Question

I’m solving a problem which requires very fast input/output. More precisely, the input data file will be up to 15MB. Is there a fast, way to read/print integer values.

Note: I don’t know if it helps, but the input file has the following form:

• line 1: a number n
• line 2..n+1: three numbers a,b,c;
• line n+2: a number r
• line n+3..n+4+r: four numbers a,b,c,d

Note 2: The input file will be stdin.

Edit: Something like the following isn’t fast enough:

void fast_scan(int &n) {
  char buffer[10];
  gets(buffer);
  n=atoi(buffer);
}

void fast_scan_three(int &a,int &b,int &c) {
  char buffval[3][20],buffer[60];
  gets(buffer);
  int n=strlen(buffer);
  int buffindex=0, curindex=0;
  for(int i=0; i<n; ++i) {
    if(!isdigit(buffer[i]) && !isspace(buffer[i]))break;
    if(isspace(buffer[i])) {
      buffindex++;
      curindex=0;
    } else {
      buffval[buffindex][curindex++]=buffer[i];
    }
  }
  a=atoi(buffval[0]);
  b=atoi(buffval[1]);
  c=atoi(buffval[2]);
}

Answer 1

General input/output optimization principle is to perform as less I/O operations as possible reading/writing as much data as possible.

So performance-aware solution typically looks like this:

1. Read all data from device into some buffer (using the principle mentioned above)
2. Process the data generating resulting data to some buffer (on place or another one)
3. Output results from buffer to device (using the principle mentioned above)

E.g. you could use std::basic_istream::read to input data by big chunks instead of doing it line by line. The similar idea with output – generate single string as result adding line feed symbols manually and output it at once.