nyu-daniel-zint / intro-to-computer-science-2024-spring / branching

class: center, middle # Branching --- # Agenda 1. [Overview](#concept) 1. [Flow Charts](#flow-charts) 1. [If/Else if/Else](#ifs) 1. [Switch/Case](#switch-case) 1. [The Ternary Operator](#ternary) 1. [Conclusions](#conclusions) --- name: concept # Overview -- ## Concept The flow of a program can be controlled such that it deviates from its '_normal_' path. -- - What is a program's 'normal' path? -- - By default, a program will execute statements in the order written, from top to bottom. -- - Some branches are _unconditional_ - the control will always break out of this sequential flow every time the program is executed. -- - Other branches are _conditional_ - certain contextual conditions must be met if the control of the program is to break out of its usual flow. -- - Conditional branches are our focus here. --- name: flow-charts # Flow Charts -- ## Concept Understanding the flow of a program's control can be visualized easily with diagrams, irrespective of the programming language and code. -- - Writing code is precarious, laborious, and error-prone. -- - Creating such flow diagrams can help understand and communicate the problem being solved without such burden. -- - The [Unified Modeling Language](https://en.wikipedia.org/wiki/Activity_diagram) (UML) has attempted to define the "Activity Diagram" - a standard for how such diagrams should be drawn. -- - In practice, people often use parts of the UML Activity Diagram style mixed with their own personal styles. --- template: flow-charts name: flow-charts-6 ## Example A virtual card game program randomly picks two cards, from 1 to 11. - If those two cards add up to 21, the program outputs the text "Blackjack!" - Otherwise, the program outputs the text, "Maybe next time!" --- template: flow-charts name: flow-charts-7 ## Example  --- template: flow-charts name: activity-diagrams ## UML Activity Diagram Rules UML Activity Diagrams follow a few basic rules: -- - program control starts at the top of the diagram and ends at the bottom. -- - the start point is indicated with just filled circle, the end with a filled circle with a line around it -- - any process is indicated with an ellipse (or a rounded rectangle) -- - if any process being diagrammed is so complex it makes the diagram complex and difficult to understand, make a separate activity diagram for that sub-process, and represent it as a single process in this diagram. -- - any decision point leading to more than one branch is indicated with a diamond shape (also called a rhombus) -- - branches should have simple annotations indicating in what decision context they are followed --- name: ifs # If/Else if/Else -- ## Isolated ifs _if_ statements on their own either execute a particular branch or not. ```java int card1 = (int) (Math.random() * 11) + 1; // number from 1 to 11 int card2 = (int) (Math.random() * 11) + 1; // number from 1 to 11 if (card1 + card2 == 21) { // if the cards add up to 21... System.out.println("Blackjack!"); } ``` --- template: ifs name: ifs-2 ## Binary choice _if_ statements followed by _else_ blocks can have control flow into one of two branches, depending on a single condition being met or not. ```java int card1 = (int) (Math.random() * 11) + 1; // number from 1 to 11 int card2 = (int) (Math.random() * 11) + 1; // number from 1 to 11 if (card1 + card2 == 21) { // if the cards add up to 21... System.out.println("Blackjack!"); } else { // otherwise... System.out.println("Maybe next time!"); } ``` --- template: ifs name: ifs-3 ## n-ary _if_ statements followed by one or more _else if_ blocks can have control flow into one of several different branches, depending on multiple conditions. ```java int card1 = (int) (Math.random() * 11) + 1; // number from 1 to 11 int card2 = (int) (Math.random() * 11) + 1; // number from 1 to 11 if (card1 + card2 == 21) { // if the cards add up to 21... System.out.println("Blackjack!"); } else if (card1 == 1 && card2 == 1) { // otherwise if the cards are both ones... System.out.println("Snake eyes!"); } else { // otherwise... System.out.println("Maybe next time!"); } ``` --- template: ifs name: ifs-4 ## UML Activity Diagram  --- name: switch-case # Switch/Case -- ## Concept In Java, an alternative to if/else if/else statements is switch/case statements. -- - these can be easier to read and write in particular situations. -- - such as when the branch followed depends upon the value of a single variable -- - and when multiple conditions lead to the same branch --- template: switch-case name: switch-case-5 ## Example Imagine a program that recommends how to dress.  --- template: switch-case name: switch-case-6 ## Not using switch / case This could be handled by if/else if/else statements with some boolean operators thrown in. ```java Scanner scn = new Scanner(System.in); String weather = scn.nextLine(); if ( weather.equals("very sunny") || weather.equals("raining") ) { // if it's raining or very sunny... System.out.println("Take an umbrella"); } else if (weather.equals("snowing") ) { // otherwise, if it's snowing... System.out.println("Wear a hat!"); } else { // otherwise... System.out.println("Good luck!"); } ``` --- template: switch-case name: switch-case-7 ## Using switch / case Using switch/case would remove the need for boolean operations and provide more readable code. ```java Scanner scn = new Scanner(System.in); String weather = scn.nextLine(); switch (weather) { case "raining": case "very sunny": // if it's raining or very sunny... System.out.println("Take an umbrella!"); break; case "snowing": // otherwise, if it's snowing... System.out.println("Wear a hat!"); break; default: // otherwise... System.out.println("Good luck!"); } ``` --- name: ternary # The Ternary Operator -- ## Concept It is often the case that we use if/else statements to control setting the value of a variable. ```javascript if (breakfastWasServed && breakfastWasEaten) { satiationLevel = "full" } else { satiationLevel = "hungry" } ``` -- The ternary operator, `?`, thankfully allows us to simplify such syntax: -- ```javascript satiationLevel = breakfastWasServed && breakfastWasEaten ? "full" : "hungry" ``` --- name: conclusions # Conclusions -- You now have a basic understanding of UML activity diagrams, if/else if/else statements, and switch/case statements.