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Converting Strings to Numbers

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Java Programming: Solving Problems with Software

듀크대학교

4.6(2,740개의 평가) | 120K명의 학생이 등록함

Object Oriented Programming in Java 전문 분야에서 4의 강좌 1

Learn to code in Java and improve your programming and problem-solving skills. You will learn to design algorithms as well as develop and debug programs. Using custom open-source classes, you will write programs that access and transform images, websites, and other types of data. At the end of the course you will build a program that determines the popularity of different baby names in the US over time by analyzing comma separated value (CSV) files. After completing this course you will be able to: 1. Edit, compile, and run a Java program; 2. Use conditionals and loops in a Java program; 3. Use Java API documentation in writing programs. 4. Debug a Java program using the scientific method; 5. Write a Java method to solve a specific problem; 6. Develop a set of test cases as part of developing a program; 7. Create a class with multiple methods that work together to solve a problem; and 8. Use divide-and-conquer design techniques for a program that uses multiple methods.

강의 계획서 보기

배우게 될 기술

Algorithms, Problem Solving, String (Computer Science), Java Programming

검토

4.6(2,740개의 평가)

5 stars
1,995 ratings
4 stars
541 ratings
3 stars
111 ratings
2 stars
33 ratings
1 star
60 ratings

DN

Aug 08, 2016

You need a little more experience in coding that just the first course, or try not to leave everything by the end of the week(as I did) to find what you need for the assignments. Great course !

JS

May 19, 2019

Really practical course content with great tutorials. The programming assignments are fun and challenging and deal with real world data and problems which makes the course all the more useful!

수업에서

CSV Files and Basic Statistics in Java

A common format for storing tabular data (any data organized into columns and rows) is in comma separated values (CSV) files. In this module, you will learn how to analyze and manipulate data from multiple CSV data files using a powerful open-source software package: Apache Commons CSV. Using this library will empower you to solve problems that could prove too complex to solve with a spreadsheet. By the end of this module, you will be able to: (1) Use the open-source Apache Commons CSV package in your own Java programs; (2) Access data from one or many CSV files using Java; (3) Convert strings into numbers; (4) Understand how to use “null” in Java programs (when you want to represent “nothing”); (5) Devise an algorithm (and implement in Java) to answer questions about CSV data; and (6) Analyze CSV data across multiple CSV files (for example, find maximums, minimums, averages, and other simple statistical results).

Hottest Day in a Year: Comma Separated Values2:29

Converting Strings to Numbers4:27

Maximum Temperature: Developing an Algorithm5:13

Java for Nothing—null: When You Don't Have an Object4:17

Maximum Temperature: Translating into Code4:42

Maximum Temperature: Testing Code3:33

Maximum Temperature from Multiple Datasets5:51

Maximum Temperature Refactored4:07

CSVMax: Summary0:41

강사:

Owen Astrachan
Professor of the Practice
Robert Duvall
Lecturer
Andrew D. Hilton
Associate Professor of the Practice
Susan H. Rodger
Professor of the Practice

스크립트

Welcome back.

As you work with data in CSV files you may encounter numerical data such as this.

Here we have a variety of numbers such as years.

However, the libraries you use to read a CSV file will read this data in as

a string, the sequence of characters 1, 4, 9, 3, not the integer 1,493.

If you want to manipulate this data numerically adding values,

finding the largest, or some other task,

you will want to work with the data as an int or a double.

So what would happen if you tried to write a line of code like this,

int value = row.get("year");?

When you try to compile it, you would get an error message here,

that the types are incompatible.

You're trying to assign a string to an int.

Let us look a bit more at this error.

If you look at the documentation for

the CSV reader, which we show here, you will see information about the get method.

In particular, you can see that its return type is string.

Because that method returns a string,

this entire expression, row.get("year") has type string.

Java knows that it will be a sequence of characters.

However, that is not the same type as int, which is the type we declared value to be.

This raises two questions we will answer for you.

First, why is this a problem?

Sometimes Java can automatically convert between two types, so

why can't it convert a string to an int?

It seems like if we have the string 1493 Java could just figure out that

that should be the number 1493.

However, it cannot do this in this case.

The second question is, of course, how do you fix it?

To see why Java cannot automatically convert a string to an int consider what

would happen if the string did not have numerical digits in it.

For example, suppose our CSV reader read the string hello instead.

What would you expect Java to do for this line of code?

The rules of Java require the compiler to reject this line of code because there

is no guarantee that you can meaningfully turn any string into a number.

Another consideration is that the string 1493 has a very

different representation from the number 1493.

Even though everything is a number, the type of a piece of data describes how

that number is represented and interpreted.

We will not delve into the details of the representational differences here but

it takes some work to convert between the two.

Accordingly we would have to execute some code to perform the conversion algorithm.

Such an algorithm would examine the digits in the string and

compute the integer that they represent.

We are not going to write this algorithm ourselves,

because it's already built into Java.

You can just call Integer.parseInt to convert a string to a number.

Even though it's built in, you still have to call it explicitly to tell Java that

that is what you want to do.

Here you can see an example of how you can use the Integer.parseInt method.

You pass in the string you want to convert and it returns the integer value.

If you need to work with numbers that have fractional parts,

you can use Double.parseDouble, which will accept a string like 42.56 and

convert it into the corresponding value of type double.

The type you use to work with non-integer numbers.

For either of these methods, the string could be invalid.

It could be something that does not represent a number.

For example, if you passed hello to Integer.parseInt,

the method cannot turn hello into an integer.

So what happens?

The method throws an exception, which indicates that an error has occurred.

If your program throws an exception, then it will crash.

There are ways to make your program handle the exceptions,

specifying what to do instead of crashing.

Well, we are not going to go into that more advanced topic here.

So, now you know that you cannot just assign a string to an int.

And that if you need to convert a string to an int,

you should use Integer.parseInt.

Likewise, for real numbers you have learned to use Double.parseDouble.

Now you are ready to manipulate CSV data with numbers in it.

Thank you.

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