Having to input value twice to work C++ - c++

Question

#include <iostream>
#include <limits>
#include <math.h>
using namespace std;
int main()
{
float startTemperature;
float endTemperature;
float loopTemperature;
float stepSize;
float i;
float numberOne;
cout << "Please enter a start temperature: " << endl;;
cin >> startTemperature;
while(!(cin >> startTemperature)){
cin.clear();
cout << "Invalid input. Try again: ";
}
cout << "Please enter an end temperature: ";
cin >> endTemperature;
while(!(cin >> endTemperature)) {
cin.clear();
cin.ignore(256, '\n');
cout << "Invalid temperature. Please try again: ";
}
cout << "Please enter a step size: ";
cin >> stepSize;
while(!(cin >> stepSize)) {
cin.clear();
cin.ignore(256, '\n');
}
for(i = startTemperature; i < endTemperature; i += stepSize) {
if(i == startTemperature) {
cout << "Celsius" << endl;
cout << "-------" << endl;
cout << startTemperature << endl;
loopTemperature = startTemperature + stepSize;
}
loopTemperature += stepSize;
if(loopTemperature > 20) {
break;
}
cout << loopTemperature << endl;
}
}
Hi, The problem with this code is that I have to input the value of the temperature twice. I have looked at other answers and I think it is something to do with the cin buffer but I don't know exactly what is wrong.

Answer 1

In the line
cin >> startTemperature; // <---problem here
while(!(cin >> startTemperature)){
cin.clear();
cout << "Invalid input. Try again: ";
}
You are taking input once, then again in the loop. That's is why you had to give input twice.
Just remove first input line, same for endTemparature and stepSize.

Answer 2

You're asking for input before the while loop, then again in the loop condition statement. Change the condition in your while statement to
while(!cin){
//error handling you already have
cin>>startTemperature; //endTemperature respectively
}

Answer 3

It's not only for the temperature but rather for every input. Change your code to the one below:
cout << "Please enter a start temperature: " << endl;;
while (!(cin >> startTemperature)){
cin.clear();
cin.ignore(std::numeric_limits<int>::max(), '\n');
cout << "Invalid input. Try again: ";
}
cout << "Please enter an end temperature: ";
while (!(cin >> endTemperature)) {
cin.clear();
cin.ignore(std::numeric_limits<int>::max(), '\n');
cout << "Invalid temperature. Please try again: ";
}
cout << "Please enter a step size: ";
while (!(cin >> stepSize)) {
cin.clear();
cin.ignore(std::numeric_limits<int>::max(), '\n');
cout << "Invalid step size. Please try again: ";
}
Reason:
You had redundant cin calls. Also use std::cin.ignore(std::numeric_limits<int>::max(), '\n'); instead of arbitrary number 256.