Improving efficiency of my function

[Alex Monari]

Hi all,

For work reasons I'm required to discuss the efficiency of the function below by manually stepping through the code. I needed some pointers as to how to go about that.

Thanks for any suggestions.
unsigned long normalise(unsigned long input_time) { bool finished; // This produces a formatted time string like: // Thu_Nov_24_18:22:48_1986 string str_time = format_time( input_time ); while( str_time.substr(1,3) != "Sun") { input_time -= 24*60*60; str_time = format_time( input_time ); } while( str_time.substr(11,2) != "00" ) { input_time -= 60*60; str_time = format_time( input_time ); } while( str_time.substr(14,2) != "00") { str_time = format_time( input_time ); input_time -= 60; } while( str_time.substr(17,2) != "00") { input_time -= 1; str_time = format_time( input_time ); } return input_time; }

[Carey Brown]

The first thing that would help is stating the requirements clearly.
You are computing the elapsed time from the start of the week, ie the previous Sunday at 0 a.m..

[Knute Snortum]

Moved thread to C/C++

[Anand Hariharan]

Carey Brown wrote:The first thing that would help is stating the requirements clearly.

Agree 100%

Carey Brown wrote:You are computing the elapsed time from the start of the week, ie the previous Sunday at 0 a.m..

No, just short of that. He is trying to establish the time (the 'unsigned long' integral equivalent) corresponding to the start of the week. Agree it is trivial to compute the elapsed time from that.

Alex Monari wrote:For work reasons I'm required to discuss the efficiency of the function below by manually stepping through the code. I needed some pointers as to how to go about that.

See if you can come up with your "'unsigned long' integral equivalent" input_time for 'Saturday July 4 23:59:59 2015' and then step through the code. Do you think the function is efficient?

[Campbell Ritchie]

That I think is C++ code because of the bool keyword, not C.
I presume the == and != operators are overloaded on Strings.
I suspect the greatest overhead is in the format time function. What complexity does that run in?

[Anand Hariharan]

Campbell Ritchie wrote:That I think is C++ code because of the bool keyword, not C.

C introduced bool albeit after C++. It is a 'typedef' and one needs to include <stdbool.h> for it to be usable.

Campbell Ritchie wrote:I presume the == and != operators are overloaded on Strings.

Actually, C does not have a native string type. In C, one uses an array of characters terminated by the NUL character to hold strings. In C++, one has the string class that comes with member functions (substr is one of them) and overloaded operators.

Campbell Ritchie wrote:I suspect the greatest overhead is in the format time function. What complexity does that run in?

The code posted by the OP does not need any while loops. Also, the OP could use a function that does the reverse of 'format_time' (i.e., one that takes the formatted string representation of the time and returns the integral equivalent.