IO

Objectives

Your Objectives:

• Write input routines for three kinds of test inputs,
• use ‘cin‘, ‘scanf‘, and ‘printf‘ properly for various types of variables, and
• write code for interactive tests.

Input Scenario: Explicit Case Count

• Often the input tells you explicitly how many test cases you will have.
• Example in C, C++

#include <stdio.h>
int main() {
  int cases,x,y;
  scanf("%d",&cases);
  while (cases>0) {
    --cases;
    scanf("%d %d",&x,&y);
    printf("%d\n",x+y);
  }
}

#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int cases,x,y;
  cin >> cases;
  while (cases>0) {
    --cases;
    cin >> x >> y;
    cout << x + y << "\n";
  }

Input Scenario: Termination Marker

• Simple. Just check for the marker and break.

int main() {
  int x,y;
  while (1) {
    scanf("%d %d",&x,&y);
    if (x==-1 && y==-1)
      break;
    printf("%d\n",x+y);
  }
}

#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int x,y;
  while (1) {
    cin >> x >> y;
    if (x==-1 && y==-1)
      break;
    cout << x + y << "\n";
  }
}

Test within the while loop

int main() {
  int x,y;
  while (scanf("%d %d",&x,&y) && x != 1 && y != 1) {
    printf("%d\n",x+y);
  }
}

#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int x,y;
  while (cin >> x >> y && x != 1 && y != 1) {
    cout << x + y << "\n";
  }
}

Input Scenario: EOF

• Two ways: test for EOF explicitly, or test for failure to read.

int main() {
  int x,y;
  while (scanf("%d %d",&x,&y) != EOF) {}
    printf("%d\n",x+y);
  }
}

#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int x,y;
  while (cin >> x >> y) {
    cout << x + y << "\n";
  }
}

Why use scanf and printf?

• There are problems that TLE if you use cin and cout.
• This happens if the problem requires a lot of output.
• Use ios_base::sync_with_stdio(false); and cin.tie(NULL); to prevent flushing the output. (Maybe put that in your TRD!)
• Similarly, endl will force-flush the output. Use \n instead.
• scanf has some regular-expression like features that can be useful.

• On the other hand, you must match the type and pass in a reference.

  Code	Meaning
  %d	Scan an integer
  %lld	Scan a long long integer
  %s	Scan a string
  %c	Scan a character

Spaces and such…

• Literal Characters

// will read "(10,20)"
scanf("(%d,%d)");

• Spaces

// will read "(10,20)", "( 10, 20 )", but not "(10 ,20)"
scanf(" ( %d, %d )");

• A binary followed by vowels

// will read "110101 eieio"
scanf("%[01] %[aeiou]");

Getting a whole line

• fgets (name, 100, stdin); will read a whole line into the string.
• getline(cin, name);
• getline(cin >> std::ws, name); to read leading whitespace first.
• Sometime you will need to parse out the line yourself afterward; this can be tricky. Avoid it if possible.

Setting number of digits.

• For printf codes:
  • %d outputs an integer
  • %5d outputs an integer, using 5 characters, leading spaces.
  • %05d outputs an integer, using 5 characters, leading zeros.
• Similar codes exist for floating point.

int n = 25;
printf("%d, %5d, %05d\n",n,n,n);
Output: 25, 25, 00025

• For cout, use e.g., cout << width(5) << n << "\n";

Interactive Tests

• Not common yet, but ICPC is starting to use them.
• Opposite advice now applies: call flush(stdout) every time you print, or else use endl with cout.

#include <stdio.h>
int main() {
  int x,y;
  while (scanf("%d %d",&x,&y) != EOF) {
    if (x==-1 && y==-1)
      break;
    printf("%d\n",x+y);
    fflush(stdout);
  }
}