[Chp 4] Control Stuctures I

This chapter goes through SELECTION CONTROL STRUCTURES.

[1] Control Structures

3 Ways for a Program to Execute:

  1. Sequence - default, from top to bottom

  2. Selection - choose a path (if / switch) ← lesson this week

  3. Repetition - loops

[2] Logical Expressions

A logical expression evaluates to:

true (1)
false (0)
// Example:
8 > 3
// -> true

[3] Relational Operators

These compare values:

⭐️ NOTE: These two are different

  • This is a common error

if (x = 5)
// This assigns 5 to x 
// Evaluates true because 5 ≠ 0, so it always runs
// This is a logic error, so be attentive

[4] Comparing char

Characters are compared using ASCII values 

'A' < 'B'  // true
'a' > 'A'  // true

⭐️ Lowercase letters have higher ASCII values

  • Uppercase Letters (A-Z): The ASCII values range from 65 to 90.

    • Ex. 'A' is 65, 'B' is 66, and so on up to 'Z' which is 90.

  • Lowercase Letters (a-z): The ASCII values range from 97 to 122.

    • Ex. 'a' is 97, 'b' is 98, and so on up to 'z' which is 122.

  • Case Difference: A lowercase letter's ASCII value is exactly 32 greater than its corresponding uppercase letter's value

[5] Comparing string

Strings compare character by character.

  • When == is used, it checks for equality and if the string is the same length

  • When >, < or compare() is used, string is compared using lexicographical comparison, going by character and referencing ASCII values

"apple" < "banana"
// a vs b -> a (value 97) < b (value 98) -> true
If "cat" < "caterpillar" 
//Shorter string is smaller

[6] One-Way Selection (‘If’ Statements)

if (age >= 18) 
    cout << "Adult";

  • If the condition is false → skip.

[7] Two-Way Selection

if (condition)
    statement1;
else
    statement2;
// Example
if (x > y)
    cout  << "x is bigger";
else
    cout << "y is bigger";

  • If condition is true, statement1 is outputted, if not, else (statement2) is outputted.

[8] Compound Statements (Blocks)

If you want multiple statements

if (age >= 18)
{
    cout << "Adult" << endl;
    cout << "Eligible to vote";
}

  • Without braces, only ONE statement belongs to if

[9] Dangling Else Problem

if (v1 >= v2)
   if (v1 == v2)
      cout << "equal";
else
   cout << "v2 larger";
// Based on this, you don't know whether the else belongs to the first or second if
//Answer: It belongs to the closest unmatched if

// Instead:
if (v1 >= v2)
{
    if (v1 == v2)
        cout << "equal";
}
else
    cout << "v2 larger";

  • That is why, ALWAYS use braces

[10] bool Data Type

bool isValid = true
// Internally 
// true = 1
// false = 0
bool result = (5 > 3)

[11] Logical Operators (Boolean Operators)

  • AND (&&)

// Syntax
condition1 && condition2

⭐️ both condtions must be true to return true, or else retruned false

  • OR (||)

condition1 || condtion2

⭐️ at least one condition must be true, to return true

  • NOT (!) 

!condition

⭐️ reverses the truth

further explainations from almighty GOOGLE

[12] Short-Circuit Evaluation

  1. AND 

expr1 && expr2

  • If expr1 if false → expr2 is NOT evaluated

  1. OR 

expr1 || expr2

  • If expr1 is true → expr2 is NOT evalutated

    • Prevents crashes like division by zero

//Example
if (x != 0 || y/x > 2)
// If x is 0
//second part is never evaluated

[13] Operator Precedence

  1. NOT !

  2. relational operators

  3. AND &&

  4. OR ||

  • Left to right associativity

[14] Formatting Errors

if (0 <= num <= 10)
// cpp evaluates left to right;
// 0 <= num
// returns 0 or 1 
// 0/1 <= 10 
// returns true because 0 or 1 is always LESS than 10

// Correct Formatting
if (num >= 0 && num <= 10)

[15] Conditional Operator (?:)

  • Also know as ternary operator

// Syntax
condition ? expr1 : expr2;

// Example
max = (a > b) ? a : b;
// Means:
// If a > b -> max = a
// Else -> max = b

  • Essentially a short, concise form of if-else statements

    • Are ideal for simple, one-line conditional assignments

[16] switch Statement

  • Alternatives to if-else statements but use a a specific set of data type 

switch (expression)
{
   case value1:
      statements;
      break;

   case value2:
      statements;
      break;

   default:        // Executes if no match is found
      statements;
}

⭐️ Expressions MUST be:

  • int

  • char

  • enum

CANNOT be double, string

[17] switch Execution Rules

  • Expression evaluated: switch expression is evaluated once, and its result is compared to each case value

  • Case Matching: The program jumps to the first case cluase that matches the expression’s value

  • Continues until break: Used to exit the switch block immediately after a case executes

[18] Fall Through

If break is missing:

case 1: 
    cout << "One";
case 2:
    cout << "Two";
// If value is 1, result is OneTwo

  • Fall-Through Behavior: If a break statement is omitted, execution proceeds to the next case clause, even if that case does not match the expression

[19] break

Ends:

  • switch

  • loop

After break → exits out

Break should be after every statement

[20] assert()

  • Used to stop program if condition false

include <cassert>

assert( x != 0);

  • If false → the program terminates

  • Used during debugging