If command problems.

I feel so bad making another topic so soon after the other one. I was just wondering if there was something wrong with my if statements here. When I choose an option other than option 1,2, or 3 it works fine the program looks back and asks for another option to be inputted.

However when I input 1, 2, or 3 as the option, it's not working out.
The positive if commands seem to be working. When I select option 1 it prints option 1, however it then prints option 3 also. When I select option 2, it prints option 2 however it then prints option 3 also. If I select option 3, it continues in the while loop even though it should pass it by.

Hopefully this is just a small thing I've done wrong and quick to solve.
Alex

Code:

//A program that calculates the reduced mass of diatomic molecules.
//The following libraries will be used.
#include <stdio.h>
#include <stdlib.h>
#include <math.h>

Manual_Input()
{

}

Textfile_Input()
{

}

int main()
{
int selection;//Defines the variable used to decide on a keyboard or text file input.
while (selection!=1||selection!=2)
{

    printf("A program to calculate the reduced mass of diatomic molecules.\n");//Intro Message.
    Data_input://Allows the program to loop if the user selects a value other than 1,2, or 3.
    printf("Input data from the keyboard, from a text\nfile, or close the program?\n");
    printf("1. From the keyboard.\n");printf("2. From a text file.\n");printf("3. Close program.\n");printf(":");
    scanf("%d",&selection);//Allows the user to choose whether or not to input from a text file or the keyboard.

    if (selection==1)
        {
            printf("Hello");
        }
    if (selection==2)
        {
            printf("how are you?");
        }
    if (selection!=1 || selection!=2 || selection!=3)
        {
            printf("Invalid Option, please choose again. ");
        }

}
return 0;
}

Code:

int selection;
while (selection!=1||selection!=2)

Tell me what this code does

> if (selection!=1 || selection!=2 || selection!=3)
You want && here, not ||

At the moment, you have say
selection = 1
which means you have
if ( false || true || true )

But then if you have
selection = 2
you have
if ( true || false || true )

Your if statement as written is always true.

Originally Posted by std10093

Code:

int selection;
while (selection!=1||selection!=2)

Tell me what this code does

Well that code was providing the loop, it seemed to be working because it kept looping back.

Originally Posted by Salem

> if (selection!=1 || selection!=2 || selection!=3)
You want && here, not ||

At the moment, you have say
selection = 1
which means you have
if ( false || true || true )

But then if you have
selection = 2
you have
if ( true || false || true )

Your if statement as written is always true.

But if it wasn't 1 it doesn't necessarily mean False || True || True, because someone could have put in the wrong number like 4. No? Then they would all be false.

When I reach for first time the line of while (line 2 of the quoted code) want is the value of selection?

Ermm it wouldn't have a value?

Exactly. So what's the behavior of the code?

Better initialize your variable

Well this is the thing, because I have a chemistry teacher and not a programming teacher who taught us, we didn't go into that depth of detail. I assumed before you assigned the value, it still existed just the value had an arbritray value of 0 or something like that.

What happens to a declared, unitialized variable in C?

You may use the do while loop, if you don't want to initialize it

Code:

//A program that calculates the reduced mass of diatomic molecules.
//The following libraries will be used.
#include <stdio.h>
#include <stdlib.h>
#include <math.h>

Manual_Input()
{

}

Textfile_Input()
{

}

int main()
{
int selection;//Defines the variable used to decide on a keyboard or text file input.
selection=4;//Sets the value of the variabe selection so it can be used for the do while loop.

do {
    printf("A program to calculate the reduced mass of diatomic molecules.\n");//Intro Message.
    Data_input://Allows the program to loop if the user selects a value other than 1,2, or 3.
    printf("Input data from the keyboard, from a text\nfile, or close the program?\n");
    printf("1. From the keyboard.\n");printf("2. From a text file.\n");printf("3. Close program.\n");printf(":");
    scanf("%d",&selection);//Allows the user to choose whether or not to input from a text file or the keyboard.

    if (selection==1)
        {
            printf("Hello");
        }
    if (selection==2)
        {
            printf("how are you?");
        }
    if (selection!=1 || selection!=2 || selection!=3)
        {
            printf("Invalid Option, please choose again. ");
        }

} while (selection!=1 || selection!=2 || selection!=3);
return 0;
}

I've changed the code to this, and also set the value of the selection variable so it has a different value to that of 1, 2 or 3.

Code:

int selection;//Defines the variable used to decide on a keyboard or text file input.
selection=4;//Sets the value of the variabe selection so it can be used for the do while loop.

However the loop doesn't seem to be working. Even when I set the value to 1, 2 or 3 the loop repeats.

Originally Posted by Salem

> if (selection!=1 || selection!=2 || selection!=3)
You want && here, not ||

At the moment, you have say
selection = 1
which means you have
if ( false || true || true )

But then if you have
selection = 2
you have
if ( true || false || true )

Your if statement as written is always true.

Never mind, I maybe misunderstood how if statements work. This is now working. Thank you very much.

Read again Salem's post

Also when you say doesn't work, specify what is happening

//you just read it :P

Originally Posted by Alex Saunders

Never mind, I maybe misunderstood how if statements work. This is now working. Thank you very much.

No, it appears that you misunderstood boolean logic, not if-statements.