Question

Really dumb so far with C++
Currently having problems with if-else statements

Answer 1

I seriously cannot get them to do what I want. I can do an if statement, but as soon as I try and else or an nested if I get errors. And if I try to just do multiple if statements I get statements to override each other and I cant get the combined effects. Just no idea how to fix it.

Answer 2

So, I guess I dont know what to put next without causing an error. I dont know how to post the code properly, so Ill just type it manually. This part is just the beginning of the if

if(num > 0 && denom > 0)
num = abs(num);
denom = abs(denom);
else

And then whatever came after that was wrong. I do have num and denom declared, so no issues with that, just...not sure. What am I supposed to do with the next statement if Im using else? Maybe im using it incorrectly? Ill post what I tried to put if needed, but I think this is more of a conceptual misunderstadning on my part then me doing something silly.

Answer 3

One problem I see right off is the lack of { }

Answer 4

I never understood the explanation of that. Lecture made it seem like it was optional, like some sort of organizational tool. Until now, we just had the whole program inside of one setof { }. So where and when do I need them in if-else?

Answer 5

They group commands. If more than one command is triggered by an if or an else, they need to be inside { }. That is probably what is causing the else to fail. It see no if rignt before it.

Answer 6

```
#include<iostream>
using namespace std;
int main()
{
int num = 1;
int denom = 0;

if(num > 0 && denom > 0) // More than one, so { }
{
num = abs(num);
denom = abs(denom);
}
else if (denom == 0)
cout << "Undefined\n"; // One command, no { }
else
{
// do
// other
// things
}
}
```

Answer 7

Oh, thats what you mean by commands. So if I only had one command the programm wouldve recognized the if?

Answer 8

Well, it would recognize the else. You had two lines of code. The first was seen as the end of the if. The second was seen as something different. That makes the else into an else without an if, which is what your error probably was.

Answer 9

Gotcha. Ill see if I can stay consistent with that and if I can get the rest of it to work that way. I wasn't sure how to get the next part to work, but I also need it to change an improper fraction into a mixed number. I figure you would need to use % for that, but I guess it didnt click in my head quite how to do it :/ Of course I was worried about getting everythign else before it correct but yeah, lol.

Answer 10

```
if(num > 0 && denom > 0)
num = abs(num); // seen as the end of the if
denom = abs(denom); // more code
else // else without an if
```

Answer 11

Ah, for ain improper to a mixed number, yah, you use %. First divide off the number to get the whole part. Then % off the remainder to get the numerator of the mixed fraction.

Answer 12

So, informally, something like

num / denom (num % denom) << "/" << denom

I havent gotten down the sequence of things yet to do all the cout << stuff properly, I just want to make sure im gettign the right idea of how to input it. I probably made it messier than what would be needed.

Answer 13

And I figure the "/" is so it doesnt actually divide again and just outputs the actual forward slash.

Answer 14

In software development they have a name for informal coding to get an idea. It is psudocode.

That would work fine in a cout for just printing them. If they are needed in other calculations:
whole_part = num/den
new_num = num%den

Answer 15

And I could just used those as declared variables?

Answer 16

Well, yah, you would have to make them as ints if you need to save them. But if you want to just print them, no need to save them. Just need to put a << between each step.

cout << num / denom << " and " << num % denom << "/" << denom;

Answer 17

This there a certain point where you want to maybe stop nested if statements and just completely restart if a new if else chain? Or is that going to cause problems if you try that? Also, if you ever use else, must an if always follow it?

Answer 18

if: means if this, do that.
else if: means if not the first if, but this, do that
else: means it did not match anything above so do this.

In many cases, if something gets too complex people will break it down into smaller peices. Then they will have a function do parts of it and return results. This can cause your ifs to nest into smaller clumps.

Answer 19

Is it wrong to have multiple if-else chains or should everything be nested if you need to have that many options? I guess I found myself at a point where I thought it might be logical to start off with another nested if chaina nd I wasnt sure.

Answer 20

The compiler won't break until things get super complex... I don't think there even is a limit in the standard. This is opposed to recursion, which does have very practical limits pretty quick. But past 4 or 5 nested levels it just gets hard to read.

Oh, and this is not nesting:
if
else if
else if
else if
else if
else if
else if
else if
else

That is just one level deep with lots of options.

Answer 21

So what is the nested if then? If thats not nested if, then I guarantee Ill get a bunch of things wrong on exams and assignments, lol x_x

Answer 22

```
if
if
if
else
else if
bla bla
else
else
```

Answer 23

Nesting is when one thing is inside another. Not after, but inside.

Answer 24

Will the first else you have only apply to the previous if or to all 3?

Answer 25

Only to the level it is at, if properly written. Which is why the depth can make it confusing and { } can become important. Here is a short example:
http://www.tutorialspoint.com/cplusplus/cpp_nested_if.htm

Answer 26

In that example, if you change the value of a, then the inner check for b never happens.

Answer 27

So its like saying if a AND if b?

Answer 28

Exactly. But what if you want "a and b or a and c or not a" So you can use the nested to first check a then find out which of b or c, but skip to else if not a.

Answer 29

Is

if(a && b)

an equivalent then?

Answer 30

Here is a modified version if you want to complie it:

```
#include <iostream>
using namespace std;

int main ()
{
// local variable declaration:
int a = 100;
int b = 200;

// check the boolean condition
if( a == 100 )
{
cout << "I checked a" << endl;
// if condition is true then check the following
if( b == 200 )
{
cout << "I checked b" << endl;
// if condition is true then print the following
cout << "Value of a is 100 and b is 200" << endl;
}
else
cout << "I skipped b" << endl;
}
else
cout << "I skipped a" << endl;
cout << endl;
cout << "Exact value of a is : " << a << endl;
cout << "Exact value of b is : " << b << endl;

return 0;
}
```

Run it. Then change b, see what happens. Then change a. You should see the messages change and skipping a should stop any messages related to b.

Answer 31

Oh wow. Okay, that makes sense for once x_x And last thing, how do you post the coding like that?

Answer 32

I put ``` above and below th blocks.

For larger bits of code I suggest dpaste.com or pastebin.com.

Answer 33

I ran the modified code just for example reasons.

Answer 34

The key part to get is in the last one where it does not say it even skipped b. The entire check/skip b is ignored because all of a was skipped. That is how nested ifs work.

Answer 35

The site also supports \(\LaTeX\) if you want to do formatted math.

\(k\begin{bmatrix}
a&b\\c&d
\end{bmatrix}=\begin{bmatrix}
ka&kb\\kc&kd
\end{bmatrix}\)

\[\int\limits_1^e\ln x \text{dx}\]

Answer 36

I thought it does say "I skipped b"

Answer 37

In the middle. But scroll to the end.

Answer 38

There are three runs there.

Answer 39

Even in the end I see where "I skipped b" is typed. What amI missing? xD

Answer 40

All I see at the very bottom is:

I skiped a

Exact value of a is : 101
Exact value of b is : 201

Answer 41

Oh, super super bottom, not just the 3rd section.
Okay, so it also works like
if #1
if #2
if #3
else #3
else #2
else #1

I like how it puts the comments to illustrate what the program is doing. Thanks for all the examples and explanations :3

Answer 42

Yah, the picture is:

code
run

code
run

code
run

And that final run, it flat out skips b entirely. The if, the else, all of it just goes by-by because it is nested inside a and a got skipped.

Answer 43

Gotcha. Yeah, never thought of it as like an and statement. Yeah, for some reason lecture is doing me less and less good in this class. Just gotta survive it, lol. Maybe itll click in my head more and more as I do it.

Answer 44

Lecture is a way to get an overview. It is rarely a way to get good detail. And c++ is not the friendliest language to start with...

Answer 45

Never really had a choice. This was the "intro" course I had to take. Where is the usual starting point?

Answer 46

C++ is where a lot of people start. Python and Java are both a little friendlier and get to graphics with built in things so they can be more interesting. C++ suffers from the linage of being an overhaul of C into object orientated programming. C, while very powerful, is exceptionally confusing to read at times.

Answer 47

Yeah, this was considered tobea course for those who know nothing about cimputers going in. Seems like that was a bit of a fib. I just need to get this assignment done with and then review something on the last assignment. I couldnt get things to align the way I needed them and I was sick, so I didnt have time to figure it out. If I got the time I can bring it back up and revisit it, but another time. Yeah, you help out a lot, so thanks again.

Answer 48

If you want to contrast how a Python class goes, you can do so for free from Rice University:
https://www.coursera.org/course/interactivepython

Answer 49

And the great thing is, if you learn the concepts in Python, you can translate them to C++. I have used things I learned in C++ in Python and Java.