What will happen if the user presses <enter> after every input? Will all work like below? Enter the values of x, y, z: Result: The sum is 17.

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1 (LN 2-1) Review-exercises Exercise 1. FUNCTION CALLING void x1() cout << 10; Exercise 2. MORE ON CIN int x2() return 20; Consider the functions on the left. What will happen if we write: (1) cout << x1(); (2) x1(); (3) cout << x2(); (4) x2(); To write a program to input 3 values into integers x,y,z: Enter the values of x, y, z: Result: The sum is 17. Which of the below is(are) correct? (1) cin >> x >> y >> z; (2) cin >> x; cin >> y; cin >> z; What will happen if the user presses <enter> after every input? Will all work like below? Enter the values of x, y, z: Result: The sum is 17. Exercise 3. (i) What if I run the following program and enter "abcdefghijkl<enter>"? (ii) What if I run the following program and enter "a<enter>b<enter>c<enter>.."? int i=0; while (i<10) char c; cin >> c; cout << int(c) << " "; i++; (LN 2-2) Flow-of-Control Matching between if-else How to correct the code: int temperature; cin >> temperature; Testing: Input 50. What will it show? if (temperature < 100) if (temperature <= 0) cout << "Water is frozen\n"; else cout << "Water is all boiled away\n"; 1/20

2 While-loop, do-while loop, for-loop int total, count, mark; total = 0; count = 1; while (count <=10) cout << "Enter marks: "; cin >> mark; total += mark; count +=1; - Why "total/10.0"? - Rewrite using do-while and for-loops. cout << "Average of 10 students = " << total/10.0 << endl; Empty statement (say, in an while-loop) Some common programming mistakes: - for (i=0;i<10;i++); - i=0; while (i<10);.. i++; continue / break (CS2331 Lec12) - Not encouraged to use them Continue means "skip to next iteration of this loop." break means "immediately halt execution of this loop" switch-case (CS2331 Lec05) - In switch(expr), expr must have integral data type (int, char and bool). - The case values (expr1, exprn) must be constant expressions and must be all different. // To print no. of days in a month int month; cin >> month; switch (month) case 1: case 3: case 5: case 7: case 8: case 10: case 12: cout << "31 days"; break; case 4: case 6: case 9: case 11: cout << "30 days"; break; case 2: cout << "28 or 29 days"; break; default: cout << "input error"; break; 2/20

3 (LN 2-3) Functions Call-by-value and Call-by-reference Suppose we want a function to increment a counter (as parameter) from time to time. We should apply Call-by-reference (see explanation below). Using call-by-value to pass the counter Using call-by-reference to pass the counter //Add one to the counter and display void addone(int c) c++; cout <<"It is now " << c << endl; int counter=0; addone(counter); addone(counter); addone(counter); addone(counter); addone(counter); //Add one to the counter and display void addone(int &c) c++; cout <<"It is now " << c << endl; int counter=0; addone(counter); addone(counter); addone(counter); addone(counter); addone(counter); Result: Can't do! Result: It works! What is call-by-value: In Call-by-value, The argument (counter) is evaluated, and its value is copied to the corresponding parameter (c - where c in addone is just like a local variable). Any changes made on the parameter (c) have no effect on the argument (counter). Pictorially, What is call-by-reference: In Call-by-reference, The argument passed must be a variable (counter). During the function call, the call-by-reference parameter (c) actually refers to this variable (counter). Therefore, changes made on a call-by-reference parameter will actually be done on the corresponding argument. Pictorially, 1 c Created upon each invocation of addone. Destroyed after the execution of addone is finished. 0 counter 5 c and counter refers to the same item in the memory. 3/20

4 Function Prototype Functions should be declared before they are used. Style 1: Define the function before calling it void DoSomething().... DoSomething();.. Style 2: Write function prototype before function calls Function call.. void DoSomething();.. DoSomething();.. Don't forget this semi-colon. Function definition.. void DoSomething().. Driver Programs Example, ".. write a driver program ( ie. just add a main() function ) to test your function." (LN 2-4) Arrays (A) What a Primitive array! An array is just primitively a memory address where a block of memory is allocated for one specific data type. Don t expect to call A.length, A.getAt, etc.. (An array is not a class/template object!!) Don t expect to set or change the array size during run-time: int total; cout << "Please input total: "; cin >> total; int A[total]; Though DEV-C++ etc.. permit this code, but it is not accepted in standard C++ lang.. Visual Studio gives compilation error!! 4/20

5 When defining an array, the size must be a constant. Example: There are 2 errors in the code below. Find them out. #define n1 30; const int n2=40; char array1[n1]; char array2[n2]; int n3; cout << "Please input a number for n3"; cin >> n3; char array3[n3]; //... some work to do on n1,n2,n3 cout << "Thank you.\n"; Storage mechanism for Arrays When defining int marks[5];, the 5 integer variables are stored in contiguous memory locations: (hypothetic) marks[0] marks[1] marks[2] marks[3] marks[4] marks[5] marks[5] is invalid (programming error), but may not prohibited by the compiler. If not prohibited, marks[5] will be interpreted as an integer; although the location may actually store other types of data. Suppose each integer takes 4 bytes, the array occupies locations 1000 to The Watch Window shows you: the value of marks is actually the address!! (B) Actual array size and Effective array size Sample run: Input n: 7 Input 7 marks: After adding bonus: Press any key to continue... < MARK_ARR_SIZE < n > #include <iostream> using namespace std; const int MARK_ARR_SIZE=100; int marks[mark_arr_size], i, n; cout << "Input n: "; cin >> n; cout << "Input " << n << " marks: "; for (i=0;i<n;i++) cin >> marks[i]; //.. Recall : the compiler must know the actual array size early at compile time. Here we - use const int MARK_ARR_SIZE for the actual array size, and - use int n for the effective array size 5/20

6 (C) Different ways to initialize an array int marks[5]=27, 31, 25, 50, 70; List fewer values than the array size => remaining array elements are initialized to zero: If you initialize an array when it is declared, you can omit the size, e.g., int marks[]=34, 87, 72, 65; is equivalent to int marks[4]=34, 87, 72, 65; If we omit the size, it is automatically calculated according to the initializing values given (here size is 4) int marks[5]=51,52,53,54,55; int marks[5]=51,52; int marks[5]=0,0,0,0,0; int marks[5]=0; int marks[5]; int marks[ ]=51,52,53,54,55; int marks[ ]=0,0,0,0,0; ????? Not initialized int marks[ ]=0; int marks[ ]; 0 If no initializer, we must NOT leave the array size empty (compilation error) (D) Passing arrays to functions Can Arrays be RETURNED? Answer: No! int some_function(..); char some_function(..); double some_function(..); bool some_function(..); int[] some_function(..); char[] some_function(..); double[] some_function(..); bool[] some_function(..); Therefore, we pass "array parameters" (The default mechanism is a weak form of call-by-reference, through which the caller's array contents can be directly changed.) Recall: the associated value of an array name (eg. marks) is actually the address of the array (eg. int marks[5];) 6/20

7 The common format for passing an array into a function: #include <iostream> using namespace std; const int MARK_ARR_SIZE=100; // read integers from console and store in an array void input_into_arr(int arr[], int size) for (int i=0; i<size; i++) cin >> arr[i]; int marks[mark_arr_size], i, n; cout << "Input n: "; cin >> n; cout << "Input " << n << " marks: "; input_into_arr(marks,n); for (i=0;i<n;i++) marks[i]+=10; cout << "After adding bonus: "; for (i=0;i<n;i++) cout << marks[i] << " "; cout << endl; Sample run: Input n: 7 Input 7 marks: After adding bonus: Press any key to continue... Array Parameter void input_into_arr(int arr[], int size) The first parameter (int arr[]) is called an array parameter int arr[n] (1) We often DON'T specify the size in the array parameter (no need actually ignored by the compiler ). (2) An array parameter is a weak form of call-by-reference parameter (**) in which the starting memory address of the array is told to the function (but the size of the array is not told). Advantage: arrays of different sizes can use the same function, as long as they have the same base type (above is int) (3) Adding & is not needed: void input_into_arr(int &arr[], int size) 7/20

8 Omitting the parameter for effective size (if array's effective size is known, eg. equal to actual size) #include <iostream> using namespace std; const int N=5; // read integers from console and store in an array void input_into_arr(int arr[], int size) for (int i=0; i<size; i++) cin >> arr[i]; N int marks[n], i; cout << "Input " << N << " marks: "; input_into_arr( marks,n ); for (i=0;i<n;i++) marks[i]+=10; cout << "After adding bonus: "; for (i=0;i<n;i++) cout << marks[i] << " "; cout << endl; ** However, if you want to make the function useful as well in other occasions (or other programs) which do not restrict to the size N, then you better keep the original approach: keep int size as parameter and pass the argument in function call. Passing arguments: array vs array element #include <iostream> using namespace std; const int MARK_ARR_SIZE=100; //swap the contents of 2 input arguments. void swap(int &x1, int &x2) int temp; temp=x1; x1=x2; x2=temp; // read integers from console and store in an array void input_into_arr(int arr[], int size) for (int i=0; i<size; i++) cin >> arr[i]; Sample run: Input n: 5 Input 5 marks: After swapping: Press any key to continue.. Passing an array ( array_name ) int marks[mark_arr_size],i,n; Passing 2 array elements (array_name [ array_index ]) cout << "Input n: "; cin >> n; cout << "Input " << n << " marks: "; input_into_arr( marks, n ); swap( marks[0], marks[4] ); cout << "After swapping: "; for (i=0;i<n;i++) cout << marks[i] << " "; cout << endl; Self-test: Why don't we add & to marks: void input_into_arr(int &arr[], int size) 8/20

9 (E) const parameter modifier If the function is not to change the array contents, add const to the array parameter like:.. // display integer items stored in an array void display_arr_items( const int arr[], int size) int i; for (i=0; i<size; i++) cout << arr[i] << " "; int marks[5]=80,81,82,83,84; display_arr_items(marks,5); We say, "I don t want the function call display_arr_items(marks,n); to modify marks. Now the array parameter has the const modifier. I feel safe." - The const modifier indicates that the array parameter should not be changed. - If violated (eg. typing "=" by mistake: "if (arr[i]=100)"), the compiler will give compilation error and the program can't run. Note: We should use the const modifier on an all-or-nothing basis, eg., the following will cause a compilation error. Line A=> // display one integer item void display_one( int arr[], int index ) cout << arr[index] << " "; Missing const Line B=> // display integer items stored in an array void display_arr_items(const int arr[], int size) for (int i=0; i<size; i++) display_one(arr,i); - In the above, const is missing at line A. - The compiler will give compilation error at line B: 'display_one' : cannot convert parameter 1 from 'const int []' to 'int []' Reason: The compiler thinks that at line B, the call to display_one may change the value of the parameter arr. But arr is a const array parameter of display_arr_items that mustn't be changed compilation error: 9/20

10 (F) Multidimentional Arrays Start-up Questions: (1) Fill in the blank below, so that the data array can be passed to the testing function. #include <iostream> using namespace std; void testing( ) //.. some processing int data[5][4][3]; testing(data); //.. some processing (2) Suppose the above data[5][4][3] array is to represent a 3D table (several sheets, each sheet has some rows/columns). Which one of the below is the best match? (a) (b) (c) (d) (e) (f) 10/20

11 2-dimensional array Pictorially: int values[6][10]; is an array of 6 rows: Row 0 (an array of 10 integers) Row 1 (an array of 10 integers) Row 2 (an array of 10 integers) Row 3 (an array of 10 integers) Row 4 (an array of 10 integers) Row 5 (an array of 10 integers) You can think of it as an array of 6 arrays (each of these 6 arrays is an array of 10 integers). Actual storage in the memory: int values[6][10]; Row 0 values[0][0] W: number of columns H: number of rows Row 1 Between values[0][0] and values[2][7], there are 2 rows + 7 elements Row 2 cout << values[2][7]; values[2][7] Suppose values[0][0] is at location And the size of an integer is 4 bytes. Row 3 From the example (or observe the picture), Row 4 The location of values[2][7] is at : = location (2* ) * 4 Row 5 = location 1108 W 4 bytes per integer Note: Calculation of values[2][7] is based on the number of columns W (2 nd dimension), but NOT the number of rows H (1 st dimension), 11/20

12 3-dimensional array int values[5][4][3]; declares a 3-dimensional array of integers 1 st index 2 nd index 3 nd index An array of 5 pages: Page 0 (a 2D array: 4x3) Page 1 (a 2D array: 4x3) Page 2 (a 2D array: 4x3) Page 3 (a 2D array: 4x3) Page 4 (a 2D array: 4x3) Actual storage in the memory: int values[5][4][3]; cout << values[3][1][2]; Suppose values[0][0][0] is at location And the size of an integer is 4 bytes. From this example (or observe the picture), Each row has 3 elements. Each page has 12 elements (ie. 4x3 elements). The location of values[3][1][2] is: = location (3*4*3 + 1*3 + 2) * 4 = location 1164 W: number of columns H: number of rows P: number of pages H * W W 4 bytes per integer Row 0 Row 1 Row 2 Row 3 Row 0 Row 1 Row 2 Row 3 Row 0 Row 1 Row 2 Row 3 Row 0 Row 1 Row 2 Row 3 Row 0 Row 1 Row 2 Row 3 Page 0 Page 1 Page 2 Page 3 Page 4 Note from the above: Calculation of values[3][1][2] is based on H and W (2 nd and 3 rd dimensions), but NOT the number of pages P (1 st dimension), Similarly, for other kinds of arrays (4D, 5D,..), the 1 st dimension is not needed. (But the remaining dimensions are needed.) As a result, the compiler doesn't need to know it for multidimensional array parameters: Omit P void display(const int data[][4][3], int size) Passing a Multidimensional Array: All the dimension sizes except the first must be specified in the function header. E.g., void display(const int data[][10], int size) void display(const int data[][4][3], int size) 12/20

13 (LN 2-5) File I/O To handle File I/O, we use the ifstream and ofstream classes (in <fstream>) Example: Read the width and height from a file and output a rectangle of asterisks (*) #include <iostream> #include <fstream> using namespace std; ifstream infile; int width, height, i, j; char filepathname[300]; cout << "Please input the file pathname: "; cin >> filepathname; infile.open(filepathname); infile >> width >> height; for (i=0;i<height;i++) for (j=0;j<width;j++) cout << '*'; cout << endl; infile.close(); Please input the file pathname: 1.txt *** *** *** *** *** Press any key to continue... To output contents to a file, write ofstream outfile; outfile.open(filepathname); /* where filepathname is a string (character array) */ outfile << "some contents to be added to the file"; outfile.close(); 13/20

14 Detecting end-of-file End Of File Member function eof() returns true if and only if the program has attempted to read from the input file beyond the end of the file: Only for input files (objects of class ifstream). We typically read a value into a variable and then process the value in the variable. Before processing with the variable, we may use.fail() to check whether the program has actually failed in reading any value into the variable. (This is essential in certain cases. Example: if the file may have extra blank line(s) at the end, then "not yet end-of-file" doesn't guarrentee a sucessful reading.) Example: Extra blank line (newline character) #include <fstream> #include <iostream> using namespace std; ifstream in_stream; int x; Code A I in_stream.open("infile.txt"); while (!in_stream.eof())// ie. if in_stream.eof() is NOT true in_stream >> x; if (!in_stream.fail()) cout << x << endl; in_stream.close(); Note: If we remove "if (!in_stream.fail())" Then we may get a wrong result (17 is displayed twice). Explanation: In Code B: After reading 25, then 4, then 17, in_stream.eof() is still false (because the reading of 17 stops at the newline character, so "end-of-file" hasn't been known), so the while-loop will run the 4 th time. while (!in_stream.eof()) Code B (Wrong) in_stream >> x; if (!in_stream.fail()) cout << x << endl; Press any key to continue... When the loop runs the 4 th time, "in_stream >> x" will encounter end-of-file. It cannot read a new value. Since no checking is done, "cout << x << endl;" will display the old value of x again => WRONG result. The expression in_stream >> x has value 0 if in_stream has no more data E.g. #include <fstream> #include <iostream> using namespace std; ifstream in_stream; int x; in_stream.open("infile.txt"); while ( (in_stream >> x)!=0 ) //or simply: while (in_stream >> x) cout << x << endl; in_stream.close(); 14/20

15 File doesn't exist? Use of.fail() after file opening: If fail, then terminate the program with exit(). ifstream infile; infile.open(filepathname); if (infile.fail()) cout << "Cannot open file\n"; exit(1); // 1 stands for error, program ended. (LN 2-6) Exercises (Test all your programs in PASS except Q5 and Q6) LN2-Q1. [ Sample Question paper Q1 ] Code Fixing In Q1 of the Sample Question paper (available at the course web), the function count_0_to_100 is wrong in logic and syntax. Download the complete program from the course web and fix it. Hint: - There are more than 10 errors. - We need to type A[][WIDTH] instead of A[][] for the function parameter. After fixing, the program should output the following result (No user input is needed): The count is: 25 You should - Submit the program to PASS for correctness checking (only 1 test case: no input) - Afterwards you may further modify the main function and values of the WIDTH and HEIGHT constants for more testing on your own. LN2-Q2. Flow of Control - Multiplication Table Write a C++ program that prints out a 9 x 9 multiplication table as shown below: Sample output: This is a 9 x 9 multiplication table Suggestion: (Approach A) nested for-loops (2 levels) Another approach (B) is to use only one for-loop, but that is a bit tricky. If you use approach B, you need to design a way to determine the line breaks, and also the changes on product factors. Tips: To align the columns properly, use setw(). setw is provided by <iomanip>. Suggested solution/approach is available at the course web (Download Area) 15/20

16 LN2-Q3. Merging two lists Write a function that merges two sorted integer arrays into one sorted integer array: Inputs Outputs source1[] name of the first sorted array size1 size of first array source2[] name of second sorted array size2 size of second array result[] name of array for storing the result size size of resultant array result[] store the merged array size store the size of the merged array Assuming each input array has size at most 10, write a driver program ( ie. just add a main() function ) to test your function. For easy debugging at the early stage, you may hard-code the input arrays to some fixed values. When the merge function is fully debugged, you may write a function to input an array and another function for display. Follow the input/output format shown below. Sample input and output: size of source1[]: 4 element(s) of source1[]: size of source2[]: 3 element(s) of source2[]: Size of result[]: 7 Element(s) of result[]: Press any key to continue... Underlined contents are input by the user Note that size of result (7) should be generated by the program (not input by user). Suggested solution/approach is available at the course web (Download Area) LN2-Q4. Minesweeper Write a program to input an H x W array of 0 s and 1 s where H and W are also user inputs. You may assume that both H and W are non-zero and at most 10. The array represents an H x W minefield so that a cell with a 1 (resp. 0) means the cell contains (does not contain) a bomb. Your program should print the minefield as follows: (1) If a cell contains a bomb, print a `#. (2) If a cell does not contain a bomb and is not adjacent to any bomb, print a `. (3) Otherwise, print the number of neighboring bombs. Sample input and output: Enter H and W: 5 5 Enter the minefield: #32#1 ## #...11 Underlined contents are input by the user Suggested solution/approach is available at the course web (Download Area) 16/20

17 LN2-Q5. File I/O Write a program to read in pairs of integers from an input file (Infile1.dat) and output the sum of each pair to an output file (Outfile1.dat). The number of pairs is specified in the first line of Infile1.dat, subsequent lines contain one pair of integers on each line. Use number of pairs to control the loop (not to use eof()). Use fail() to check for file-opening and reading. Here s a sample input and output: ** Please find the test cases at the course web. ** Sample Infile1.dat Corresponding Outfile1.dat should contain: [ Testing is NOT AVAILABLE in PASS for this question. ] LN2-Q6. File I/O Same as last question, but the number of pairs is not specified. Use eof() to detect end of input, and fail() to check for file-opening and reading. ** Please find the test cases at the course web. * Sample Infile2.dat Corresponding Outfile2.dat should contain: [ Testing is NOT AVAILABLE in PASS for this question. ] 17/20

18 LN2-Q7. Call-by-reference Simple Statistics Example 1 Please input the total count: 6 Please input a value: 2 Please input a value: 3 Please input a value: 3 Please input a value: 4 Please input a value: 4 Please input a value: 4 Result: average= , median=3.5, mode=4 Example 2 Please input the total count: 5 Please input a value: 2 Please input a value: 3 Please input a value: 3 Please input a value: 4 Please input a value: 5 Result: average=3.4, median=3, mode=3 Underlined contents are input by the user Write a C++ function that finds the average, median, and mode of the integers in an array. The function should return nothing: void function_name(..) The function should have the following parameters: (1) array (2) total count of items in the array (3) [call-by-reference] average, median, mode values. (To find out what are average, median, and mode, you my use the help in Microsoft Excel.) Implement the main function that calls the above function to work like the examples above: Assumptions and guidelines: - All input values are within 0 to The input numbers are already in non-descending order. - If the total count is even, the median should be the average of the middle 2 values. Eg. the median of 2,3,4,5,5 is 4 the median of 3,3,4,5 is Assume that the mode value is unique. That is, no 2 integers have the same maximum count. - The default maximum precision for floating point numbers in C++ is 6. For this question, we just leave it in the normal way that C++ behaves. Suggested solution/approach is available at the course web (Download Area) LN2-Q8. Poker game Write a program to input five integers in the range [1,13] and determine if they are consecutive. Hint: Define an array of size 13 to count the frequency of each value in [1,13]. Sample input and output: Enter 5 integers in [1,13]: A straight Underlined contents are input by the user Sample input and output: Enter 5 integers in [1,13]: Not a straight Underlined contents are input by the user Suggested solution/approach is available at the course web (Download Area) 18/20

19 LN2-Q9. Rounding Off Write a function to round off a floating-point number n (of type double) to the nearest 10 k th where k is another function parameter. The layout of the driver program is given as follows: (Available for download at the course web) // Round up n to the nearest 10^kth #include <iostream> #include <math.h> using namespace std; int funcround(double n, int k) // Precondition : n >= 0, k >= 0 // Postcondition: return n rounded off to the nearest 10^k (return n // rounded off to the nearest integer if k == 0) // put your code here double parn; // parameter n int park; // parameter k cout << "Enter a floating-point number n: "; cin >> parn; cout << "Enter an integer k: "; cin >> park; cout << parn << " rounded to the nearest 10^" << park << " = " << funcround(parn, park) << endl; Sample input and output: Input n Input k Output Tips: You may use library functions provided by <math.h>, e.g. ceil(), pow() etc. Mind the data types!!! For example, suppose k and x are integers, then: - pow(10,k) is wrong. It should be pow(10.0,k). - ceil(x/2) is wrong. It should be ceil(x*0.5) or ceil(7/2.0). Reason: pow has several forms for float and double. If you pass an integer argument (eg. 10), then it can't resolve to which specific version. Visual Studio will give compilation error on pow(10,k): It won t determine for you whether to use the double form or the float form. Suggested solution/approach is available at the course web (Download Area) 19/20

20 LN2-Q10. Palindrome detection A palindrome is a word that is spelled the same forward and backward. For example, radar and noon are palindromes. Write a program that repeatedly accepts a string of letters (all in lowercase) and determines whether or not the string is a palindrome. Each input string is at most 20 letters long and is terminated by a period '.' while the program is terminated by an empty string: '.'. Input and output Sample 1: radar.abc.. true false Underlined contents are input by the user Input and output Sample 2: abcdefghijjihgfedcba.dddd.. true true Underlined contents are input by the user Suggested solution/approach is available at the course web (Download Area) --- end /20