#include<iostream>
#include<ctime>
#include<string>
using namespace std;
int main()
{
string NowTime;
time_t now;
struct tm nowLocal;
now=time(NULL); // get the time from the OS
nowLocal=*localtime(&now);
NowTime = nowLocal.tm_hour + ':' + nowLocal.tm_min + ':' + nowLocal.tm_sec;
cout<< NowTime;
}
When I run the program, it display nothing, can someone help me? I am totally new in programming.
If you try
cout << nowLocal.tm_hour + ':' + nowLocal.tm_min + ':' + nowLocal.tm_sec;
you'll see an integer, not anything resembling a time.
This is because it's the sum of five integers - the characters are promoted to integers, and then it's all added up.
The simplest fix is to not build a string at all but to output the parts individually:
cout << nowLocal.tm_hour << ':' << nowLocal.tm_min << ':' << nowLocal.tm_sec;
Otherwise, you need to convert those numbers to strings:
NowTime = std::to_string(nowLocal.tm_hour) + ':' + std::to_string(nowLocal.tm_min) + ':' + std::to_string(nowLocal.tm_sec);
or, you can use a std::ostringstream, which works just like std::cout and other streams, but writes to a std::string:
std::ostringstream ss;
ss << nowLocal.tm_hour << ':' << nowLocal.tm_min << ':' << nowLocal.tm_sec;
NowTime = ss.str();
The line:
NowTime = nowLocal.tm_hour + ':' + nowLocal.tm_min + ':' + nowLocal.tm_sec;
is adding the hour, minute, and second to the numeric value of the colon symbol, since char is implicitly coerced to an int. That value is then being interpreted as a char in the string assignment operator.
Instead, you can simply output the values directly to cout. They will be formatted appropriately by cout's stream insertion operator <<.
#include<iostream>
#include<ctime>
#include<string>
using namespace std;
int main()
{
string NowTime;
time_t now;
tm nowLocal;
now=time(NULL); // get the time from the OS
nowLocal=*localtime(&now);
cout << nowLocal.tm_hour << ':' << nowLocal.tm_min << ':' << nowLocal.tm_sec;
return 0;
}
If you would instead want to store them in a string, read up about stringstreams. They have a similar syntax to cout and can make formatting strings much easier.
Instead of having to put the result in a variable, you could output it like this:
cout << nowLocal.tm_hour << ':' << nowLocal.tm_min << ':' << nowLocal.tm_sec;
Also, if you want to keep the variable, do this:
NowTime = std::to_string(nowLocal.tm_hour) + ':' + std::to_string(nowLocal.tm_min) + ':' + std::to_string(nowLocal.tm_sec);
cout << NowTime;
Try this:
#include<iostream>
#include<ctime>
#include<string>
#include <sstream>
using namespace std;
int main()
{
string NowTime;
time_t now;
struct tm nowLocal;
now=time(NULL); // get the time from the OS
nowLocal=*localtime(&now);
stringstream s;
s<<nowLocal.tm_hour;
s<<":";
s<<nowLocal.tm_min;
s<<":";
s<<nowLocal.tm_sec;
NowTime = s.str();
cout<< NowTime;
}
You cannot cas directly from int to string and you need put values into stream and then to string.
What about using iostringstream to build the string you want?
#include<iostream>
#include<ctime>
#include<string>
#include <sstream>
using namespace std;
int main()
{
ostringstream NowTime;
time_t now;
struct tm nowLocal;
now=time(NULL); // get the time from the OS
nowLocal=*localtime(&now);
NowTime << nowLocal.tm_hour << ":" << nowLocal.tm_min << ":" << nowLocal.tm_sec;
cout<< NowTime.str() << endl;
}
Or for the purposes of your program you could simple use std::cout which also happens to be an output stream.
cout << nowLocal.tm_hour << ":" << nowLocal.tm_min << ":" << nowLocal.tm_sec << endl;
Since it looks like you're pre-c++11 and can't use std::to_string. Here's a C-like way of doing it, sticking to the includes you're currently using.
#include<iostream>
#include<ctime>
#include<string>
using namespace std;
#define STR_LEN 128
int main()
{
string nowTime;
time_t now;
struct tm nowLocal;
now = time( NULL ); // get the time from the OS
nowLocal = *localtime( &now );
char hour[ STR_LEN ], min[ STR_LEN ], sec[ STR_LEN ];
sprintf( hour, "%d", nowLocal.tm_hour );
sprintf( min, "%d", nowLocal.tm_min );
sprintf( sec, "%d", nowLocal.tm_sec );
nowTime = string( hour ) + ':' + string( min ) + ':' + string( sec );
cout << nowTime << endl;
}
How append system time to a string?
My answer is to build a convenience function.
If you really only need hour, minute, second, then you need not use the relatively slow localtime(). (on the other hand, if you do need more, I think you should prefer localtime_r() for the conversion).
For an embedded system several contracts back, I found this conversion to be a relatively slow function and chose to avoid it. The algorithms to handle leap days, centuries, etc. appear simple enough. I suspect I considered it slow simply because it calculates more than I needed in that application that was trying to do the conversion many times per second.
There exists a simpler (and probably still faster) approach involving modular arithmetic. It starts the same - with a time(0) (and thus I suspect what I am doing here is 'hidden' in the localtime_r() function). Side note 1 - on my older Dell running Ubuntu 15.10, time(0) is simply the fastest access to the wall clock, measuring about 6 or 7 ns 'typical' duration. Side note 2 - time_t may change someday. "The time_t Wikipedia article article sheds some light on this. The bottom line is that the type of time_t is not guaranteed in the C specification."
The code I currently use to conveniently generate a time stamp string:
std::string hhCmmCssGet(time_t tt = 0) // tt has default value
{
time_t now = ( tt ? tt : time(0) );
static time_t prev = 0;
static char hhmmss[] = "hh:mm:ss";
if (prev != now)
{
prev = now;
const int32_t CST = -6;
int64_t hr = ((now / 3600) % 24) + CST; // hr of day, CST offset
if (hr < 0) hr += 24;
uint64_t min = ((now / 60) % 60); // min of day
uint64_t sec = (now % 60); // sec of day
std::stringstream ss;
ss << std::dec
<< std::setfill('0') << std::setw(2) << hr << ":"
<< std::setfill('0') << std::setw(2) << min << ":"
<< std::setfill('0') << std::setw(2) << sec;
for (size_t i=0; i<8; i++) // transfer new value
hhmmss[i] = ss.str()[i]; // into static hhmmss
}
std::string retVal(hhmmss);
return(retVal);
}
The static items and "if (prev != now)" clause, allow this function to be invoked thousands of times per second ... with much reduced effort. The second, after all, only updates 1ce per second. And note that the std::stringstream stuff and modular arithmetic operations only run 1ce per second.
