The string class, the math class, and the DateTime class

String class

String a = "abcdef123456"; Note: The length of the string is counted from 0 onwards, such as: 0,1,2,3,4,5,6,7,8,9 ...

A.length; is a property that represents the length of the string ★
A[2]; A is a string in the beginning of a, 2 is the position in the string, starting from 0, [] is the index number, a[2] the output is C

A=a.trim (); Remove the left and right side of the space, you can not remove the space inside the string ★
A=a.trimstart (); Remove the left space
A=a.trimend (); Remove the space on the right

A=a.tolower (); Converts uppercase English characters in a string to lowercase
A=a.toupper (); Converts lowercase English characters in a string to uppercase

A=a.startswith ("a"); Match the beginning to see if it starts with a and returns TRUE or False
A=a.endswith ("6"); Match end, see if it ends with 6, returns TRUE or False
A=a.contains ("CD"); Matches whether the entire string contains a CD, returns TRUE or false★

A=a.indexof ("1"); Where a string appears in Str (where it first appears)
A=a.lastindexof ("2"): The last occurrence of a string in str

A=a.substring (5,4); Intercept string from fifth bit, intercept 4 bit ★

A=a.replace ("CD", "GG"); Replace all strings that match the string condition of the specified segment (Find Replace function), replace the CD in the string with GG

Math class:

Double n = 3.1415;
Math.Round (n,2); Rounds N, retains 2 digits after the decimal point
Math.ceiling (n); Take the smallest integer greater than the decimal
Math.floor (n); Take the largest integer less than the decimal number
Math.sqrt (a); Open square, square root
Math.pow (2,4); 2 of the 4-time Square

DateTime class:

DateTime d =datetime.now Gets the current time of the computer

DateTime d=new datetime (); Defines a new time, initially zero
int a = D.year; Year of extraction
int b = D.month; Extract Month
int c = D.day; Extract Date
int e = D.hour; Extraction hours
int f = d.minute; Extract minutes
int g = D.second; Extract seconds
int h = d.millisecond; Extracting milliseconds
int k = D.dayofyear; Date obtained is the day ordinal of the year
DayOfWeek L = d.dayofweek; The date is the week of the week

System.TimeSpan da = new TimeSpan (1,0,0,0);
D = D.add (DA);

D = d.addyears (1); Add one year
D = D.addmouth (1); Add one months
D = d.addday (1); Add a day
D = d.addhour (1); Add an hour
D = D.addminute (1); Add a minute
D = d.addseconds (1); Add one second

ToString ("yyyy mm month DD Day HH when mm minutes ss seconds") according to "Month Day and minute minute" to show out (hh means 12-hour system; HH denotes 24-hour system, and the year-to-date can be replaced by a surrogate, or can be replaced by several )

The string class, the math class, and the DateTime class