Hello fellow members,
it's me again with the silly questions.

I was give a scenario to write a program for and I finished it (I think).
Although is pretty simple (first program) I had a hard time trying to assemble it.
It’s exactly as the scenario I was given ( validations, clear screen, return, etc.) apart from the page titles, headers and little visual display (for what I know), which is just something I had fun doing it.

If you run the program you are presented with two choices. Either one you choose after that you are asked to enter a number. If by mistake you enter anything else than a number the program crashes. Apart from that it works.

I was wondering maybe you know there is a way I can validate the cin choice.
I hope is not to confusing to read (man, I lost myself a few times in it).

Thanks anyway

//---------------------------------------------------------------------
//					T H E  C A L C U L A T O R
//---------------------------------------------------------------------


#include <iostream.h>
#include <iomanip.h>	// contains the declarations for the data manipulators.
#include <stdlib.h>		// system("cls"); used for clearing the screen. 


void HD1();             // program front main page heading 
int ProcessLoop();		// processing loop.

//---------------- MULTIPLICATION FUNCTION PROTOTYPE ---------------------
void HD_M();			// multiplication table heading
int Process_M();		// ask user for input
float Calculation_M();	// compute and display result

//---------------- CALCULATION FUNCTION PROTOTYPE -------------------------
void Process_C();		// Calculation Loop
void Menu();            // Displays menu options.
float input();          // Ask user to enter data.
float Add(), Sub();     // Calculation functions. 
float Mul(), Div();
int Mod();              // Modulos function.
void LoopBack();		// return control to function.

//---------------- FRONT MAIN PAGE TITLE --------------------------------------

char PT1[] ="\n\t\t ----------------------------------------------"
			"\n\t\t T H E  G U I L D S  P R I M A R Y  S C H O O L"
			"\n\t\t ----------------------------------------------";
char PT2[] ="\n\n\n\t\t          T H E   C A L C U L A T O R        \n\n";

//---------------- CALCULATION PAGE TITLE ----------------------------------

char HD_C1[] = "\n\t\t\tC A L C U L A T I O N\n\n";
char HD_C2[] = "\t\t  -->  Only Numbers between 0 and 1000  <--\n\n\n";

//----------------- GLOBAL VARIABLES ---------------------------------------

char choice1;						// Multiplication Table
float row, column, product;			// variables.


float firstNum, secondNum, result ; // Calculator  
char choice;                        // variables.  

//----------------- MAIN CONTROL ------------------------------------------- 

int main(){
	ProcessLoop();
	return 0;
}

//----------------- PROCESSING LOOP -----------------------------------------

int ProcessLoop(){
	cout << PT1 << PT2 ;
	HD1();
	
	while (choice1 !='C' && choice1 !='M'){
		cout << "\a\n\t\tERROR...Enter (M) or (C) only: ";
		cin >> choice1;
	}
		if (choice1 == 'M'){
		Process_M();
		}
		else if (choice1 == 'C'){
		Process_C();
		}
		return 0;
}

// ---------------- PRINT PROGRAM FRONT PAGE HEADING ---------------------------------- 

void HD1(){
	cout << "\n\n\t\tTo Create a Multiplication Table --> Enter (M)"
		 <<	"\n\n\n\t\t\tTo Perform a Calculation --> Enter (C)\n"
		 << "\n\n\n\t\tEnter M or C only: ";
	cin >> choice1;
}

//------------------------------------------------------------------------------
//							MULTIPLICATION PROCESS					
//------------------------------------------------------------------------------


// ---------------- PRINT MULTIPLICATION TABLE FRONT PAGE ---------------------- 

void HD_M(){
	int row, column;
	cout << "\n                  M U L T I P L I C A T I O N   T A B L E\n\n";
	
	cout << "        | 1 2 3 4 5 6 7 8 9 10 11 12  13  14  15  16  17  18  19  20\n";
	cout << "        |                                                           \n";
	cout << "      ===============================================================\n";
	for (row = 1; row <=15; row++){
		cout << setw(7)<< row <<" |" ;

		for (column = 1; column <= 20; column++){
			cout<<"__|";
			}
			cout << "\n";
	}
}

//----------------- INPUT DATA --------------------------------------------------

int Process_M(){
	system("cls");
	HD_M();
	cout << "\n\tEnter Table Number reqired: ";
	cin >> row ;
	while (row < 0 || row > 20){
		cout << "\a\tERROR...Enter Table Number 1 ---> 20: ";
		cin >> row;
	}
	system("cls"); 
	HD_M();
	cout << "\n\tEnter Lines required 1 --> 15: ";
	cin >> column;
	while (column < 0 || column > 15){
		cout << "\a\tERROR...Enter Lines Number 1 ---> 15: ";
		cin >> column;
	}
	Calculation_M();
	return 0;	
}

//----------------- CALCULATE TABLE -------------------------------------------

float Calculation_M(){
	system("cls");
	char exit;			// Local 
	int i;				// variables.
	
	cout << "\n\t\t\t MULTIPLICATION TABLE OF " << row <<"\n";
	cout << "\t\t\t===========================\n\n";
	for (i = 1; i <= column; i++){
		cout <<"\t\t\t  "<< setw(2) << i <<" __|" << "_____________ ";
		product = row * i;
		cout << "" << setw(3) << setprecision(6) << product <<"\n\t\t\t       |\n";
		}
		cout <<"\n\n\t\tType \'E\' to Exit or Any other Key to continue...";
		cin >> exit;
		if (exit != 'E' && exit != 'e'){
			system("cls");
			Process_M();
		}
		else{
			system("cls");
			ProcessLoop();
		}
		return 0;
}

//-------------------------------------------------------------------------------
//						CALCULATION PROCESS
//-------------------------------------------------------------------------------


//---------------- CALCULATION LOOP -------------------------------------------

void Process_C(){
	system("cls");
	Menu();
	while (choice != 'E' && choice != 'e'){
		switch(choice) {
		case '+': Add(); break;
		case '-': Sub(); break;
		case '*': Mul(); break;
		case '/': Div(); break;
		default: Mod(); break;
		}
		system("cls");
		Menu();
	}
}

//--------------- DISPLAY CALCULATION MENU --------------------------------------

void Menu(){
    cout << HD_C1 << endl;
	cout << "\t\t\t (+)  <--  Addition\n\n";
	cout << "\t\t\t (-)  <--  Subtraction\n\n";
	cout << "\t\t\t (*)  <--  Multiplication\n\n";
	cout << "\t\t\t (/)  <--  Division\n\n";
	cout << "\t\t\t (%)  <--  Modulos(%)\n\n\n";
	cout << "\t\t\t (E)  <--  Exit\n\n";
	cout << "\n\t\t\tEnter an Operator for\n";
	cout << "\t\t\tthe Calculation of Your Choice: ";
	cin >> choice;

	while (choice == 'E' || choice == 'e'){
		system("cls");
		ProcessLoop();
	}
	while (choice!='+' && choice!='-' && choice!='*' && choice!='/' && choice!='%'){
		cout <<"\a\t\t\tERROR...enter correct sign: ";
		cin >> choice;
	}
}

//--------------- CALCULATION FUNCTIONS -------------------------------------------		
	
	float Add(){
		system("cls");
		cout << "\n\n\t\t\t\t(+)ADDITION\n\n\n";
		cout  << HD_C2;
		input();
		result = firstNum + secondNum;
		cout << "\n\n\t\t\t"<<firstNum<<" + "<<secondNum<<" = "<<result <<endl;
		LoopBack();
		return result;
	}
	float Sub(){
		system("cls");
		cout << "\n\n\t\t\t\t(-)SUBTRACTION\n\n\n";
		cout << HD_C2;
		input();
		result = firstNum - secondNum;
		cout << "\n\n\t\t\t"<<firstNum<<" - "<<secondNum<<" = "<<result <<endl;
		LoopBack();
		return result;
	}
	float Mul(){
		system("cls");
		cout << "\n\n\t\t\t\t(*)MULTIPLICATION\n\n\n";
		cout << HD_C2;
		input();
		result = firstNum * secondNum;
		cout << "\n\n\t\t\t"<<firstNum<<" * "<<secondNum<<" = "<<result <<endl;
		LoopBack();
		return result;
	}
	float Div(){
		system("cls");
		cout << "\n\n\t\t\t\t(/)DIVISION\n\n\n";
		cout << HD_C2;
		input();
		result = firstNum / secondNum;
		cout << "\n\n\t\t\t"<<firstNum<<" / "<<secondNum<<" = "<<result <<endl;
		LoopBack();
		return result;
	}
	int Mod(){
		system("cls");
		cout << "\n\n\t\t\t\t(%)MODULOS\n\n\n";
		cout << HD_C2;
		cout << "\t\t\tNote: The modulos operator (%) is used\n"; 
		cout << "\t\t\tto calculate the remainder from\n"; 
		cout << "\t\t\tdividing two \"integer numbers\".\n\n";
		int firstNum, secondNum, result;         // local variables
		cout << "\n\t\t\tEnter First Number: ";
		cin >> firstNum;
		cout << "\n\t\t\tEnter Second Number: ";
		cin >> secondNum;
		result = firstNum % secondNum;
		cout << "\n\n\t\t\t"<<firstNum<<" % "<<secondNum<<" = "<<result <<endl;
		LoopBack();
		return result;
	}

//-------------- INPUT DATA ----------------------------------------------------
	
	float input(){
		cout << "\n\t\t\tEnter First Number: ";
		cin >> firstNum;
		cout << "\n\t\t\tEnter Second Number: ";
		cin >> secondNum;
		while ((firstNum < 0 || firstNum > 1000)||(secondNum < 0 || secondNum > 999)){
      		cout << "\t\t\t\aERROR...numbers of digits exceed --> (1 - 1000)\n";
            input();
		}
		return 0;
	}
	
//-------------- RETURN CONTROL TO FUNCTION --------------------------------------
	
void LoopBack(){
char exit;    // Local variable
cout <<"\n\n\n\t\tType \'E\' to Exit or Any other Key to continue...";
	cin >> exit;
	if (exit != 'E' && exit != 'e'){
		system("cls");
		Process_C();
	}
	else{
		system("cls");
		ProcessLoop(); 
	}
}

the problem is caused because you are trying to store a character in a float...

you should make the input a string, so you can test the data to make sure its valid.

char crows[255];

cin >> crows;

row = atof(crows);

....
while (row < 0 || row > 20){

btw....
why are you using a float. rows are always going to be an integer... it is bad programming practice to use so many glocal variables.
atof = float, atoi = integer (stlidb.h or math.h)...

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