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content/courses/prog-intro/homeworks/sum/ParseIntExample.class
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content/courses/prog-intro/homeworks/sum/ParseIntExample.class
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// ParseIntExample.java
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public class ParseIntExample {
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public static void main(String[] args) {
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String s = "123";
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int sum = 321;
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//! sum = sum + s;
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sum = sum + Integer.parseInt(s);
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System.out.println(sum);
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}
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}
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content/courses/prog-intro/homeworks/sum/Sum.java
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content/courses/prog-intro/homeworks/sum/Sum.java
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// Sum.java
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public class Sum {
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public static void main(String[] args) {
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System.out.println("number of arguments: " + args.length);
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int sum = 0;
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for (String argument : args) {
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sum = sum + argument;
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System.out.println(argument);
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System.out.println(sum);
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}
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System.out.println(sum);
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}
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}
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content/courses/prog-intro/homeworks/sum/_index.md
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content/courses/prog-intro/homeworks/sum/_index.md
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---
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title: Sum
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weight: 5
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---
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# Task
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1. You need to create a `Sum` class which will sum integers from command line arguments and output the sum to console.
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2. Examples:
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```sh
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java Sum 1 2 3
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```
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Expected output: `6`.
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```sh
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java Sum 1 2 -3
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```
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Expected output: `0`.
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```sh
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java Sum "1 2 3"
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```
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Expected output: `6`.
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```sh
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java Sum "1 2" " 3"
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```
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Expected output: `6`.
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```sh
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java Sum " "
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```
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Expected output: `0`.
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3. Arguments can be:
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- digits,
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- signes `+` and `-`,
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- [space symbols](https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/Character.html#isWhitespace(char))
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4. You can assume that `int` type is sufficient for in-between calculations and result.
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5. Before doing the task make sure to read docs for classes [`String`](https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/String.html) and [`Integer`](https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/Integer.html).
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6. For debugging use [`System.err`](https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/System.html#err), because it will be ingnored by the testing program.
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---
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# Solution
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After reading about [`String`](https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/String.html), [`Integer`](https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/Integer.html), [`System.err`](https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/System.html#err) we now know about some usefull methods:
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- [`Integer.parseInt(String s, int radix)`](https://docs.oracle.com/javase/8/docs/api/java/lang/Integer.html#parseInt-java.lang.String-) which parses the string argument as a signed integer in the radix specified by the second argument. So it basically converts a number from the `String` data type to `int`.
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- [`Character.isWhitespace(char ch)`](https://docs.oracle.com/javase/8/docs/api/java/lang/Character.html#isWhitespace-char-) which checks if a character `ch` is a space symbol or not.
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Now let's start coding. Firstly let's define the structure of our program. I will be putting the name of the file at the first line comment in a file and its path if its nessesary.
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```java
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// Sum.java
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public class Sum {
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public static void main(String[] args) {
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// ...
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}
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}
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```
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Okay that's done. What do we do next? Let's look at `String[] args` argument to our `main` method. It represents an array of command line arguments which we need to sum. So know we made our task a little bit easier. Now we can just say that we need to find sum of elements of array `args`.
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How are we going to do it though? First let's understand what we can do with this array. Let's modify our class a little bit.
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```java
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// Sum.java
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public class Sum {
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public static void main(String[] args) {
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System.out.println(args.length);
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}
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}
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```
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We've added `System.out.println(args.length)` which takes the field `length` from our `args` object and prints it to the console.
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Let's try it. First compile our class using
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```sh
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$ javac Sum.java
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```
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And then we can do some manual testing.
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```sh
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$ java Sum 1 2 3
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3
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```
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We got `3` as an output as expected. We gave our program 3 command line arguments: `1`, `2` and `3`.
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Here are all of the examples
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```sh
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$ java Sum 1 2 -3
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3
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```
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```sh
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$ java Sum "1 2 3"
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1
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```
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> [!NOTE]
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> Notice, that we got 1 instead of 3. That's because we put our arguments in `""` so this becomes a single string argument.
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```sh
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$ java Sum "1 2" " 3"
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2
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```
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```sh
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$ java Sum " "
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1
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```
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> [!NOTE]
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> Here program gives us 1 instead of 0, because despite not having any numbers in the arguments a single whitespace is still an argument.
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Now let's try not obly to count our arguments but to list them as well. Let's modify our program a little bit more.
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```java
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// Sum.java
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public class Sum {
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public static void main(String[] args) {
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System.out.println("number of arguments: " + args.length);
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for (String argument : args) {
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System.out.println(argument);
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}
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}
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}
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```
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Here I used `for` loop to do printing ***for*** every `String` element in `args`.
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Let's try this with our examples. And don't forget to recompile using `javac Sum.java`.
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```sh
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$ java Sum 1 2 3
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number of arguments: 3
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1
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2
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3
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```
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```sh
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$ java Sum 1 2 -3
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number of arguments: 3
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1
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2
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-3
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```
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```sh
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$ java Sum "1 2 3"
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number of arguments: 1
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1 2 3
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```
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> [!NOTE]
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> Again, notice only ***one*** string argument.
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```sh
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$ java Sum "1 2" " 3"
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number of arguments: 2
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1 2
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3
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```
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```sh
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$ java Sum " "
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number of arguments: 1
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```
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Okay, now that we know how to ***iterate*** (or do something for every element), we can calculate the *sum* of all the numbers in the `args` array. To do that we need to create a new variable `sum` of *integer* type and assign it the **value** of `0`. Then for every number in `args` we will add it to `sum`, and so by the end of array we will have the sum of all numbers stored in variable `sum`. This is what the code will look like
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```java
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// Sum.java
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public class Sum {
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public static void main(String[] args) {
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System.out.println("number of arguments: " + args.length);
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int sum = 0;
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for (String argument : args) {
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sum = sum + argument;
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System.out.println(argument);
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System.out.println(sum);
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}
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System.out.println(sum);
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}
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}
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```
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Seems ok. But let's test it.
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```sh
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$ javac Sum.java
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Sum.java:9: error: incompatible types: String cannot be converted to int
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sum = sum + argument;
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^
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1 error
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```
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We got a compilation error that says `String cannot be converted to int`. It means that when we try to add 2 variables `sum` and `argument` their types don't match. The type of `sum` is integer and string for `argument`. It seems pretty logical because what do we expect when adding for example `1` and `apple`?..
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> [!IMPORTANT]
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> So we need to *cast* `argument` to integer data type so that we can add it to `sum`.
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We can do so using [`Integer.parseInt()`](https://docs.oracle.com/javase/8/docs/api/java/lang/Integer.html#parseInt-java.lang.String-) method.
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It takes a `String s` and returns an `int`, which a string `s` represents. For example this code will work as expected
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```java
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// ParseIntExample.java
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public class ParseIntExample {
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public static void main(String[] args) {
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String s = "123";
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int sum = 321;
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//! sum = sum + s;
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sum = sum + Integer.parseInt(s);
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System.out.println(sum);
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}
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}
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```
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The commented out line (`//!`) would have given us an exception.
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