Code from lecture

https://github.com/ucsb-cs16-f18-mirza/cs16-f18-lectures/tree/master/lec-03

Topics

Boolean Expressions

An expression that evaluates to either true or false.
You can build boolean expressions with relational operators comparing values:

==  // true if two values are equivalent
!=  // true if two values are not equivalent
< // true if left value is less than the right value
<=  // true if left value is less than OR EQUAL to the right value
> // true if left value is greater than the right value
>=  // true if left value is greater than OR EQUAL to the right value

Integer values can be used as boolean values
- C++ will treat the number 0 as false and any non-zero number as true.

bool x = 5 == 1;  // x = 0
bool x = 3 != 2;  // x = 1

Combine boolean expressions using Logical Operators

!   // inverts true to false or false to true
&&  // boolean AND
||  // boolean OR

Example

bool x = true;
bool y = true;
x = !x;     // x = false
x = x && y    // x = false
x = x || y    // x = true

Control Structures

Boolean expressions are fundamental pieces that provide control flow within your program.

If-else statements

Ability to execute two alternative blocks of C++ statements based on the value of a boolean expression.

if (BOOLEAN_EXPRESSION) {
  // code1
} else {
  // code2
}

If the BOOLEAN_EXPRESSION evaluates to true, then code1 is executed. Otherwise code2 is executed.
Example

int x = 4;
if ((x > 3) && (x < 6)) {
  cout << “x is either 4 or 5” << endl;
} else {
  cout << “x is not 4 or 5” << endl;
}

Notice the “{ … }” . Also known as block statements.
- This allows many statements to be executed.
- Without “{ … }”, only the following statement will be executed, and other statements are considered outside the block statement.
- Example

int x = 4;
if ((x > 3) && (x < 6))
  cout << “x is either 4 or 5” << endl;
else
  cout << “x is not 4 or 5” << endl;
// Will have the same output as the last statement.

int x = 6;
if ((x > 3) && (x < 6))
  cout << “1” << endl;
  cout << “2” << endl; // outside if block
  cout << “3” << endl; // outside if block

The last two statements will always execute because it’s considered outside of the code block.
A syntax error will appear if you try to insert an “else” after the statements since “else” can only be used after an if code block.

Multi-way If-else Statements

Programs may require more than simply two paths of code execution.
Multiple if-else statements can allow the program to execute many branches.
Example

int x = 3;
if (x == 1)
  cout << “x equals 1” << endl;
else if (x == 2)
  cout << “x equals 2” << endl;
else if (x == 3)
  cout << “x equals 3” << endl;
else
  cout << “x does not equal 1, 2, or 3” << endl;

User Input

Example of interacting with the console using the cin function

#include <iostream>
#include <string>

using namespace std;

int main() {  
// Example receiving a string from the user
  string name;
  cout << "What is your name? ";
  cin >> name;
  cout << "Hello " << name << endl;

  // Example receiving a number from the user
  int i;
  cout << "Enter a number: ";
  cin >> i;
  cout << "The number entered is " << i << endl;

  cout << i / 2 << endl; // what value / type is this if i == 11? 
}

Example of using command line arguments

We can pass information into a C++ program through the command line when executing the program.
Note: You may have to convert command line argument information into the proper type (i.e. convert it to an integer type) if necessary.
The main function will need to have the following:

int main(int argc, char *argv[]) {

int argc is the number of “arguments” the program has, including the executable name.
char* argv[] is the “list” of arguments passed into the program.
- Don’t worry if the syntax doesn’t make too much sense for now, we’ll cover this later in the quarter.

#include <iostream>
#include <cstdlib>

using namespace std;

int main(int argc, char *argv[]) {

  cout << "Number of arguments: " << argc << endl;

  cout << argv[0] << endl;
  cout << argv[1] << endl;
  cout << argv[2] << endl;

  // how to use these arguments as numbers?
  // We can convert them using the atoi function
  // in the cstdlib standard library

  int x = atoi(argv[1]) + atoi(argv[2]);
  cout << x << endl;
  return 0;
}

Code from lecture

Topics

Boolean Expressions

Control Structures

If-else statements

Multi-way If-else Statements

User Input

Example of interacting with the console using the cin function

Example of using command line arguments

In class demo: Fizzbuzz!