C++ Programming (Loops and Files) Notes

Increment and Decrement Operators

• Increment Operator (++)

• Function: Adds one to a variable.

• Examples:

  • val++; is equivalent to val = val + 1;
  • Can be used in prefix form ++val; or postfix form val++;.

• **Decrement Operator (--)

• Function: Subtracts one from a variable.

• Examples:

  • val--; is equivalent to val = val - 1;
  • Can also be used in prefix form --val; or postfix form val--;.

Usage in Programming

• The following program demonstrates usage of increment and decrement operators:

  #include <iostream>
  using namespace std;
  int main() {
      int num = 4;
      cout << "The variable num is " << num << endl;
      cout << "I will now increment num. \n\n";
      num++;  // Postfix increment
      cout << "Now the variable num is " << num << endl;
      ++num;  // Prefix increment
      cout << "Now the variable num is " << num << endl;
      num--;  // Postfix decrement
      cout << "Now the variable num is " << num << endl;
      --num;  // Prefix decrement
      cout << "Now the variable num is " << num << endl;
      return 0;
  }

• Output:
  • The variable num is 4
  • Now the variable num is 5
  • Now the variable num is 6
  • Now the variable num is 5
  • Now the variable num is 4

Prefix vs Postfix

• Prefix Mode (e.g. ++val)
• Increments/decrements the value, then returns it.
• Postfix Mode (e.g. val++)
• Returns the value, then increments/decrements.
• Example:

  int val = 12;
  cout << val++;  // Outputs 12; val becomes 13
  cout << ++val;  // val becomes 14; outputs 14

Introduction to Loops

• Loop Definition: A control structure that repeats a set of statements.
• Types of Loops:
• while loop: repeats as long as a condition is true.
• do-while loop: executes at least once, then checks condition.
• for loop: typically used for counter-controlled iterations.

The while Loop

• Structure:

  while (expression) { statement; }

• Execution Flow:

1. Evaluate expression.
2. If true, execute statement; repeat.
3. If false, exit loop.

• Example:

  int number = 1;
  while (number <= 5) {
      cout << "Hello\n";
      number++;
  }

• Output: Prints "Hello" 5 times.

Input Validation with while Loops

• Purpose: Ensures user input meets specific criteria before proceeding.
• Approach:

  while (input is invalid) {
      display error;
      read input again;
  }

• Example:

  cout << "Enter a number less than 10: ";
  cin >> number;
  while (number >= 10) {
      cout << "Invalid Entry!\n";
      cout << "Enter a number less than 10: ";
      cin >> number;
  }

Counters

• Counter Definition: A variable that is incremented or decremented with each loop iteration. It controls the number of loop executions.
• Initialization: Must be initialized before entering the loop.

The do-while Loop

• Structure:

  do statement; while (expression);

• Execution: Code executes first, then checks condition. Always executes at least once.

• Example:

  do {
      cout << "Hello\n";
  } while (number < 0);

The for Loop

• Structure:

  for (initialization; test; update) { statement; }

• When to use: When you know exactly how many iterations are needed.

• Example:

  for (int count = 1; count <= 5; count++) {
      cout << "Hello" << endl;
  }

Breaking and Continuing Loops

• break Statement: Immediately terminates the loop. Use cautiously as it may complicate code readability.
• continue Statement: Skips the current iteration and proceeds to the next. Use cautiously for similar reasons.