C++ cout won't work inside for and if? - c++

I have those two pieces of code as my home assignment. The code looks all fine to me, but it won't print out what I want, no matter what. In fact, the console output remains completely empty.
The first program is supposed to print out all numbers that fulfil the ladna() function requirements and are between 1 and a:
#include <iostream>
using namespace std;
int a;
int i = 1;
bool ladna(int a)
{
if((((a>>4)*5+a*2)%3)==1)
return true;
else
return false;
}
int main()
{
cerr << "Podaj liczbe: " << endl;
cin >> a;
while (i <= a){
if (ladna(a)){
cout << i << " ";
}
i++;
}
}
the ladna() function is premade and I have to use it as is.
I tried changing while into do...while and for, didn't help. Doesn;t work with cerr either.
The second code has to print out all the natural divisors of number a.
#include <iostream>
using namespace std;
int main()
{
int a;
cerr << "Podaj liczbe" << endl;
cin >> a;
for (int i = 0; i >= a; i++){
if (a % i == 0){
cout << i << endl;
}
}
return 0;
}
Doesn't work either.
To me it looks like both pieces of code have the same issue, because they are written in the same way, based on the same principle, and the error is the same. Hence my assumption, that the cause is the same as well.
Unfortunately, for the love of me, I simply can't see what said error is...

For the first code:
I think you should call ladna function with i, like ladna(i)
For the second code:
In for it should be i<=a
'%' is the modulo operator, during the execution of (a%i) you divide a with i and take the remainder, since i start with zero you will get "Floating point exception (core dumped)" due to division by zero. So, for should start with 1. This should work:
for (int i = 1; i <= a; i++){
if (a%i == 0){
cout << i << endl;
}
}

Related

Can I do in c++ something more than just 'x++', or 'x--'?

I made here a program to do 3x+1 math problem. So I am asking, if I could write in a c++ code something like x/2;, or x*3+1. These stuff what i put here are with mistakes. Then, is it possible in c++ to do that? If yes, how?
Here's the code:
#include <iostream>
using namespace std;
int main() {
cout << "Write an integer.\n"; int x; cin >> x;
// I made here a program to do 3x+1 math problem.
while (x==1) {
if ((x%2)==0) {
x/2; cout << x << ", ";
} else if ((x%2)==1) {
x*3+1; cout << x << ", ";
}
}
return 0;
}
The output there was:
/tmp/GjudkYOaE4.o
Write an integer.
9
But I was waiting it to write the number 28, 14, and more, but it did nothing.
I can see that you are new to coding, I would suggest you to please read a good amount of information about if-elseif-else and while loop that you are using
I can show you a few corrections here
#include <iostream>
using namespace std;
int main() {
cout << "Write an integer.\n";
int x; cin >> x;
// I made here a program to do 3x+1 math problem.
while (x!=1) { // x!=1 could be one of the terminating condition so when your x becomes 1, the loop would end. But of course the terminating condition depends on what you are trying to achieve
if (x % 2 == 0) {
x = x/2; //you need to assign the value back to x
cout << x << ", ";
} else { // you dont need else-if here because there could only be 2 possibilities for %2 operaion here
x = x*3+1;
cout << x << ", ";
}
}
return 0;
}
Give value to the same variable with assignment operator ('='):
x = x*3+1;
or
x = x/2;

User inputs N, find all pairs of a and b co-primes where a<N & b<N

#include <iostream>
using namespace std;
void gcd(){
int a,b,hcf;
for (int i = 1; i <= b; ++i) {
if (a % i == 0 && b % i ==0) {
hcf = i;
}
}
}
void pirm(){
int a,b,n,hcf;
for (int a=1;a<n;a++){
for (int b=1;b<n;b++){
gcd();
if (hcf==1) {
cout << a << " and " << b << endl;
}
}
}
}
int main(){
int n,a,b,i,hcf;
cout << "Enter a natural n, less than 100" << endl;
cin >> n;
if (n>=1 && n<100){
pirm();
}
else
cout << "You didn't enter a natural number" <<endl;
}
When I run it and enter a number, it doesn't do anything. The task is as follows:
User enters a natural N, that is less than 100. Find and output all pairs of co-primes that are < 100.
As you can tell by the code, I'm a complete newb at C++. Just wondering why the program "stops" or where I messed up the code in general. Any help is greatly appreciated.
There are a number of issues here.
First, the direct answer to your question: your program never prints because it never executes a print statement. If you enter a natural number, but hcf is never equal to 1, you will never print anything.
Now the question becomes why is hcf never 1? And that's where we start to run into more issues with your code. The main misunderstanding seems to be about scope. If you declare variables inside a function, they are within the function's "scope" -- the function can use them, and any scope inside the function can use them (e.g. loops and conditionals), but no outer scope can use them. You declare a, b, and hcfseveral times, but you only ever use them in gcd(). Not only is this unnecessary, but critically, these are different variables. You change the hcf in the scope of gcd(), but the hcf in pirm() is unchanged!
There are many ways to do what you're trying to do. The one that's closest to what you already have is to have gcd() take parameters by reference. If you change your declaration of gcd() to void gcd(int a, int b, int &hcf), and do not redeclare those variables in the first line of the function, then you are able to modify hcf in a way that will stick even when the function goes out of scope. You will then need to change your call on line 15 to gcd(a,b,hcf);.
Your algorithm could also be improved, though I believe it will still work. If you're interested in a more canonical way to find the gcd, try looking into Euclid's algorithm.
You should learn how to pass arguments to a function. Corrected your code for you, seems to be working:
#include <iostream>
void gcd(int &hcf, int a, int b)
{
for (int i = 1; i <= b; ++i)
{
if (a % i == 0 && b % i == 0)
{
hcf = i;
}
}
}
void pirm(int n)
{
int hcf;
for (int a = 1; a<n; a++)
{
for (int b = 1; b<n; b++)
{
gcd(hcf, a, b);
if (hcf == 1)
{
std::cout << a << " and " << b << std::endl;
}
}
}
}
int main()
{
int n;
std::cout << "Enter a natural n, less than 100" << std::endl;
std::cin >> n;
if (n >= 1 && n<100)
{
pirm(n);
}
else
std::cout << "You didn't enter a natural number" << std::endl;
system("pause");
return 0;
}

Finding prime numbers and then putting the prime numbers into an output file

So I have an assignment for class in which I have to ask the user for an integer in between 1 and 3000. Then my program should be able to tell if the integer is a prime number or not. Lastly, I would have to put that integer into a file, but only if it is a prime number. But my issue is my syntax, I am not sure if it's right(well actually obviously it's not because I keep getting errors). Is it possible to open a file in a function? and if so does it become a parameter?
I have been going through my textbook and googling as much as possible for some guidance but I'm still feeling lost. Any advice would help.
Edit: My logic as far as the numbers work, however when I add the code to write to a file, I'm now getting errors.
The two errors are
C2440 initializing: cannot convert from constant char to int (line 18)
C2079 myfile: uses undefined class'std::basic_fstream <>char,std::char_traits>'
Here's my code so far!
// Project 5.cpp : Defines the entry point for the console application.
//
#include <fstream>
#include "stdafx.h"
#include <iostream>
using namespace std;
//functions
void prime(int x);
//variables
int x=0;
int i;
char answer;
fstream myfile("castor_primes.txt");
int main()
{
do
{
cout << "Enter an integer between 1 and 3000 \n";
cin >> x;
if (x == 1)
{
cout << x << " is not a prime number.\n";
}
else if (x < 1 || x>3000)
{
cout << x << " is an invalid number. \n";
}
else
{
prime(x);
}
cout << "Do you want to enter another number? Y/N \n";
cin >> answer;
} while (answer == 'y' || answer == 'Y');
myfile.close();
return 0;
}
void prime(int x)
{
if (x == 2)
{
cout << "Yes, " << x << " is Prime\n";
}
else
{
for (i = 2; i < x; i++)
{
if (x%i == 0)
{
cout << x << " is not a prime number\n";
break;
}
}
if (x == i)
{
cout << "Yes, " << x << " is Prime\n";
myfile << x ;
}
}
}
#include "stdafx.h" includes a pre-compiled header file when using Microsoft Visual Studio. Check to see if that file exists and if it is correct (depending on the errors you see).
There is no reason to pass the outputFile into the prime() function since you are opening and closing it in the function. Although it works just fine if you do.
It is a parameter of the function call (if that is your question) and is a global variable since it is defined outside the main() function.
As some one else suggested the code functions if the aforementioned #include "stdafx.h" is removed, but I am not sure how that affects the compilation in Visual Studio.

Super basic code: Why is my loop not breaking?

for(int i=0;i<50;i++,size++)
{
cin >> inputnum[i];
cout << size;
if(inputnum[i] == '.')
{
break;
}
}
The break breaks the input stream but the size keeps outputting.
The output of size is 012345678910111213...474849.
I tried putting size++ inside the loop but it made no difference. And size afterwards will be equal to 50, which means it went through the full loop.
I forgot to explain that I added the cout << size within the loop to debug/check why it outputted to 50 after the loop even if I only inputted 3 numbers.
I suspect that inputnum is an array of int (or some other numeric type). When you try to input '.', nothing actually goes into inputnum[i] - the cin >> inputnum[i] expression actually fails and puts cin into a failed state.
So, inputnum[i] is not changed when inputting a '.' character, and the break never gets executed.
Here's an slightly modified version of your code in a small, complete program that demonstrates using !cin.good() to break out of the input loop:
#include <iostream>
#include <ostream>
using namespace std;
int main()
{
int inputnum[50];
int size = 0;
for(int i=0;i<50;i++,size++)
{
cin >> inputnum[i];
if (!cin.good()) {
break;
}
}
cout << "size is " << size << endl;
cout << "And the results are:" << endl;
for (int i = 0; i < size; ++i) {
cout << "inputnum[" << i << "] == " << inputnum[i] << endl;
}
return 0;
}
This program will collect input into the inputnum[] array until it hits EOF or an invalid input.
What is inputnum ? Make sure t's a char[]!! with clang++ this compiles and works perfectly:
#include <iostream>
int main() {
int size = 0;
char inputnum[60];
for(int i=0;i<50;i++,size++) {
std::cin >> inputnum[i];
std::cout << size;
if(inputnum[i] == '.') {
break;
}
}
return 0;
}
(in my case with the following output:)
a
0a
1s
2d
3f
4g
5.
6Argento:Desktop marinos$
Your code seams OK as long as you're testing char against char in your loop and not something else.. Could it be that inputnum is some integral value ? if so, then your test clause will always evaluate to false unless inputnum matches the numerical value '.' is implicitly casted to..
EDIT
Apparently you are indeed trying to put char in a int[]. Try the following:
#include <iostream>
int main() {
using namespace std;
int size = 0;
int inputnum[50];
char inputchar[50];
for(int i=0;i<50;i++,size++) {
cin >> inputchar[i];
inputnum[i] = static_cast<int>(inputchar[i]); // or inputnum[i] = (int)inputchar[i];
cout << size << endl; // add a new line in the end
if(inputchar[i] == '.') break;
}
return 0;
}
Then again this is probably a lab assignment, in a real program I'd never code like this. Tat would depend on the requirements but I'd rather prefer using STL containers and algorithms or stringstreams. And if forced to work at a lower-level C-style, I'd try to figure out to what number '.' translates to (simply by int a = '.'; cout << a;`) and put that number directly in the test clause. Such code however might be simple but is also BAD in my opinion, it's unsafe, implementation specific and not really C++.

Is there a way in C++ to only return the the last instance of a for loop?

Like for example:
#include <iostream>
using namespace std;
int main()
{
for (int n=10; n>0; n--){
cout<< n <<", ";}
}
This will output the numbers 10,9,8,7,6,5,4,3,2,1
So is there a new way so I just get the last instance of the loop, the 1?
I new at this and google isn't giving me any answers.
There is no direct way to detect whether the current iteration of a for loop is the last one. But if the behavior of the loop is predictable, you can usually write code that can detect when you're on the last iteration.
In this case, you could do something like:
if (n == 1) {
cout << n << "\n";
}
in the body of the loop. (Of course it would be simpler in this case to replace the entire loop with cout << "1\n";, but I presume this is an example of something more complex.)
In more complicated cases, you can save whatever information you need in the body of the loop:
int value_to_print:
for ( ... ) {
value_to_print = i;
}
std::cout << value_to_print << "\n";
On each iteration, value_to_print is replaced by the current value of i. The final value is the value of i on the last iteration.
You could create a variable (outside the loop) to hold the "current" value of n; whatever happens to the loop (exit condition reached, break, an exception is thrown...) the value will stay there:
int last_n;
for (int n=10; n>0; n--) {
last_n = n;
cout<< n <<", ";
if (something) {
break; // works in this case
} else if (something else) {
throw some_random_error; // works in this case too
}
}
cout << "The last value of 'n' was " << last_n << endl;
You can use a simple if statement for that.
int main()
{
for (int n=10; n>0; n--) {
cout << n << ", ";
if( n == 1 ) {
return n;
}
}
}
The simplest way to accomplish this is: -
#include <iostream>
using namespace std;
int main()
{
int x;
for (int n = 10; n > 0; n--){
x = n;
}
cout << x;
return 0;
}
I'm new to programming too and was trying to figure out something which will allow me to get the last instance of my loop as output.
I tried something and got the output, see if it can help you (if there's a mistake please let me know).
Here user input string is being replaced by "*" and instead of giving output of every instance i have made so only last instance is given as output.
#include <iostream>
#include <string>
using namespace std;
int main()
{
string str;
int string_length;//string length
cout<<"Enter your Email-ID: ";
cin>>str;
string_length = str.length(); //to give the length of input string and use it for the loop
cout<<"lentgh of the string: "<<string_length <<endl;
for(int x = 0; x <= string_length; x++){
str[x] = '*';
while(x==string_length) //string_length is the last instance of the loop
{
cout<<"Here's your Encrypted Email-ID: " <<str<<endl;
break;
}
}
return 0;
}