I/O Streams File I/O 2-D array review Discussion 7 I/O Streams File I/O 2-D array review

Discussion Plan File Input/Output (File I/O) 1-D and 2-D array review Practice problems

Projects going forward For projects 3 and 4 you can work with a partner! After that, there is the Final Project, which is in groups of 4 Start thinking now about people you would want to work with A good group can make the project experience better!

File Streams Up until now you’ve needed to #include <iostream>. Why?

File Streams Up until now you’ve needed to #include <iostream>. Why? To be able to use the input stream cin and the output stream cout What is <fstream>, and why do we include it?

File Streams Up until now you’ve needed to #include <iostream>. Why? To be able to use the input stream cin and the output stream cout What is <fstream>, and why do we include it? If we want to use a file stream and read in data/input from a file, or write data (output) to a file

Careful with input streams Do NOT use cin.eof() or my_ifstream.eof() as a condition Can sometimes cause undefined behavior INSTEAD: while (inFile >> x){…} // while reading into the file is still successful while (!inFile.fail()){…} //while reading into the file does not cause a fail state You can use cin.clear() or inFile.clear() or outFile.clear() to reset a fail state Where “outFile” is the variable name of your ostream A useful function to do this all was provided in lecture Maybe add a screenshot of this function

Questions about file streams?

Moving to review of 2D Arrays Any questions about 1-Dimensional arrays?

2 Dimensional Arrays You can visualize these like a matrix, or game board! ALWAYS: arr[row][col] You’ll often see ‘col’ in place of column col stands for column

Initializing all to 0 1-dimensional array: int board[3] = {0}; Both of these initialize every element in the array to 0 THIS IS ONLY THE CASE WITH 0 – if any other number is between the brackets, it sets only the first element to that number, and sets all the rest to zero This isnt true if you put a number into the brackets – only the first element will be set Feel free to try it to see exactly what happens!

Initializing directly const int HEIGHT = 3; const int WIDTH = 4; int board[HEIGHT][WIDTH] = { {1, 2, 3, 4}, {5, 6, 7, 8}, {9, 10 , 11, 12} }; You can also initialize all to the same thing, using nested loops We’ll do a lot with nested loops in a few minutes

When initializing… Which of these is invalid? int data[10][2]; int data [4][]; int data [][3] = {{1, 2, 3}, {4, 5, 6}};

When initializing… Which of these is invalid? int data[10][2]; int data [4][]; //compile error int data [][3] = {{1, 2, 3}, {4, 5, 6}};

When initializing… int data[10][2]; int data [4][]; //compile error With a 2-dimensional array, the compiler absolutely needs to know the column (the second parameter) size at compile time The row is optional

How do we declare and initialize our own 2-d array Let’s say we want the values to increase: 1, 2, 3, 4, 5, 6, 7, 8, 9 Do on board first, try only student input

How do we declare and initialize our own 2-d array Let’s say we want the values to increase: 1, 2, 3, 4, 5, 6, 7, 8, 9 int my_arr[3][3] = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9} }; You can see how you can write the code to be easy to understand visually You will have done this for Assignment 3 and for project3

How do we iterate through a 2-d array What if we want to print every element, as in the picture? Do on the board with only student input

How do we iterate through a 2-d array What if we want to print every element, as in the picture. We print each element in each row, and start a new line for each row so it looks like a matrix. for (int i = 0; i < 3; ++i){ for (int j = 0; j < 3; ++j){ cout << my_arr[i][j] << " "; } cout << endl; Do on the board with only student input

How do we iterate through a 2-d array Now what if we want to print the values backwards, keeping the matrix form? int my_arr[3][3] = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9} }; Do on the board with only student input

How do we iterate through a 2-d array Now what if we want to print the values backwards, keeping the matrix form? int my_arr[3][3] = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9} }; for (int i = 2; i >= 0; --i){ for (int j = 2; j >= 0; --j){ cout << my_arr[i][j] << " "; } cout << endl; Do on the board with only student input

How do we iterate through a 2-d array Now what if we want to only reverse each row, not the entire array? int my_arr[3][3] = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9} }; Do on the board with only student input

How do we iterate through a 2-d array Now what if we want to only reverse each row, not the entire array? int my_arr[3][3] = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9} }; for (int i = 0; i < 3; ++i){ for (int j = 2; j >= 0; --j){ cout << my_arr[i][j] << " "; } cout << endl; Do on the board with only student input

How do we iterate through a 2-d array Now what if we want to only reverse each column, not the entire array? This is the same as doing what? (in terms of rows) int my_arr[3][3] = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9} }; Do on the board with only student input

How do we iterate through a 2-d array Now what if we want to only reverse each column, not the entire array? This is the same as doing what? (in terms of rows) Reversing the order of the rows int my_arr[3][3] = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9} }; Do on the board with only student input

How do we iterate through a 2-d array Now what if we want to only reverse each column, not the entire array? int my_arr[3][3] = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9} }; for (int i = 2; i >= 0; --i){ for (int j = 0; j < 3; ++j){ cout << my_arr[i][j] << " "; } cout << endl; Do on the board with only student input

2-dimensional arrays wrap up There are a lot of examples from last lecture – go through them in detail! Any questions? Understanding arrays is very important for project 3

Feel free to ask any questions after class!