#define N 10;
int main()
{
int x;
for (int i=0; i<N; i++)
x = i;
return 0;
}
Result of compiling this in g++:
test-define.cpp: In function ‘int main()’:
test-define.cpp:7:22: error: expected primary-expression before ‘;’ token
test-define.cpp:7:22: error: expected ‘)’ before ‘;’ token
test-define.cpp:7:24: error: name lookup of ‘i’ changed for ISO ‘for’ scoping [-fpermissive]
test-define.cpp:7:24: note: (if you use ‘-fpermissive’ G++ will accept your code)
test-define.cpp:7:27: error: expected ‘;’ before ‘)’ token
But it compiles fine when I change line 7 to for (int i=0; i<10; i++).
Why is this and how can I use the #define directive to accomplish what I want?
Remove the semicolon - you will be good - the semicolon is included in the substitution
Sometimes it is useful to get the compiler to run the preprocessor only. With gcc/g++ you can do something like
gcc -E file.c > result.txt
This will show you how the macro expanded (hint start at the end of the file and work up)
I recommend replacing the macro with a constant:
const int N = 10;
It's best to avoid macros when you can. Macros don't have any scope. They are a global text substitution. The compiler never sees them, so if you use a debugger it won't know about them. There are probably other reasons not to use them that I'm forgetting.
Related
I'm new to Sycl/DPC++ language. I wrote a sample vector addition code using Unified shared memory (USM):
#include<CL/sycl.hpp>
#include<iostream>
#include<chrono>
using namespace sycl;
int main()
{
int n=100;
int i;
queue q{ };
range<1>(n);
int *a=malloc_shared<int>(n,q);
int *b=malloc_shared<int>(n,q);
int *c=malloc_shared<int>(n,q);
for(i=0;i<n;i++)
{
a[i]=i;
b[i]=n-i;
}
q.parallel_for(n,[=](auto &i){
c[i]=a[i]+b[i];
}).wait();
for(i=0;i<n;i++){
std::cout<<c[i]<<std::endl;
}
free(a,q);
free(b,q);
free(c,q);
return 0;
}
When I compile it I get the following error:
warning: parentheses were disambiguated as redundant parentheses around declaration of variable named 'n' [-Wvexing-parse]
range<1>(n);
^~~
vec_add.cpp:11:1: note: add enclosing parentheses to perform a function-style cast
range<1>(n);
^
( )
vec_add.cpp:11:9: note: remove parentheses to silence this warning
range<1>(n);
^ ~
vec_add.cpp:11:10: error: redefinition of 'n' with a different type: 'range<1>' vs 'int'
range<1>(n);
^
vec_add.cpp:8:5: note: previous definition is here
int n=100;
^
1 warning and 1 error generated.
How to fix this error?
error: redefinition of 'n' with a different type: 'range<1>' vs 'int'
Two variables with the same name within the same scope create confusion to the compiler, so it might be the reason for the error which you are getting. You can try defining the value of n globally say for eg: #define N 100 in this case, set
range<1>(n);
to
range<1> (N);
and use that in your code.
If you want to declare the size locally then assign another variable (r) to the range as
range<1> r (n);
Now you can directly pass the 'r' variable as a parameter to the parallel_for.
In C++, the following is valid and I can run it without a problem:
int main(){
if (int i=5)
std::cout << i << std::endl;
return 0;
}
However, even though the following should also be valid, it gives me an error:
if ((int i=5) == 5)
std::cout << i << std::endl;
Error:
test.cpp: In function ‘int main()’:
test.cpp:4:10: error: expected primary-expression before ‘int’
if ((int i=5) == 5)
^
test.cpp:4:10: error: expected ‘)’ before ‘int’
test.cpp:5:36: error: expected ‘)’ before ‘;’ token
std::cout << i << std::endl;
^
Furthermore, in C++17 the below code must be valid too, but it gives me a similar error again:
if (int i=5; i == 5)
std::cout << i << std::endl;
Error:
test.cpp: In function ‘int main()’:
test.cpp:4:16: error: expected ‘)’ before ‘;’ token
if (int i=5; i == 5)
^
test.cpp:4:18: error: ‘i’ was not declared in this scope
if (int i=5; i == 5)
^
I am trying to compile with g++ test.cpp -std=c++17. g++ --version gives me g++ (Ubuntu 5.4.0-6ubuntu1~16.04.12) 5.4.0 20160609. What am I missing here?
if ((int i=5) == 5) is a syntax error. It does not match any supported syntax for if statements. The syntax is init-statement(optional) condition, where condition could either be an expression, or a declaration with initializer. You can read more detail about the syntax on cppreference.
if (int i=5; i == 5) is correct. However, you are using an old version of GCC that dates from before C++17 was standardized. You would need to upgrade your compiler version. According to C++ Standards Support in GCC this feature was added in GCC 7.
For starters, I believe your compiler is right to reject
if ((int i=5) == 5)
because this is not legal C++ code. A variable declaration statement isn’t an expression, so you can’t treat (int i = 5) as an expression.
For the second one, I suspect you just need to update your compiler. g++ 5.6 is a fairly old version at this point, and I believe more updates versions of g++ will handle that code with no problem.
I have recently downloaded MinGW into my computer but on using certain containers and iterators like unordered_map and auto it shows an unexpected error.
my code is as follows :
#include <bits/stdc++.h>
#include<unordered_map>
using namespace std;
int main()
{
unordered_map<string, int> umap;
umap["GeeksforGeeks"] = 10;
umap["Practice"] = 20;
umap["Contribute"] = 30;
for (auto x : umap)
cout << x.first << " " << x.second << endl;
return 0;
}
it gives the following error :
C:\Users\naima\Documents\cpp>g++ -o try2 try2.cpp
In file included from C:/TDM-GCC-64/lib/gcc/x86_64-w64-mingw32/5.1.0/include/c++/unordered_map:35:0,
from try2.cpp:2:
C:/TDM-GCC-64/lib/gcc/x86_64-w64-mingw32/5.1.0/include/c++/bits/c++0x_warning.h:32:2: error: #error This file requires compiler and library support for the ISO C++ 2011 standard. This support is currently experimental, and must be enabled with the -std=c++11 or -std=gnu++11 compiler options.
#error This file requires compiler and library support for the \
^
try2.cpp: In function 'int main()':
try2.cpp:9:5: error: 'unordered_map' was not declared in this scope
unordered_map<string, int> umap;
^
try2.cpp:9:25: error: expected primary-expression before ',' token
unordered_map<string, int> umap;
^
try2.cpp:9:27: error: expected primary-expression before 'int'
unordered_map<string, int> umap;
^
try2.cpp:11:5: error: 'umap' was not declared in this scope
umap["GeeksforGeeks"] = 10;
^
try2.cpp:15:15: error: 'x' does not name a type
for (auto x : umap)
^
try2.cpp:19:5: error: expected ';' before 'return'
return 0;
^
try2.cpp:19:5: error: expected primary-expression before 'return'
try2.cpp:19:5: error: expected ';' before 'return'
try2.cpp:19:5: error: expected primary-expression before 'return'
try2.cpp:19:5: error: expected ')' before 'return'
The compiler told you exactly what was wrong. It usually will.
This file requires compiler and library support for the ISO C++ 2011 standard. This
support is currently experimental, and must be enabled with the -std=c++11 or
-std=gnu++11 compiler options.
You just have to compile with the proper flag, -std=c++11. I don't know if you are version-matching against what graders use or what, but there are very few good reasons to be on a minGW compiler where support for an 8 year old standard is still considered experimental.
You can see that it works as expected here: https://godbolt.org/z/JQxL00
If you remove the -std=c++11 flag, it will fail to compile and give you the same error message.
You might also notice that I altered the includes to only include what I use. This results in a much faster compile time, smaller executable, and an easier to understand piece of code (Since it is plain to see what standard features are being used). You also avoid polluting your namespace.
I am trying to cin a loop index's value in the loop itself using lambda expression:
#include<iostream>
using namespace std;
int main(){
for(int a, ([](int & b){cin>>b;})(a); a < 2; ++a);
return 0;
}
These are the errors when i compile using g++ 4.5 on ubuntu:
forLoopAndCinTest.c++: In function ‘int main()’:
forLoopAndCinTest.c++:5:14: error: expected unqualified-id before ‘[’ token
forLoopAndCinTest.c++:5:14: error: expected ‘)’ before ‘[’ token
forLoopAndCinTest.c++:5:34: error: expected primary-expression before ‘)’ token
forLoopAndCinTest.c++:5:34: error: expected ‘;’ before ‘)’ token
forLoopAndCinTest.c++:5:40: error: name lookup of ‘a’ changed for ISO ‘for’ scoping
forLoopAndCinTest.c++:5:40: note: (if you use ‘-fpermissive’ G++ will accept your code)
forLoopAndCinTest.c++:5:50: error: expected ‘;’ before ‘)’ token
If i use a normal function instead of the lambda, program compiles fine.
Using -fpermissive doesnt help either.
Any ideas?
That's not how the for look works. You are trying to call a lambda where the compiler expects you to declare an int:
for( int a, int2, ...; a < 2; ++a );
Now,
If i use a normal function instead of
the lambda, program compiles fine
Yes, but it's probably not doing what you think it does.
void f(int& b)
{
cin >> b;
}
// ...
for( int a, f(a); a < 2; ++a );
Here, the loop declares two int variables, named a and f. The loop doesn't call f() as you might expect.
Try this instead:
for( int a; cin >> a && a < 2; ++a );
The first part of the for is interpreted as a declaration. We get the very same error when replacing your code by the (almost) equivalent :
int main(){
int a, ([](int & b){cin>>b;})(a); // This produces the same error
for(; a < 2; ++a);
return 0;
}
To answer a comment you made, for (int a, foo() ; ... works, but not like you think it does. It is in fact declaring a function (inside the for scope) that returns an int, and has the name foo. As in :
int a, foo();
Which you should read as :
int a;
int foo();
After this: for( int a, compiler expects some name (of the variable) - unqualified-id.
But in your case it is not so.
// File test.cpp
#include <my_global.h>
#include <algorithm>
int main()
{
return 0;
}
Compiled with: g++ -c -I /usr/local/mysql/include/mysql/ test.cpp, where /usr/local/mysql is the mysql install directory.Then the compiler report the following errors:
In file included from /usr/include/c++/4.4/algorithm:61,
from test.cpp:3:
/usr/include/c++/4.4/bits/stl_algobase.h:232:56: error: macro "min" passed 3 arguments, but takes just 2
/usr/include/c++/4.4/bits/stl_algobase.h:253:56: error: macro "max" passed 3 arguments, but takes just 2
In file included from /usr/include/c++/4.4/bits/stl_algo.h:61,
from /usr/include/c++/4.4/algorithm:62,
from test.cpp:3:
/usr/include/c++/4.4/bits/algorithmfwd.h:353:41: error: macro "max" passed 3 arguments, but takes just 2
/usr/include/c++/4.4/bits/algorithmfwd.h:364:41: error: macro "min" passed 3 arguments, but takes just 2
In file included from /usr/include/c++/4.4/algorithm:61,
from test.cpp:3:
/usr/include/c++/4.4/bits/stl_algobase.h:186: error: expected unqualified-id before ‘const’
/usr/include/c++/4.4/bits/stl_algobase.h:186: error: expected ‘)’ before ‘const’
/usr/include/c++/4.4/bits/stl_algobase.h:186: error: expected ‘)’ before ‘const’
/usr/include/c++/4.4/bits/stl_algobase.h:186: error: expected initializer before ‘const’
/usr/include/c++/4.4/bits/stl_algobase.h:209: error: expected unqualified-id before ‘const’
/usr/include/c++/4.4/bits/stl_algobase.h:209: error: expected ‘)’ before ‘const’
/usr/include/c++/4.4/bits/stl_algobase.h:209: error: expected ‘)’ before ‘const’
/usr/include/c++/4.4/bits/stl_algobase.h:209: error: expected initializer before ‘const’
/usr/include/c++/4.4/bits/stl_algobase.h:232: error: ‘std::min’ declared as an ‘inline’ variable
/usr/include/c++/4.4/bits/stl_algobase.h:232: error: template declaration of ‘const _Tp& std::min’
/usr/include/c++/4.4/bits/stl_algobase.h:235: error: expected primary-expression before ‘if’
/usr/include/c++/4.4/bits/stl_algobase.h:235: error: expected ‘}’ before ‘if’
/usr/include/c++/4.4/bits/stl_algobase.h:237: error: expected unqualified-id before ‘return’
/usr/include/c++/4.4/bits/stl_algobase.h:253: error: ‘max’ declared as an ‘inline’ variable
/usr/include/c++/4.4/bits/stl_algobase.h:253: error: template declaration of ‘const _Tp& max’
/usr/include/c++/4.4/bits/stl_algobase.h:256: error: expected primary-expression before ‘if’
/usr/include/c++/4.4/bits/stl_algobase.h:256: error: expected ‘}’ before ‘if’
/usr/include/c++/4.4/bits/stl_algobase.h:258: error: expected unqualified-id before ‘return’
/usr/include/c++/4.4/bits/stl_algobase.h:259: error: expected declaration before ‘}’ token
I think that there's some name conflict between my_global.h and algorithm, so I wrap my_global.h in a namespace:
// File test.cpp
namespace MYSQL_NAMESPACE {
#include <my_global.h>
}
#include <algorithm>
int main()
{
return 0;
}
But it doesn't help, the compiler still report the same errors. Then I change the include order as following:
// File test.cpp
#include <algorithm>
#include <my_global.h>
int main()
{
return 0;
}
Every thing goes well now.
Does Anybody Really Know What The Problem It Is?
TKS!
It seems that the mysql header defines a macro min.
#if !defined(max)
#define max(a, b) ((a) > (b) ? (a) : (b))
#define min(a, b) ((a) < (b) ? (a) : (b))
#endif
This has been reported to MySQL as bug 28184. The bug is marked as closed, so try updating to the newest version. According to the bug page it should be fixed in version 5.1.23, version 6.0.4 and newer versions.
Apparently the namespace trick does not work because min/max are macros and the preprocessor does not look at namespace scope.
This might fix the problem:
#include <my_global.h>
#undef min
#undef max
#include <algorithm>
The whole thing looks horrible though :)
It looks like my_global.h defines some name used by algorithm as a preprocessor macro, causing compilation to fail. With the ordering that works, you won't be able to use whatever it is that my_global.h clobbers, but your code will at least compile unless you need that feature. Since preprocessor macros are not namespaced, the namespace wrapping will not help, as you have observed.
Therefore, it sounds like my_global.h is broken, but if everything works just use the include order that works and go with it. That's my preferred include order anyway - standard library headers first, followed by external library headers, followed by internal headers.