appreciated.
You may find this post useful,
How to check if input is numeric in C++
Basically you can check the input, whether it is numeric value or not. After checking whether the given input is numbers, then you can add a while loop in main to ask user to repeat if input is not a valid number.
Every input is a string. If you want to know if a entered string can convert to a number, you have to read in a string and try to convert it yourself (eg with strol).
An alternative would be to check if the reading from cin failed, but personally i don't like it because cin.fail() covers more error situations than just a failed type conversion.
There's a library function may help,you can check it after input:
int isdigit(char c);
Tips:
1.You should include such files :
# include <ctype.h>
2.If c in 0 ~ 9 ,return 1 ; else return 0.
I am new to C++ and am in a class. I am trying to finish the first project and so far I have everything working correctly, however, I need the user to input a number to select their level, and would like to validate that it is a number, and that the number isn't too large.
while(levelChoose > 10 || isalpha(levelChoose))
{
cout << "That is not a valid level" << endl;
cout << "Choose another level:";
cin >> levelChoose;
}
That is the loop I made, and it sometimes works. If I type in 11 it prints the error, and lets me choose another level. However if the number is large, or is any alpha character it floods the screen with the couts, and the loop won't end, and I have to force exit. Why does it sometimes stop at the cin and wait for user input, and sometimes not? Thanks for the help!
This is an annoying problem with cin (and istreams in general). cin is type safe so if you give it the wrong type it will fail. As you said a really large number or non-number input it gets stuck in an infinite loop. This is because those are incompatible with whatever type levelChoose may be. cin fails but the buffer is still filled with what you typed so cin keeps trying to read it. You end up in an infinite loop.
To fix this, you need to clear the fail bit and ignore all the characters in the buffer. The code below should do this (although I haven't tested it):
while(levelChoose > 10 || isalpha(levelChoose))
{
cout << "That is not a valid level" << endl;
cout << "Choose another level:";
if(!(cin >> levelChoose))
{
cin.clear();
cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}
}
Edit: numeric_limits<> is located in the limits include:
#include<limits>
From your description, it seems likely (nearly certain) that levelChose is some sort of numeric type, probably an integer.
When you use operator>> to read a number, anything that couldn't be part of a number (e.g., most letters) will be left in the input buffer. What's happening is that you're trying to read the number, it's failing and leaving the non-digit in the buffer, printing out an error message, then trying to read exactly the same non-digit from the buffer again.
Generally, when an input like this fails, you want to do something like ignoring everything in the input buffer up to the next new-line.
levelChoose appears to be an integer type of some form (int, long, whatever).
It's not valid to input a character into an integer directly like that. The input fails, but leaves the character in the incoming buffer, so it's still there when the loop comes around again.
Here's a related question: Good input validation loop using cin - C++
I suspect the part while(levelChoose > 10..... This does not restrict level to less than 10 (assuming greater than 10 is a large number in your context). Instead it probably should be while(levelChoose < 10...
To check that an expression is not too large, the following could be a possibility to validate (brain compiled code!!)
const unsigned int MAX = 1000;
unsigned int x;
cin >> x;
while(x < MAX){}