Hi guys i am new to c++, I just wrote this code to find min/max of a array of numbers.
I just want to know how can I make the no. of entries flexible(I mean the user should be able to enter as many entries as possible without specifying how many in the starting)
Here's the code, but its not working, can someone please help?
Thanks
code:
#include <iostream>
using namespace std;
int main(){
cout<<"Program to calculate max/min/second max\n";
int *A;
A=new int[5];
bool flag=false;
int x=0,i=0;
cout<<"Enter the numbers\n";
do{
cin>>x;
if(x=='\0'){
flag=true;
}
*(A+i)=x;
i++;
}
while(!flag);
for(int j=0;j<i;j++){
cout<<*(A+j)<<"\n";
}
return 0;
}
You are confused that pressing enter will just give you the null terminator. It will not. Depending on your platform it will give you a carriage return/line-feed (\r\n in Win, \n in *ix). The best way to do this is to just have them use a letter like 'q' for quit or a number like -1, and then compare on that.
Dynamic Memory Allocation is a bit of a tricky subject for a beginning programmer (in C and C++ in any case.) The easiest way is to have the user specify how many entries, but you don't want this.
Otherwise using the vector class over an array is probably a better (and easier to grapple with than directly using pointers.)
http://www.cplusplus.com/reference/stl/vector/ is a good place to start. Look at the syntax for creating the vector and the push_back() member function to accomplish your task.
Good Luck,
SegFaults McGee
You will have to use some sort of variable length datastructure and Vector is the best choice for this since you are working in C++. Therefore, instead of your fixed length array:
int *A;
A=new int[5];
use a vector like this:
std::vector<int> input;
And then to add values to this, use the following:
input.push_back(10);
There is an example on this page about using vectors.
mean the user should be able to enter as many entries as possible without specifying how many in the starting
With the above requirement and with the code you have, you can enter no more than 5 elements to an array.
do{
cin>>x;
if(x=='\0'){
flag=true;
}
*(A+i)=x;
i++;
}while(!flag);
Use std::vector instead for the requirement where it implicitly manages memory for you.
If you use
std::vector<int> a;
then the input becomes simply
while (std::cin >> x)
a.push_back(x);
Then the user can press ^D (Unix/Linux/etc) or ^Z (DOS/Win) when they've entered all the numbers, or use the program as in:
echo 1 4 22 | program
program < input_file
If you want to have an empty line denote the end of input, then with input validation:
std::string line;
while (getline(std::cin, line))
{
char c;
std::istringstream iss(line);
int x;
if (iss >> x)
{
a.push_back(x);
char c;
if (iss >> c)
{
std::cerr << "unexpected character '" << c << "' in line '" << line << "', terminating\n";
exit(EXIT_FAILURE);
}
}
else if (!iss.eof())
break; // empty line...
else
{
std::cerr << "unexpected characters in line '" << line << "', terminating\n";
exit(EXIT_FAILURE);
}
}
Related
I am trying to write a code in C++ reading a text file contains a series of numerics. For example, I have this .txt file which contains the following series of numbers mixed with a character:
1 2 3 a 5
I am trying to make the code capable of recognizing numerics and characters, such as the 4th entry above (which is a character), and then report error.
What I am doing is like
double value;
while(in) {
in >> value;
if(!isdigit(value)) {
cout << "Has non-numeric entry!" << endl;
break;
}
else
// some codes for storing the entry
}
However, the isdigit function doesn't work for text file. It seems when I am doing in >> value, the code will implicitly type-cast a into double.
Can anyone give me some suggestion?
Thanks a lot!
Your while loop doesn't do what you think it does.
It only iterates one statement:
in >> value;
The rest of the statements are actually outside the loop.
Using curly braces for the while body is always recommended
I created a small mini script where I would be reading in a file through a standard fstream library object as I was a little unsure on what your "in" represented.
Essentially, try to read in every element as a character and check the digit function. If you're reading in elements that are not of just length 1, a few modifications would have to be made. Let me know if that's the case and I'll try to help!
int main() {
std::fstream fin("detect_char.txt");
char x;
while (fin >> x) {
if (!isdigit(x)) {
std::cout << "found non-int value = " << x << '\n';
}
}
std::cout << '\n';
return 0;
}
Try reading the tokens into string and explicitly parsing it
ifstream infile("data.txt");
string token;
while (infile >> token) {
try {
double num = stod(token);
cout << num << endl;
}
catch (invalid_argument e) {
cerr << "Has non-numeric entry!" << endl;
}
}
Since it looks like the Asker's end goal is to have a double value for their own nefarious purposes and not simply detect the presence of garbage among the numbers, what the heck. Let's read a double.
double value;
while (in) // loop until failed even after the error handling case
{
if (in >> value) // read a double.
{
std::cout << value; // printing for now. Store as you see fit
}
else // failed to read a double
{
in.clear(); // clear error
std::string junk;
in >> junk; // easiest way I know of to read up to any whitepsace.
// It's kinda gross if the discard is long and the string resizes
}
}
Caveat:
What this can't handle is stuff like 3.14A. This will be read as 3.14 and stop, returning the 3.14 and leave the A for the next read where it will fail to parse and then be consumed and discarded by in >> junk; Catching that efficiently is a bit trickier and covered by William Lee's answer. If the exception handling of stod is deemed to expensive, use strtod and test that the end parameter reached the end of the string and no range errors were generated. See the example in the linked strtod documentation
Just a heads up: My c++ programming skills and terminology is intermediate at best. So please be gentle ;).
I am working on a multi-sort algorithm for a college class. Originally, I built the program to take in an array of 20 integers, since that was as big as the .txt files were. The final lab is now asking to take in files that have 10, 100, 1000, 10000, 100000 and 1000000 different numbers. I originally used an ifstream inside a for loop to read in the ints. Now that I need to read a variable amount of ints from a file, I have run into issues with this code. I have extensively searched this site and Google to find an answer to this problem. I have tried dozens of different code snippets, to no avail. Here is the code I am currently running that works for 20 ints.
int i;
int A[20];
int length;
char unsortedFilename[200];
ifstream unsorted;
cout << "Please type the full name of the file you would like sorted.\n* ";
cin >> unsortedFilename;
unsorted.open(unsortedFilename);
length = (sizeof(A) / sizeof(*A));
for( i = 0; i < length; i++ )
{
unsorted >> A[i];
cout << A[i] << "\n";
}
insertionSort();
I do have other code mixed in there, but it's error checking, selection of duplicate number removal, etc. I would like it so that code like this would run "i" number of times, where "i" is actually the number of ints in the file. Also, as I mentioned earlier, I will need to input a file that has 1,000,000 numbers in it. I don't believe that an int array will be able to hold that many numbers. Is it going to be as easy as swapping all my ints over to longs?
Thanks for any help you could provide.
As suggested in the comments, use std::vector<int> instead of an array.
Instead of a for loop, use a while loop. Break out of the while loop when there are no numbers to read.
The while loop:
std::vector<int> A;
int item;
while ( unsorted >> item )
{
A.push_back(item);
}
You can sort the std::vector by using std::vector::iterator or simply access the data through the int* returned by A.data().
You can simply read all the numbers into a vector. Then use the vector as you would have used the array.
#include <iostream>
#include <fstream>
#include <vector>
#include <string>
int main()
{
std::string unsortedFilename;
std::cout << "Please type the full name of the file you would like sorted.\n";
std::cin >> unsortedFilename;
std::ifstream is(unsortedFilename);
std::istream_iterator<int> start(is), end;
std::vector<int> A(start, end);
std::cout << "Read " << A.size() << " numbers" << std::endl;
}
What you want is a vector.
try this,
int i;
vector<int> A;
int length;
string unsortedFilename;
ifstream unsorted;
cout << "Please type the full name of the file you would like sorted.\n* ";
cin >> unsortedFilename;
unsorted.open(unsortedFilename);
int temp;
for( i = 0; unsorted >> temp; i++ )
{
A.push_back(temp);
cout << A[i] << "\n";
}
insertionSort();
A vector is basically a dynamic array. It automatically grows as more space is needed. That way it doesn't matter if you have 10, 100, or even 100000 items, it'll automatically grow for you.
Also use a string for your file name, some file names are longer than 200 characters.
Good Luck!
will need to input a file that has 1,000,000 numbers in it. I don't believe that an int array will be able to hold that many numbers.
Sure it can. 1 Million ints is ~4Mb of memory, which is a trivial amount. You can even declare it static just as you do now int A[1000000];.
But real problem is that you're assuming a fixed length in your code, rather than determine the length from the input. I guess this is what your assignment is trying to teach you, so I won't show you the solution. But consider using ifstream::eof and make your sort accept the length as an argument...
The first line contains an integer n (1 ≤ n ≤ 100). Each of the following n lines contains one word. All the words consist of lowercase Latin letters and possess the lengths of from 1 to 100 characters.
(Source: http://codeforces.com/problemset/problem/71/A)
How would you get input from the user given n? I tried using a while loop but it doesn't work:
#include <iostream>
using namespace std;
int main()
{
int n;
cin>>n;
int i;
while (i<=n) {
cin>>i ;
i++;
}
}
You probably meant to have something like:
#include <iostream>
int main() {
int n;
cin>>n;
int theInputNumbers[n];
for(int i = 0; i<n; ++i) {
cin >> theInputNumbers[i];
}
}
Your loop is really quite far off of what you need. What you wrote is extremely wrong such that I cannot provide advice other than to learn the basics of loops, variables, and input. The assistance you need is beyond the scope of a simple question/answer, you should consider buying a book and working through it cover to cover. Consider reading Programming Principles and Practice Using C++
Here is a working example of something approximating your question's requirements. I leave file input and output as an exercise up to you. I also make use of C++11's front and back std::string members. You would have to access via array index in older versions.
#include <iostream>
#include <string>
#include <sstream>
using namespace std;
int main(){
int totalWords;
cin >> totalWords;
stringstream finalOutput;
for (int i = 0; i < totalWords; ++i){
string word;
cin >> word;
if (word.length() > 10){
finalOutput << word.front() << (word.length() - 2) << word.back();
}else{
finalOutput << word;
}
finalOutput << endl;
}
cout << endl << "_____________" << endl << "Output:" << endl;
cout << finalOutput.str() << endl;
}
With that said, let me give you some advice:
Name your variables meaningfully. "int i" in a for loop like I have above is a common idiom, the "i" stands for index. But typically you want to avoid using i for anything else. Instead of n, call it totalWords or something similar.
Also, ensure all variables are initialized before accessing them. When you first enter your while loop i has no defined value. This means it could contain anything, and, indeed, your program could do anything as it is undefined behavior.
And as an aside: Why are you reading into an integer i in your example? Why are you then incrementing it? What is the purpose of that? If you read in input from the user, they could type 0, then you increment by 1 setting it to 1... The next iteration maybe they'll type -1 and you'll increment it by 1 and set it to 0... Then they could type in 10001451 and you increment by 1 and set it to 10001452... Do you see the problem with the logic here?
It seems like you are trying to use i as a counter for the total number of iterations. If you are doing this, do not also read input into i from the user. That completely undermines the purpose. Use a separate variable as in my example.
i am experiencing much trouble reading in files from input into an array struct. here is the code if someone can tell me what im doing wrong i can figure it out. the loop is supposed to be reading 2 strings, and 1 int, and skipping possible blank lines. but when i run it, it reads the first set and doesnt read nothing after that.
struct Instruments
{
string model;
string maker;
int year;
};
int main()
{
int size;
Instruments data[20];
int i =0;
ifstream fin;
fin.open("input.txt");
for (i=0; i<20; i++)
{
do{
getline(fin, data[size].model);
getline (fin, data[size].maker);
fin >> data[size].year;
size++;
}
while (data[size].model.length() > 0);
}
fin.close();
for(int i=0;i<size; i++)
{
cout << data[i].model << "model"<<endl;
cout << data[i].maker << "maker" << endl;
cout << data[i].year<< " year" << endl;
}
return 0;
}
There are multiple issues here:
Your first 'for' loop is using i as the loop counter but size as the array index.
After this call:
fin >> data[size].year
it will read to the end of the number and any whitespace that follows will form part of your next read, so if you are expecting to start the next record at the next line, do a blank getline() here too.
Aside from that.
Use vectors not arrays
Have a method to read from a stream into your struct, and if that succeeds, use push_back() to add it to your vector.
That doesn't necessarily mean you have to loop until the read fails, it may be that you know in advance how many you wish to read. But you should still do it this way.
size variable is not initialized. In C++, variables are not automatically initialized.
You must add:
int size = 0;
This is just a guess. In addition to the missing initialization of size, the following:
do{
.....
}
while (data[size].model.length() > 0);
looks also quite suspect to me: as soon as data[size].model has some content (which it does after the first read, this will evaluate to true and you probably have an infinite loop.
If you craft the for loop correctly, you don't need the do-while loop.
This is what I have to do:
A teacher has asked all her students to line up single file according to their first name. For example, in one class Amy will be at the front of the line and Yolanda will be at the end. Write a program that prompts the user to enter the number of students in the class, then loops to read in that many names. Once all the names have been read in it reports which student wourld be at the front of the line and which one would be at the end of the line. You may assume that no two students have the same name. Input Validation: Do not accept a number less than 1 or greater than 25 for the number of students.
This is what I have so far:
#include <iostream>
#include <iomanip>
#include <string>
using namespace std;
int main()
{
int StudentNum;
cout << "How many student are in the class?\n";
cin >> StudentNum;
char sname[StudentNum + 1][25];
if (StudentNum < 1 || StudentNum > 25)
{
cout << "Please enter a number between 1-25 and try again\n";
return 0;
}
for (int i = 1; i <= StudentNum; i++);
{
cout << "Please enter the name of student #" << i << endl;
cin >> sname[i];
}
for (int output = 0; output <=StudentNum; output++);
{
cout << endl << sname[output] << endl;
}
system ("pause");
return 0;
}
Am I missing something about arrays??
You cannot create such an array because its length has to be known at compile time (i.e., it cannot be the result of an expression such as StudentNum + 1).
You can solve this issue because by the problem definition you know an upper bound for the array size, so you can use that as a compile time constant.
However, this problem can be solved without using an array at all. Read the wording carefully.
Hint for the solution without arrays: Think of the array as a single piece of paper (variable) with all the names written one after another. Not using an array then means that you have to be able to solve the problem without looking at all the names at once. How would you come to the answer if I only allowed you to see the names one by one?
Another hint: The problem is still solvable if there were several trillion students in the class (with unique names no less), i.e. more than could possibly fit in the computer's memory at any one time.
C++ array dimensions must be known at compile time (ie not dependent on user-entered variables at run-time). Use strings instead:
string sname[25];
If you were using something besides char arrays, you could also use a vector.
Think about what the problem statement is actually asking for. Your program only needs to output the first and last names alphabetically. Do you actually need to store all the names to do that?
Just for fun, here's how I would do it. Don't turn this in unless are ready to explain to your teacher how it works.
struct MinMax {
std::string min;
std::string max;
MinMax& operator+(const std::string& kid) {
if( min.empty() || kid < min) min = kid;
if( max.empty() || kid > max) max = kid;
return *this;
}
};
int main() {
int nKids;
std::cout << "How many students? " << std::flush;
std::cin >> nKids;
std::cout << "Enter students' names, followed by EOF\n";
MinMax mm(std::accumulate(
std::istream_iterator<std::string>(std::cin),
std::istream_iterator<std::string>(),
MinMax()));
std::cout << mm.min << ", " << mm.max << "\n";
}