README-en.md edited online with Bitbucket

README-en.md

@@ -131,6 +131,143 @@ The **frequency of occurrence** is defined as the ratio of times that a digit ap
 
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+This lab requires you to read data from a text file. You can skip the next section if you feel that your file reading skills are competent. Otherwise, read on.. 
+
+
+## Reading data from text files in C++
+
+
+C++ provides functions to read and write data to/from files.  In this lab you will be using one of the most rudimentary file input/output schemes provided in C++ to read/write from **text** files. Text files consist exclusively of ASCII characters which represent data in any of the primitive types provided by C++. Typically, the values are separated by spaces. For instance lets asumme that the file `nameAge.txt` contains some data about names and ages.
+
+```
+Tomas 34
+Marta 55
+Remigio 88
+Andrea 43
+```
+
+To **read** a text file in C++, we need to have a sense of how it is organized and what type of data you would like to read. The example `nameAge.txt` file contains four lines, each consisting of a string and an integer.  Here is a simple program to read that file entirely while printing its content. Read the comments to understand the various parts.
+
+```
+
+#include <iostream>
+
+// fstream is the header file that contains classes, functions and 
+// objects to deal with file input and output.
+#include <fstream>  
+
+using namespace std;
+
+int main(){
+
+    // We shall use these two variables to assign the values read
+    // from each line in the file.
+    string name;
+    int age;
+    
+    // This is the object that will represent the file.
+    ifstream inFile;
+    
+    // We call the open function to open the input file `nameAge.txt` 
+    inFile.open("nameAge.txt");
+    
+    
+    // We check if the file was correctly opened
+    if (!inFile.is_open()) {
+        cout << "Error openning file nameAge.txt\n";
+        exit(1);
+    }
+
+    // While there is data in the file, read a string and an int.
+    // Notice how the `>>` symbol is used, similar to when using cin
+ 
+    while (inFile  >> name >> age) {
+        cout << name << " : " << age << endl;
+    }
+    
+    // Close the file. 
+    inFile.close();
+    
+    return 0;
+}
+```
+
+The ifstream object is used for reading a text file **sequentially**. It keeps track of the next position in the file that should be read. Each time that a data is read from the file (using `inFile >> ____`) it advances its position so that the next `inFile >> ___` reads the next data and so forth.
+
+Notice the line `inFile  >> name >> age`. This instruction accomplishes several tasks:
+
+* It reads a string and an int from the file (if available) and assigns them to the variables `name` and `age`.
+* If both data were read, the expression evaluates to `true`, thus entering the while block.
+* If both data could not be read, the expression evaluates to `false` thus ending the while block.  
+
+
+Here are some code snippets for common reading tasks. Observe that all of them:
+
+1. Create a ifstream object, call the `open` function and check if the file is opened correctly.
+2. Create one or more variables to assign the values that are read from the file. 
+3. Implement a loop which repeats until no more data is available in the file. 
+3. `close` the file at the end.
+
+
+**Example 1**: Read a file that consists only of integers, accumulate their values into a sum.
+
+```
+    ifstream inFile;
+    int n;
+    int accum = 0;
+    
+    inFile.open("nums.txt");
+
+    if (!inFile.is_open()) {
+        cout << "Error openning file nums.txt\n";
+        exit(1);
+    }
+
+    while (inFile  >> n) {
+        accum = accum + n;
+    }
+
+    cout << "Total: "  << accum << endl;
+    
+    inFile.close();
+```
+
+**Example 2**: Count the number of lines in a file that consists of names. Then choose the name at the center line.
+
+```
+    ifstream inFile;
+    string name;
+    int ctr = 0;
+    
+    inFile.open("names.txt");
+
+    if (!inFile.is_open()) {
+        cout << "Error openning file names.txt\n";
+        exit(1);
+    }
+
+    while (inFile  >> name) {
+        ctr++;
+    }
+
+    cout << "Total number of lines: " << ctr << endl;
+
+    // These two commands "rewind" the file so that we can start
+    // reading again from the beginning. 
+    inFile.clear();
+    inFile.seekg(0);
+
+    for (int i = 0; i <= ctr / 2; i++) {
+        inFile >> name;
+    }
+
+    cout << "The name at the position " << ctr / 2 << ": " << name << endl;
+
+    inFile.close();
+```
+
+
+
 ##Laboratory session
 
 ###Exercise 1: Understand the data files and the provided code