Uninitialized local variable?

[FakezZ]

Okay so I have been experimenting a bit in c++ and have ended up with this code:

#include <iostream>#include <cmath>#include <conio.h>#include <string> int main(){long double dx, uzero, t, a;char* e = "uzero";char* c = "dx";char* d = "t";char* v = "a";char* b ;std::cout << "Which is the unknown variable?" << std::endl;std::cin >> b; if (b=e){std::cout << "Now input the stuff you know in the following order: dx, t, a" << std::endl;std::cin >> dx >> t >> a;uzero = dx / t - 1 / 2 * a*t;std::cout << uzero;} if (b=c);{std::cout << "Now input the stuff you know in the following order: u0, t, a" << std::endl;std::cin >> uzero >> t >> a;dx = uzero*t + 1 / 2 * a*pow(t, 2);std::cout << dx;} if (b=v){std::cout << "Now input the stuff you know in the following order: dx (then press enter),t,u0 " << std::endl;std::cin >> dx >> t >> uzero;a = 2 * dx / pow(t, 2) - 2 * uzero / t;std::cout << a;} if (b=d){std::cout << "I am not that smart, yet!";} _getch();return 0;  }

Everything looks fine, but I get the error  C4700: uninitialized local variable 'b' used at line 15...

Edited April 1, 2014 by FakezZ
added code tags

[Nineshadow]

You haven't initialized b.You just named it,but you haven't assigned it a value.It doesn't matter which because you overwrite it later.

[FakezZ]

Please use the code tag.

oops sorry ill edit it

[FakezZ]

You haven't initialized b.You just named it,but you haven't assigned it a value.It doesn't matter which because you overwrite it later.

Yeah but when I do that I get an error asking me to either break or continue

[JamieF]

Try b = ""

[Nineshadow]

Yeah but when I do that I get an error asking me to either break or continue

does char* b ="b" not work? 

[FakezZ]

does char* b ="b" not work? 

will try that now

[FakezZ]

Try b = ""

nope the same thing

[FakezZ]

does char* b ="b" not work? 

didn't work ...

[Ciccioo]

You haven't initialized b.You just named it,but you haven't assigned it a value.It doesn't matter which because you overwrite it later.

actually no, b should be a pointer to the memory where the input should be saved

but b is not defined, so the program writes to a random memory location, which is a bug

 

your problem is that you're using a weird mix of c and c++

i actually know about nothing of c++, but you should use the standard namespace and strings instead of char*

especially if you don't know what the asterisk is there for, just don't do things to make the compiler happy and not prompt errors

[Nineshadow]

didn't work ...

What did it say?And why are you using chars?Why not strings?

[FakezZ]

actually no, b should be a pointer to the memory where the input should be saved

but b is not defined, so the program writes to a random memory location, which is a bug

 

your problem is that you're using a weird mix of c and c++

i actually know about nothing of c++, but you should use the standard namespace and strings instead of char*

especially if you don't know what the asterisk is there for, just don't do things to make the compiler happy and not prompt errors

But if I use strings how will i be able to compare them?

[FakezZ]

What did it say?And why are you using chars?Why not strings?

It prompts me to break... I am not using strings because I don't know how to compare them

[Ciccioo]

But if I use strings how will i be able to compare them?

with a simple ==

that's the power of strings, and of the standard namespace: you will be able to use all the "built-in functions and behaviours"

the namespace knows how to compare strings, so

if(string1 == string2)

works just the way you would expect it to

[FakezZ]

with a simple ==

that's the power of strings, and of the standard namespace: you will be able to use all the "built-in functions and behaviours"

the namespace knows how to compare strings, so

if(string1 == string2)

works just the way you would expect it to

Oh ok thank you. When I googled it it said something like str1.compare(str2) == 0 and it didn't work

[Ciccioo]

Oh ok thank you. When I googled it it said something like str1.compare(str2) == 0 and it didn't work

actually that should work too

 

i guess it's because you're not using the namespace

after the includes, you need to add a

using namespace std;

i'm not sure if the semicolumn is needed

[WanderingFool]

I agree with Ciccioo....with that said, if your heart is content on using char* instead of string keep reading (which I can understand, I still prefer that method but just because I built up a collection of stuff that needs char*).

 

So to address a few things that others have said.

b is uninitialized...so it point to a random part of memory.  std::cin will try writing to where b is pointing to (or from my recollection)...so if b is pointing to a random place it is trouble.

The next problem is why @Nineshadow and @JamieF 's solutions didn't work.  The reason is simple "b" and "" are both a dedicated part of memory....you should not be manipulating it at all.  "b" and "" are effectively constant pointers....their memory location is fixed.  This can't be said enough NEVER try to manipulate characters that have been added using quotes.

 

The better solution to this type of problem is creating the actually memory space (as follows)

char b[1024]; //This creates room for 1023 characters, not 1024...the array has 1024 slots...this is because the last input is always 0 to tell end of lineor another syntaxchar* b = char[1024]; //Wrong syntax sorry.  Thanks Ciccioo for correcting me

This creates a problem though, only 1024 characters can be written...although this is hardly a problem (if you are expecting the user will never do this)...just something to keep a note of...as there are ways to solve this, but I really don't feel like writing them out.

 

 

Now we approach a new problem.  You will be getting errors in your code due to if(b=e) and all your other b=___.  There are a few flaws in this, first b=e will simply makes b = e...and since e points to uzero in memory it will always return true.  As a note even b == e is incorrect.  b == e will compare where the memory is pointing.  e.g. e points to uzero at location 0x222222 but b points to uzero at location 0x12121212...while b == e returns false it still is equal.  strcmp was built for this (fyi strcmp is actually a dangerous function to use, but in this case it would be okay).

 

 

So...after writing all of this, go with Ciccioo's solution of using string....I don't think you are ready to handle the concepts of pointers at the moment....but you are welcome to try, and I can always help if you get more errors

[Nineshadow]

@WanderingFool

I don't know much c++,just the very basics.

My experience comes from Java.Being an OOT language,they are based around almost the same principles.

[Ciccioo]

char b[] = char[1024]; //This creates room for 1023 characters, not 1024...the array has 1024 slots...this is because the last input is always 0 to tell end of lineor another syntaxchar* b = char[1024];

i agree with your agreement with my statement, but i disagree with yours

 

isn't it

char b[1024];

to declare a static array, or

char* b = new char[1024];

to allocate it at runtime?

[WanderingFool]

i agree with your agreement with my statement, but i disagree with yours

 

isn't it

char b[1024];

to declare a static array, or

char* b = new char[1024];

to allocate it at runtime?

Oh yes....my bad...I have been away from C++ for too long, I have forgotten some of the syntax

 

It is char b[1024];

 

I shall edit my last post to correct that

[FakezZ]

I agree with Ciccioo....with that said, if your heart is content on using char* instead of string keep reading (which I can understand, I still prefer that method but just because I built up a collection of stuff that needs char*).

 

So to address a few things that others have said.

b is uninitialized...so it point to a random part of memory.  std::cin will try writing to where b is pointing to (or from my recollection)...so if b is pointing to a random place it is trouble.

The next problem is why @Nineshadow and @JamieF 's solutions didn't work.  The reason is simple "b" and "" are both a dedicated part of memory....you should not be manipulating it at all.  "b" and "" are effectively constant pointers....their memory location is fixed.  This can't be said enough NEVER try to manipulate characters that have been added using quotes.

 

The better solution to this type of problem is creating the actually memory space (as follows)

char b[1024]; //This creates room for 1023 characters, not 1024...the array has 1024 slots...this is because the last input is always 0 to tell end of lineor another syntaxchar* b = char[1024]; //Wrong syntax sorry.  Thanks Ciccioo for correcting me

This creates a problem though, only 1024 characters can be written...although this is hardly a problem (if you are expecting the user will never do this)...just something to keep a note of...as there are ways to solve this, but I really don't feel like writing them out.

 

 

Now we approach a new problem.  You will be getting errors in your code due to if(b=e) and all your other b=___.  There are a few flaws in this, first b=e will simply makes b = e...and since e points to uzero in memory it will always return true.  As a note even b == e is incorrect.  b == e will compare where the memory is pointing.  e.g. e points to uzero at location 0x222222 but b points to uzero at location 0x12121212...while b == e returns false it still is equal.  strcmp was built for this (fyi strcmp is actually a dangerous function to use, but in this case it would be okay).

 

 

So...after writing all of this, go with Ciccioo's solution of using string....I don't think you are ready to handle the concepts of pointers at the moment....but you are welcome to try, and I can always help if you get more errors

Ok so I tried both ways and they worked, but with a problem. If I type for example t, the second if statement runs and not the last one. Am I doing something wrong? Also after all the editing the code now looks like:

#include <iostream>#include <cmath>#include <conio.h>#include <string>int main(){	long double dx, uzero, t, a;	char b[1024];	std::cout << "Which is the unknown variable?" << std::endl;	std::cin >> b;	if (b == "u0")	{		std::cout << "Now input the stuff you know in the following order: dx, t, a" << std::endl;		std::cin >> dx >> t >> a;		uzero = dx / t - 1 / 2 * a*t;		std::cout << uzero;	}	if (b == "dx");	{		std::cout << "Now input the stuff you know in the following order: u0, t, a" << std::endl;		std::cin >> uzero >> t >> a;		dx = uzero*t + 1 / 2 * a*pow(t, 2);		std::cout << dx;	}	if (b == "a")	{		std::cout << "Now input the stuff you know in the following order: dx ,t , u0 " << std::endl;		std::cin >> dx >> t >> uzero;		a = 2 * dx / pow(t, 2) - 2 * uzero / t;		std::cout << a;	}	if (b == "t")	{		std::cout << "I am not that smart, yet!";	}		_getch();	return 0;	}

[WanderingFool]

Sorry I am in a rush so this will be very quick....but you have a ; on the if statement

if (b == "dx");

so it will always run the second one since the if statement is being consumed by the ;

[Ciccioo]

-snip-

i don't know how you're compiling it and if actually works on your machine, but it doesn't on mine

 

in fact, it's not supposed to work, comparing char arrays that way is wrong and shouldn't work, it should compile giving warnings and the if statements shoud always see a "false" result

the correct way to compare char arrays with string literals is via strcmp

char test[10];std::cin >> test;if(strcmp(test, "hey") == 0){        // stuff if they're equal}else{        // other stuff if they're not}

this is also mixing C and C++, which is not the best, especially while learning

 

anyway, the == comparison only works with c++ strings

[FakezZ]

Sorry I am in a rush so this will be very quick....but you have a ; on the if statement

if (b == "dx");

so it will always run the second one since the if statement is being consumed by the ;

Oh that evil semicolon > Thanks!

[FakezZ]

i don't know how you're compiling it and if actually works on your machine, but it doesn't on mine

 

in fact, it's not supposed to work, comparing char arrays that way is wrong and shouldn't work, it should compile giving warnings and the if statements shoud always see a "false" result

the correct way to compare char arrays with string literals is via strcmp

char test[10];std::cin >> test;if(strcmp(test, "hey") == 0){        // stuff if they're equal}else{        // other stuff if they're not}

this is also mixing C and C++, which is not the best, especially while learning

 

anyway, the == comparison only works with c++ strings

I am using visual studio express 2013. It works on my machine, although I did follow your advice and now the code is only c++ and not a mix