1. File Reading

Some graphs have a large amount of data and are suitable for file reading.

int main(){
//If there is an oj system (online decision), file reading is ignored, otherwise files are used as standard input.
#ifdef ONLINE_JUDGE                
#else    
    freopen("data.txt", "r", stdin);   //Enter data from data.txt
#endif 
	....	
	return 0;
}

Under the same folder as the. cpp file, create a txt text named data to store the input data.

2. About scanf() and printf()

2-1 parameter:

1. Type conversion specifiers:% d,% lld,% f,% lf (nothing special, the format of input and output)
2. The transformation specification modifier (modifier 1, added between% and d), such as:

Modifier	Effect
number	Specified width (see Example 2-1-1 if less than actual width is automatically updated upward)
x.y	Limit precision (same number, x is integer partial digit, y decimal partial digit): Output is a little useful??? Is this to be verified?
*	For scanf(): suppress assignment, for printf(): character width wildcard

• For printf():

Modifier	Effect
+	Displays a +/- sign: If it is positive, it shows a + sign.
Blank space	Positive: Symbol bits show a space, negative: - sign
0	0 Fill
*	Specifies character width

• For scanf():

Modifier	Effect
*	Input suppression, see Example 2-1-2

• Example 2-1-1:

int main(){
	int in, other;
	scanf("%4d", &in);		//Input: 123, 123456789 
	printf("%d\n", in);		//Obtained: 123, 1234	 
	scanf("%d", &other);	//123: Continue to input, 123456789: other directly receives the rest (56789) 
	printf("%d", other); 	//123456789: Direct Output Residual 
	int out = 1234; 
	printf("|%2d|\n", out); 	//Output: | 1234| 
	printf("|%5d|", out);		//Output: | 1234| 
	return 0;
}

• Example 2-1-2

int main(){
	char s[5];
	int in;
	scanf("%*d %d", &in);		//Input: 123 12345
	printf("|%*d|", 8, in);		//Get: | 12345|
	scanf("%*s %d", &in);		//Input: abc 12345
	printf("|%*d|", 8, in);		//Get: | 12345|
	return 0;
}

2-2 return value:

1. About scanf():

Returns the number of items successfully read in and returns 0 when an input mismatch occurs.

If the end of the file is read, then EOF is returned: for cases where the number of inputs is not fixed, such as Example 2-2-1

• Example 2-2-1:

while(scanf("%d", &in) != EOF){}	//Read until the end
while(~scanf("%d", &in)){}	//Equivalent to the previous one: because EOF is -1 (11111111111) and 0 (00000000) after the inverse.

1. About printf();

Returns the number of characters successfully printed on the screen:

int main(){
	int x = 103;
	char s[10] = "abcde";
	cout << printf("%d %s", x, s);	//Output: 103 abcde9 - > 9:9 characters (space wrap, digit) 
	return 0;
}

3. Characters and strings:

3-1 characters:

1. Read in single character: getchar(); output single character: putchar();

ch = getchar();
putchar(ch);

1. Fill out the blanks and change lines:

int main(){
	char ch;
	char valid[5];
	//Output 1: balabala this_is_valid 
	//Enter 2: | this_is_valid | The first place is a space. 
	while((ch = getchar()) != ' ') continue;	//Filter out blanks: Input before discarding the first blank (skipping the blank) can also be filtered \n... 
	scanf("%s", valid);
	//Output 1: this_is_valid
	//Output 2: this_is_valid
	printf("%s", valid); 
	return 0;
}

3-2 Read in a line:

getline(cin, line);

Regarding the return value of geline(): getline (cin, stri) returns cin, CIN is converted to bool value true (read success) or false (read failure), see Example 4.-

4. Stream Input and Output: Control Format

4-1 sprintf() and sscanf():

1. sscanf(): can achieve multiple segmentation at one time, such as:

/*Write the contents of character array STR in the format of "% d:%lf,%s" to int variable n, double variable db, char array variable str2.*/
int main(){
	string line;
	int father, child;
	//Input: 12 is the father of 3 
	getline(cin, line);
	sscanf(line.c_str(), "%d is the father of %d", &father, &child);
	//Output: father:12 child:3
	cout << "father:" << father << " child:" << child;
	return 0;
}

1. Format conversion (and format control), such as:

sscanf(str, "%lf", &tmp);
//Write str to tmp in floating point form. If the format does not match, do not write
sprintf(str, "%.2f", tmp);
//Write tmp (floating point form) into str in. 2f format. If tmp does not match. 2f format, such as char tmp []: output 0.00, but instead of "% d", output 7405072 does not know what the principle is.

4-2 stringstream:

• Effect:

1. For example, read in a sentence: think of a word, a word processing (divided by spaces): 3 is my favorite number

2. Want to process a row of data in a unified way (e. g. 1234............................................ And, average and so on)

#include <iostream>
#include <string>
#include <sstream>
using namespace std;

int main() {
  string line;
  //Input: 3 is my favorite number
  while(getline(cin,line)){	//Note 1: See 3-2 
    string x;
    stringstream ss(line);
    while(ss>>x) {
        //Take one word in each round: 3, is, my...
    	cout << x;
	}
  }
  return 0;
} 

• About String: Details can be consulted: String Summary