CS 101 – Oct. 5 Computer problem solving Chapter 6: read pp Problem-solving procedure Structure of a solution Examples!

Review: what is CS ? The study of how we … –Solve problems –Represent information In problem solving, we’d like: –Find systematic ways of going about solution –Correct, quick and informative solutions. Information can be: –Different types: numbers, text, images, sound, the software itself –The input and output to a computer program

Why create software? Computational power –Excel is limited to so many rows/columns –Can convert data to images –Can create games and useful applications –With one “language”, can solve virtually any problem. Built-in features of languages –Many common calculations are pre-defined for you such as sorting, opening files, surfing the Web, creating a button, etc.

Software Powerful! –We get to tell the machine exactly what we want. –Sometimes, existing programs like Excel or Photoshop are not enough. Program = sequence of instructions for CPU to obey. –Works like a recipe. –A recipe has: ingredients, steps, and result is food! –A program has: input, calculations, output. When we start to look at programs, be on the lookout for these 3 parts.

Recipes Cooking may be a good analogy, because it’s solving a specific problem “I’m hungry.” What do we see in recipes? Here’s one: –Brown the beef 15 min. Drain grease. –Dice carrot, celery, onion (aka “mirepoix”) –Cut up and boil 6 potatoes until soft. –Mash potatoes –Add flour, spices, sauce, mirepoix to beef. –Put meat mixture into casserole, top with potatoes. –Bake in oven at 400 for 30 minutes.

Recipes (2) A computer program has some of the same elements as a recipe… In recipes we see: –Ingredients (the “nouns” of the problem) –Steps to perform (the “verbs”) –In some steps, we continue/wait for something –Although it’s not obvious, sometimes we check things: Are potatoes fully mashed? Should I add more _____ to the mixture?

Recipes (3) But we don’t eat the same stuff every day. Once we know a few recipes, we can put together a menu for choices. if (have all ingredients), make Shepherd’s pie. if (no potatoes), just make soup instead. if (no veggies), make hamburger. if (no beef), make pasta. When you view a whole menu as a program, then “making soup” becomes a sub-program. –A large program is composed of several parts. –In industry, sometimes each part implemented by different people. A kitchen may have many chefs.

Problem-solving 1.Understand problem; inputs and outputs 2.Write solution in English “pseudocode” 3.Write code in a programming language 4.Compile 5.Run and test When program works, can refine.

Problems The earliest problems given to a computer were mathematical. Sometimes there is no clean formula –Many equations can’t be solved analytically. For example, cos(x) = x. Need to solve numerically. –Ex. Heat equation is a partial differential equation (PDE). Most PDEs have to be solved numerically. –Ex. Calculating a square root. And even if there is a clean formula, a computer can help you automate the calculations.

Problems (2) “Mathematical” problems may at first sound boring. But they include many useful applications –Ex. Finding directions Other kinds of problems for the computer –Games –Record keeping, managing investments, … –Networking, communication –Multimedia (e.g. image processing) –Of course, much more!