Lab 3: Control Flow / Conditional Statements. Based on the presentation made by Graham Northup

Transcription

1 Lab 3: Control Flow / Conditional Statements Based on the presentation made by Graham Northup

2 Control Flow PC #include <iostream> using namespace std; int main() { int a, b, c; a = 3; cout << "Enter factor: "; cin >> b; Normally, a programs code will execute line after line, in the order it appears in the source. (1) PC is the program counter a piece of data on every processor core that indicates what the core is running or looking at. c = a * b; cout << c << "\n"; return 0; Naturally, as a program runs, the PC usually goes from one statement to the next. This change is the most basic form of control flow. (1) In modern optimizing compilers, this is often an illusion.

3 Showing Control Flow (start) a = 3; Control flow is most easily demonstrated with a graphical aid the flowchart. In this example, it s clear that these three statements are always executed in sequence. cout << "Enter factor: "; cin >> b; (end)

4 Modifying Control Flow #include <iostream> using namespace std; int main() { int a, b, c; a = 3; cout << "Enter factor: "; cin >> b; There are exceptions to the general insequence rule; in C and C++, these are called the control structures, and they change control flow what the processor executes next in various useful ways. The most obvious one is only doing the next statement if a condition is true. PC PC PC if(b < 0) { cout << "Factor cannot be negative!\n"; return 1; c = a * b; cout << c << "\n"; return 0;

5 If Statement (start) b < 0 True All of the control structures we will study today have decision points in them where a condition is checked. I will occasionally also call them branches, because like a tree s branch from its trunk the flowchart (pictures here) splits in two. False cout << "Factor cannot be negative!;" return 1; (end)

6 Relational Operators These can be used as a condition wherever a truth is expected. A == B /* True when A is equal to B */ A!= B /* True when A is not equal to B */ A < B /* True when A is less than B */ A > B /* True when A is greater than B */ A <= B /* True when A is less than or equal to B */ A >= B /* True when A is greater than or equal to B */

7 Activity: Try an If Statement #include <iostream> using namespace std; int main() { int number; cout << "Enter an integer: "; cin >> number; if(number > 0) { cout << number << " is positive\n"; return 0;

8 If/Else Statement #include <iostream> using namespace std; int main() { int foobar; With our knowledge of these relations and their inverses, it is easy to construct branches that are always mutually exclusive. However, this is quite a bit of typing... cout << "What is your foobar? "; cin >> foobar; if(foobar == 7) { cout << "Your foobar is a 7? Mine too!\n"; if(foobar!= 7) { cout << "Aww, my foobar is a 7.\n"; return 0;

9 If/Else Statement #include <iostream> using namespace std; int main() { int foobar; cout << "What is your foobar? "; cin >> foobar;...so one can also just use else for this purpose. Control flows to the statement(s) following the else when the if would be skipped. Exactly one block is always executed never both. Note that an else must always directly follow an if. if(foobar == 7) { cout << "Your foobar is a 7? Mine too!\n"; else { cout << "Aww, my foobar is a 7.\n"; return 0;

10 If/Else Statement (start) In terms of a flowchart... False foobar == 7 True cout << "Your foobar is a 7? Mine too!\n"; cout << "Aww, my foobar is a 7.\n"; (end)

11 Activity: Try an If/Else Statement #include <iostream> using namespace std; int main() { int number; cout << "Enter an integer: "; cin >> number; if(number > 0) { cout << number << " is positive\n"; else { cout << number << " is non-positive\n"; /* NB: Why not "negative"? */ return 0;

12 Nesting and Ladders The simplified flowchart for the main() function of the previous program looks approximately like this. Note that there is only one branch.

13 Nesting and Ladders The simplified flowchart for the main() function of the previous program looks approximately like this. Note that there is only one branch. It is worth repeating that the control structures are, themselves, statements, which means they can be nested inside of other control structures.

14 Nesting and Ladders The simplified flowchart for the main() function of the previous program looks approximately like this. Note that there is only one branch. It is worth repeating that the control structures are, themselves, statements, which means they can be nested inside of other control structures. This ability to create arbitrary structure is the hallmark of any non-trivial computer program; if you peruse C++ source, you will likely find some deeply-nested and very long structures!

15 Nesting and Ladders #include <iostream> What this code does, then, should be no surprise. using namespace std; int main() { int number; cout << "Enter an integer: "; cin >> number; (Although this is based on the activities, you don t need to type this.) However, while nesting can be used, note the growing indent for each nested if... if(number > 0) { cout << number << " is positive\n"; else { if(number == 0) { cout << number << " is zero\n"; /* "0 is zero"! */ else { cout << number << " is non-positive\n"; return 0;

16 #include <iostream> using namespace std; int main() { int number; Nesting and Ladders cout << "Enter an integer: "; cin >> number;...most programmers prefer this version, so as to avoid the code running off the right side of their editor :). This version is frequently called a ladder of ifs. Both of them have the same flowchart and same behavior. Choose whichever you prefer. if(number > 0) { cout << number << " is positive\n"; else if(number == 0) { cout << number << " is zero\n"; /* "0 is zero"! */ else { cout << number << " is non-positive\n"; return 0;

17 #include <iostream> using namespace std; int main() { int number; Switch/Case cout << "Magic Eight Ball\n"; /* TODO: Generate random numbers */ cout << "Provide an integer (1-6): "; cin >> number; if(number == 1) { cout << "MAYBE LATER\n"; else if(number == 2) { cout << "ASK AGAIN\n"; else if(number == 3) { cout << "YES\n"; else if(number == 4) { cout << "UNCERTAIN\n"; There s one more, mostlyhistorical caveat to talk about; another statement less general than if/else, and can help when you re finding yourself testing if something is equal to plenty of other values, as here... else if(number == 5) { cout << "NO\n"; else if(number == 6) { cout << "VERY LIKELY\n"; else { cout << "I said 1-6!\n"; return 1; return 0;

18 #include <iostream> using namespace std; int main() { int number; Switch/Case cout << "Magic Eight Ball\n"; /* TODO: Generate random numbers */ cout << "Provide an integer (1-6): "; cin >> number; switch(number) { case 1: cout << "MAYBE LATER\n"; break; case 2: cout << "ASK AGAIN\n"; break; case 3: cout << "YES\n"; break; case 4: cout << "UNCERTAIN\n"; break; case 5: cout << "NO\n"; break; This is the switch/case statement. The switch part takes something that evaluates to an integer value, and the cases are labeled with possible values. You can provide a default case, which is like the last else of an if ladder. The strange syntax is mostly a remnant from the past. You can use an if ladder wherever you would use a switch/case. Although switch/case can sometimes be a little faster, I don t recommend using it while you re learning, as its syntax is not obvious. You should at least note its existence for the sake of future exams (hint). case 6: cout << "VERY LIKELY\n"; break; default: cout << "I said 1-6!\n"; return 1; break; return 0;

19 Task 1 You need to submit Grade.cpp file into your directory /afs/ad.clarkson.edu/class/cs141/students/username/lab3/

20 Task 2 You need to submit Arithmetic.cpp file into your directory /afs/ad.clarkson.edu/class/cs141/students/username/lab3/

21 Please no other files in lab3 directory except Grade.cpp and Arithmetic.cpp