Problems with std::cin and isdigit in c++ - c++

When I run the code and enters for instance 1 in the terminal it goes to the 'else' condition and breaks. But I'm providing it with a digit so I have trouble understanding why it behaves like that. Could someone help clarify?
int main()
{
vector<int> positions;
int p;
for(int i = 0; i <= 3; i++){
cout << "Enter a number: ";
cin >> p;
if(isdigit(p)){
positions.push_back(p);
} else
{
cout << "Please provide numbers from 0 to 100!" <<"\n";
break;
}
}
return 0;
}

the function is defined for the character, it would be true if you wrote isdigit('1').
Also isdigit(49) = true, because in ascii 49 is digit 1, so isdigit(49) = true;
check the reference of isdigit(): http://www.cplusplus.com/reference/cctype/isdigit/

Related

How can I throw a error if the user enters more than one integer

So this is my code:
#include <iostream>
#include <vector>
#include <string>
using namespace std;
long int iterFunc(int);
long int recurFunc(int);
int main() {
int n;
while(true){
try{
cout << "Enter: ";
if (!(cin >> n))
throw("Type Error");
if (n < 0)
throw n;
else
if (n == 0)
break;
cout << "Iterative: " << iterFunc(n) << endl;
cout << "Recursive: " << recurFunc(n) << endl;
}
catch(int n){
cout << "Error. Enter positive number." << endl;
}
catch(...){
cin.clear();
cin.ignore(100, '\n');
cout << "Error. Please enter a number" << endl;
}
}
cout << "Goodbye!";
return 0;
}
long int iterFunc(int n){
vector<long int> yVec = {1, 1, 1, 3, 5};
if (n <= 5)
return yVec[n - 1];
else
for(int i = 5;i < n; i++){
long int result = yVec[i - 1] + 3 * yVec[i- 5];
yVec.push_back(result);
}
return yVec.back();
}
long int recurFunc(int n){
switch (n) {
case 1:
case 2:
case 3:
return 1;
break;
case 4:
return 3;
break;
case 5:
return 5;
break;
default:
return recurFunc(n - 1) + 3 * recurFunc(n - 5);
break;
}
}`
The program shoud accept only one integer and return the y of the function using both iterative and recursive implemetations. Ex.: 30, 59, 433. How can I throw an error message if the user enters more then one integer, separated by space? Ex.: '3 45 32'.
I tried using if (cin.getline == ' ') throw("Error name") but the program still executes and return the y of the function for number in the input
Something like this works:
int main()
{
std::string str;
std::cout << "? : ";
std::getline(std::cin, str);
std::string::size_type pos(0);
int i = std::stoi(str, &pos);
if (pos != str.length())
return 1;
}
I found a part of my old code that might come in handy.
int val;
do
{
cin>>val;
if(!cin){ //you can add more conditions here
cin.clear();
cin.sync();
/* additional error handling */
}
else{
break; //input is correct - leaving loop
}
}while(true); //or here
Basically what !cin does is - it checks what type of value you actually want to write to, because it's needed anyway to figure out if data type is written to the correct type of our val. This means, that "30" or "433" etc. are integers (correct), "s" or "string" etc. are strings (or char*, correct me if I am wrong) (incorrect).
This also means, that "3 45 32" should be interpreted as string, which should result in another loop run.
Note: I didn't really test this code, so it might be completely wrong.
Edit: Okay now after some tests I realised this code needs some retweaking.
Firstly, "3 45 32" is not interpreted as string (now understandable). Instead, first number (before whitespace) is saved as an integer and all other numbers are stored in the buffer (next cin will be filled with it), which we can avoid using cin.clear() and cin.sync() once again.
The question is - is it okay for you to accept the first integer and ignore everything after the first whitespace? If not, you will have to save the input as string and extract whatever data you want from it.
I am leaving the original answer as is for simplicity of finding references in this edit.

Visual Studio shows warning C6330: 'char' passed as _Param_(1) when 'unsigned char' is required in call to 'isdigit'. when I try to build

This is only a small part from my code. What I'm trying to do is writing at the end of the file (add record) which in this case is "books.txt" that already has 40 records. But when I debug, it would still prompt the user to enter isbn code but after entering, (process 3296) exited with code 3. came out. Which part am I doing wrong? The counter() function is to count how many records I already have in my file. And I'm also using array of struct to store my records.
int add_record(DATA book[])
{
int count = counter();
system("CLS");
cout << "\t\t\t\t\t\t\t\t : :Add Book Record: :\n\n";
bool cont;
ofstream outfile("books.txt", ios::app);
if (outfile.is_open() && !outfile.eof())
{
do
{
cont = true;
cout << "ISBN Code: ";
cin.getline(book[++count].isbn_code, 14, '\n');
//cin.ignore(numeric_limits<streamsize>::max(), '\n');
int length = strlen(book[++count].isbn_code);
for (int i = 0; i <= length; i++)
{
if (!isdigit(book[++count].isbn_code[i]))
{
cont = false;
cout << "Your input is invalid. Enter again.\n";
break;
}
}
} while (cont == false);
do
{
cont = true;
cout << "Author: ";
cin.getline(book[++count].author, 50, '\n');
int length = strlen(book[++count].author);
for (int i = 0; i <= length; i++)
{
if (isdigit(book[++count].author[i]))
{
cont = false;
cout << "Your input is invalid. Enter again.\n";
break;
}
}
} while (cont == false);
outfile << book[++count].isbn_code << "," << book[++count].author ;
outfile.close();
}
else
cout << "File is not open\n";
return 0;
}
Yes, the error message is completely correct. This is a rare case where using a cast is the correct thing to do
if (isdigit(static_cast<unsigned char>(book[++count].author[i])))
Reference, https://en.cppreference.com/w/cpp/string/byte/isdigit
But this has nothing to do with your crash which is caused by other errors. For instance
cin.getline(book[++count].isbn_code, 14, '\n');
//cin.ignore(numeric_limits<streamsize>::max(), '\n');
int length = strlen(book[++count].isbn_code);
You definitely don't want to increment count twice. I would guess the correct code is
cin.getline(book[count].isbn_code, 14, '\n');
int length = strlen(book[count].isbn_code);
and to increment count once later in your loop.
Remember ++count is not the same as count + 1. The first increments the count variable, that is it changes the value of the count variable, but count + 1 just adds one to count and does not change the value of the count variable.
This is also wrong
for (int i = 0; i <= length; i++)
In C++ string indexes start at zero and go upto the length of the string minus one, so the correct code is
for (int i = 0; i < length; i++)
Also not part of your question but X can be a legal character in an ISBN.

Checking for letters when reading an integer

I'm making an app that requires the user to input a production order (7 digits long) like this:
int order = 0;
cout << "Insert the order number: ";
cin >> ordem;
How can I prevent the user from entering a letter? Like "I2345G789"?
Doing that, my app just enters an infinite loop. I was thinking to use a function like this:
bool isLetter(int a)
{
string s = to_string(a);
for (int i = 0; i < s.size()-1; i++)
{
if (isdigit(s[i]))
{
return false;
}
else
return true;
}
}
And then:
if (isLetter(order))
{
cout << "Insert only numbers \n";
}
But it doesn't work. Why? And how can I improve the code?
PS: I'm very new to programming, so, sorry for any beginner mistakes.
I guess you have a loop around your code in order to ask for the order number again in case it contains non-digits, for example:
while(...)
{
int order = 0;
cout << "Insert the order number: ";
cin >> order;
}
If you enter something that cannot be parsed into an integer, then the input stream will go into failure mode and that might be the reason why you end up in an infinite loop. In order to overcome your problem in a simple way, you could read a string instead:
string order;
while (true)
{
cout << "Insert the order number: ";
cin >> order;
if (isLetter(order))
cout << "Insert only numbers" << endl;
else
break;
}
The function isLetter() now takes a string and looks like this:
bool isLetter(string s)
{
// Return true if the given string contains at least one letter.
for (size_t i = 0; i < s.size(); i++)
if (!isdigit(s[i]))
return true;
// Return false if there are only digits in the given string.
return false;
}
Please note, that it should be i < s.size() and not i < s.size()-1. And maybe you should rename your function isLetter() to hasLetter(), because that would be a bit more correct.

C++ Excluding non-integer user inputs

I am making a number-guessing game where the user is asked to input a four-digit number. It is possible, however, that the user inputs less or more than four digits and/or a non-integer input (i.e. invalid input). My code stores the user input into an integer-type array. I just realized now that my code will still recognize "invalid inputs" as valid since the array where the input is being stored is declared as an integer-type. Below is a portion of my code:
#include <iostream>
using namespace std;
void guess(int num_guess[], int size);
int main(){
int list[4];
guess(list, 4);
for(int i = 0; i < 4; i++){
cout << list[i];
}
cout << endl;
}
void guess(int num_guess[], int size){
int number;
cin >> number;
for(int i = size-1; i >= 0; i--){
num_guess[i] = number%10;
number /= 10;
}
}
cout << list[i]; isn't really part of the original code, but this was how I found out that invalid inputs are still accepted. I encountered a similar problem before when I was making a rational roots calculator program in Python, but it was much easier then to detect and exclude unwanted inputs. My question is, how do I fix my code so that it can detect invalid inputs and output something like "Invalid input" and then proceed to ask the user for another input.
The following is a function to check if a string is a 4 digit positive integer. If the number could be negative, you just need to check if the s[0] == '-'.
bool check(string &s){
if(s.size() != 4) return false;
for(int i=0; i < 4; i++){
if(s[i] < '0' || s[i] > '9') return false;
}
return true;
}
The following is a function to convert a string to an int:
#include <stringstream>
int strToInt(string &s){
stringstream ss(s);
int ans;
ss >> ans;
return ans;
}
To exclude non integer inputs try the following:
void skip_to_int(){
// if is not an integer
if(cin.fail()){
// check character type
cin.clear();
char ch;
while(cin>>ch){
// throw away non digits
if(isdigit(ch)){
// put back if digit to be written
cin.unget();
return;}
}
}
else error ("no input");
}
And your input prompt function will look like this:
cout << "Please enter an integer" << endl;
int n=0;
if(cin>>n){
// integer OK, proceed
}
else{
cout << "That was not a numeric value, try again." << endl;
skip_to_int();}
Here's my solution. Beware, it uses C++11. Certainly not necessary if you use std::stringstream, but this should work pretty well.
I presume you don't want negative numbers. I also presume that any number of 0's in front doesn't make the number a 4-digit number. It will cut off padded 0's, so 01234 is a 4 digit number, but 0123 isn't.
void guess(int num_guess[], int size)
{
int number;
// if the length of the number isn't 4, try again until it is
do {
std::cin >> number;
if(std::to_string(number).length() != size)
std::cout << "You messed up the input. How hard could it be? Try again..." << std::endl;
} while(std::to_string(number).length() != size);
// by now, the size is definitely 4. insert it by digit into num_guess
for(int i = size-1; i >= 0; i++) {
num_guess[i] = number%10;
number /= 10;
}
}
#include <iostream>
#include <limits>
int main() {
int i = 0;
std::cout << "Please enter a number with four digits: ";
while( !(std::cin >> i) || !(i / 1000.0f >= 1.0f) )
{
std::cin.clear();
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
std::cout << "Invalid entry." << std::endl;
std::cout << "Please enter a number with four digits: ";
}
}
the std::cin.clear() clears all errors flags on current stream structure and std::cin.ignore() cleans up the input stream itself. Once we don't know the size of stream 'til this operation I have used the maximum possible value of a stream size to make sure any stream length could be cleaned.
add #include "math.h"
and change guess
void guess(int num_guess[], int size){
int number = 0;
bool firstTime = true;
do
{
if (!firstTime)
cout << " Error, try again " << endl;
firstTime = false;
cin >> number;
} while (number<pow(10, size-1) || number>=pow(10, size));
for(int i = size-1; i >= 0; i--){
num_guess[i] = number%10;
number /= 10;
}
}

How can I check if the user has entered valid value?

I'm writing a project and I have to check if the user has entered a valid value for specific things. For example, first case is to check whether he inputted an integer, which is no longer than 6 length and >0. I am working with struct, so my code is this:
#include <iostream>
using namespace std;
int dolzina_int(int vlez);
int main()
{
struct pole{
int sifra; // sifra na artiklot
string opis; // opis na artiklot
float cena; // edinecna cena
int vlez_kol; // vlezna kolicina
int izlez_kol; // izlezna kolicina
float dan_stapka; // danocna stapka
float iznos; // iznos
int datum; // datum na vlez i izlez (GGMMDD)
}artikli[100]; // maksimalen broj na artikli e 100
for (int i = 0; i < 5; i++){
cin >> artikli[i].sifra;
while(!(cin >> artikli[i].sifra) ||
(artikli[i].sifra < 0 || (dolzina_int(artikli[i].sifra) > 6)))
{
cout << "Error" << endl;
cin.clear();
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}
}
return 0;
}
// funkcija za dolzina na integer
int dolzina_int(int vlez){
int dolzina = 1;
while(vlez > 0){
dolzina++;
vlez /= 10;
}
return dolzina;
}
So, my code seems to work, the first part, it does check if it's >6 or <0, but the second else if, it doesn't work properly to check if it's an integer or not. So my question is the following, how can i make sure that it's an integer value that the user enters and how can I make sure the length of integer is not >6 or the value is <0 and meanwhile, if it is, to make him re-enter it, and also check the newly entered value, as my code doesn't do that currently.
for (int i = 0; i < 5; i++){
artikli[i].sifra = 1;
do{
cout << "Enter a code: ";
while(!(cin >> artikli[i].sifra)){
cin.clear();
cin.ignore();
cout << "Invalid entry. Enter a new value: ";
}
}while(artikli[i].sifra > 0 && dolzina_int(artikli[i].sifra) > 6);
}
You should note that the extraction will only succeed if the contents read from the file can be used as a correct value of the type of the object (in this case the integer sifra). If it doesn't work, ios_base::failbit will be set in the stream state.
So what needs to be done is that when an incorrect value is given, we discard the wrong text, clear the stream state, and re-try the extaction. For example:
#include <iostream>
#include <string>
#include <limits>
int main()
{
for (int i = 0; i < 5; ++i)
{
while (!(std::cin >> artikli[i].sifra) ||
((artikli[i].sifra < 0) || (artikli[i].sifra > 6)))
{
std::cout << "Entered an incorrect value, try again.\n";
std::cin.clear();
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}
// ...
}
}
First you should check this out:
http://www.cplusplus.com/reference/cctype/
Second, I think your over complicating it...
1) Read in int
2) check value of int
3) repeat
So I would do this
for (i = 0 to i < 5 add one to i)
while (entered value is not digit)
get digit until value is valid
Something like this:
for(i = 0; i < 5; i++) {
cout << "Enter the code" << endl;
cin >> artikli[i].sifra;
while(!is_digit) {
cout << "Invalid, please repeat" << endl;
cin >> artikli[i].sifra;
}
}
Assuming you want to restrict your input to integers and have them in a specific [min, max] range, you would do it like this:
int input = -1;
do
{
std::cout << "Enter a value in the range [min, max]: ";
while (!(std::cin >> input)) // while the input is invalid
{
std::cin.clear(); // clear the fail bit
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); // ignore the invalid entry
std::cout << "Invalid Entry! Please Enter a valid value: ";
}
} while (min > input || max < input);
Where min and max are whatever values you are looking to keep the value between.