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content/courses/prog-intro/homeworks/sum/SumHex/_index.md
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content/courses/prog-intro/homeworks/sum/SumHex/_index.md
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---
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title: SumHex
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weight: 6
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---
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# Task
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- The input is in decimal and hexadecimal numbers
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- Hexademical numbers has the prefix `0x`
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- The input is case insensitive
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- Class should be named `SumHex`
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## Solution
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This modification doesn't seem too complicated either.
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Let's modify our initial `Sum.java` and apply all our improvements from a previous modification. (A good exercise would be to try doing that yourself and then comeback to check)
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```java
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// SumHex.java
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public class SumHex {
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public static void main(String[] args) {
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int sum = 0;
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for (String argument : args) {
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StringBuilder number = new StringBuilder();
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for (char c : argument.toCharArray()) {
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if (!Character.isWhitespace(c)) {
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number.append(c);
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} else {
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sum += parseNumber(number);
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number = new StringBuilder();
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}
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}
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sum += parseNumber(number);
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}
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System.out.println(sum);
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}
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static int parseNumber(StringBuilder number) {
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return number.isEmpty() ? 0 : Integer.parseInt(number.toString());
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}
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}
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```
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> [!NOTE]
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> The importance of these changes, is that in all further modifications we will only be changing our parseNumber method and leaving `main` method almost unchanged. The only modifications to `main` would be changing the data type of `sum`.
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Now `parseNumber` thinks that every number is decimal, but we don't know what to do with hexadecimal numbers. Let's try that
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```bash
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$ java SumHex 0x1 0x2 0x3
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Exception in thread "main" java.lang.NumberFormatException: For input string: "0x1"
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at java.base/java.lang.NumberFormatException.forInputString(NumberFormatException.java:67)
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at java.base/java.lang.Integer.parseInt(Integer.java:662)
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at java.base/java.lang.Integer.parseInt(Integer.java:778)
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at SumHex.parseNumber(SumHex.java:25)
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at SumHex.main(SumHex.java:18)
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```
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So we need to split decimal and hexadecimal processing inside `parseNumber`. Our code would look something like this.
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```java
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static int parseNumber(StringBuilder number) {
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if (!number.isEmpty()) {
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if (/* number is hexadecimal */) {
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return /* parse hexadecimal */;
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} else { // if the number if decimal
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return Integer.parseInt(number.toString());
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}
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}
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return 0;
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}
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```
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Here we have 2 major questions:
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- How to check if the number is hexadecimal?
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- How to parse the hexadecimal number?
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We can answer the 1st question, because we know that hexadecimal number starts with `"0x"`. And we know that the input is not case sensitive, so `"0X"` is also a valid option.
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So we can change our *pseude-code* like this
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```java
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static int parseNumber(StringBuilder number) {
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if (!number.isEmpty()) {
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if (/* number starts with "0x" or "0X" */) {
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return /* parse hexadecimal */;
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} else { // if the number is decimal
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return Integer.parseInt(number.toString());
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}
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}
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return 0;
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}
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```
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But how to check if the string starts with some prefix? There is a [`String.startsWith(String prefix)`](https://docs.oracle.com/javase/8/docs/api/java/lang/String.html#startsWith-java.lang.String-) that returns `true` if the string starts with `prefix` and `false` otherwise.
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So we can change our code using this information.
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```java
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static int parseNumber(StringBuilder number) {
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if (!number.isEmpty()) {
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String numberString = number.toString();
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if (
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numberString.startsWith("0x") ||
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numberString.startsWith("0X")
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) {
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return /* parse hexadecimal */;
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} else { // if the number is decimal
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return Integer.parseInt(number.toString());
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}
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}
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return 0;
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}
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```
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> [!NOTE]
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> `||` here is a logical ***or***.
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We still have one problem. How to parse the hexadecimal number?
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Using [`Integer.parseInt()`](https://docs.oracle.com/javase/8/docs/api/java/lang/Integer.html#parseInt-java.lang.String-int-) method. It turnes out, that it has the second argument `radix` where we can specify the base of the numerical system (in our case it's `16`).
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So our code will look like this.
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```java
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static int parseNumber(StringBuilder number) {
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if (number.isEmpty()) {
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String numberString = number.toString();
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if (
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numberString.startsWith("0x") ||
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numberString.startsWith("0X")
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) {
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return Integer.parseInt(numberString.substring(2), 16);
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} else {
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return Integer.parseInt(numberString);
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}
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}
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return 0;
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}
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```
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> [!NOTE]
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> Here we used [`String.substring(int beginIndex)`](https://docs.oracle.com/javase/8/docs/api/java/lang/String.html#substring-int-) method to cut off first two symbols (`"0x"`/`"0X"`) in order for `parseInt` to parse number correctly and not give us exceptions. In our case `beginIndex = 2` because it's the first index of the substring that we want to get. Remember that ***indexing starts with 0*** (not 1).
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> [!IMPORTANT]
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> It's important to know that we can't store all hexadecimal numbers in `int` data type. Let's look at this example
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> ```bash
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> $ java SumHex 0x80000000
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> Exception in thread "main" java.lang.NumberFormatException: For input string: "80000000" under radix 16
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> at java.base/java.lang.NumberFormatException.forInputString(NumberFormatException.java:67)
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> at java.base/java.lang.Integer.parseInt(Integer.java:662)
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> at SumHex.parseNumber(SumHex.java:30)
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> at SumHex.main(SumHex.java:18)
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> ```
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> That's why we will need to use `long` data type for storing hexademical values.
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Here's the updated code
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```java
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static long parseNumber(StringBuilder number) { // int -> long
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if (!number.isEmpty()) {
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String numberString = number.toString();
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if (numberString.startsWith("0x") || numberString.startsWith("0X")) {
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// Integer.parseInt -> Long.parseLong
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return Long.parseLong(numberString.substring(2), 16);
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} else {
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return Integer.parseInt(numberString);
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}
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} else {
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return 0;
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}
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}
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```
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Let's present the full solution
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```java
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// SumHex.java
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public class SumHex {
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public static void main(String[] args) {
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int sum = 0;
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for (String argument : args) {
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StringBuilder number = new StringBuilder();
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for (char c : argument.toCharArray()) {
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if (!Character.isWhitespace(c)) {
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number.append(c);
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} else {
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sum += parseNumber(number);
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number = new StringBuilder();
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}
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}
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sum += parseNumber(number);
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}
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System.out.println(sum);
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}
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static long parseNumber(StringBuilder number) {
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if (!number.isEmpty()) {
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String numberString = number.toString();
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if (numberString.startsWith("0x") || numberString.startsWith("0X")) {
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return Long.parseLong(numberString.substring(2), 16);
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} else {
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return Integer.parseInt(numberString);
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}
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} else {
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return 0;
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}
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}
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}
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```
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> [!IMPORTANT]
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> Notice, that we didn't change `main` method at all, because we have a separate function handling parsing. That's a good sign because our `main` method doesn't know what number parsing algorithm is used and therefore not dependent on it.
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