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lect06, Thu 04/18

Loops, switch, functions + makefile

This lecture mainly focused on the following topics:

The first example illustrated how for loop can be written using a while loop and highlighted the major things we need to keep in mind when writing a while loop.

// main.cpp
#include <iostream>
using namespace std;

// A function to print the star * n times
void print_star( int n );

void print_star_while( int n );

int main()
{
    int n = 5;
    print_star(n);
    cout << " Now, with a while" << endl;
    print_star_while(n);

     return 0;
}

// A function to print the star * n times
void print_star( int n )
{
    for (int j = 0; j < n; j=j+2)
    {
        cout << endl;
        for (int i = 0 ; i < n; i++)
        {
            cout << "*";
        }
    }
    cout << endl;
}

void print_star_while( int n )
{

   int loop_cntr = 0;

    while ( loop_cntr < n)
    {
        cout << "*" ;
        ++loop_cntr;
    }
    cout << endl;
}

To trace though the code above, we used the following notes to count the values of the variables: j = 0 i = 0 j = 0 i = 1 … j = 0 i = 4 j = 2 i = 0 j = 2 i = 1 … j = 2 i = 4 j = 4 i = 0 … j = 4 i = 4 j = 6

We wrapped up this example by writing a very simple Makefile for compiling and removing our print_star executable:

print_s: main.cpp
	g++ main.cpp -o print_star

clean:
	rm -f print_star *.out *.o

To use this makefile, we can now type make, which by default always builds the first target in the makefile (in our case, it would be print_s). make clean will find the clean target and execute its command, which would remove the executable and any .out or .o files in the directory.

In general, a makefile rule has this structure:

target_name: dependency_1 dependency_2 ...
    command_to_make_target_1
    command2

You can find a detailed walkthrough and explanation of makefiles on this page: https://ucsb-cs16.github.io/topics/makefile/.

Switch statements

We then looked at how to write a switch statement and what happens when you don’t include a break inside the case statements.

// switch.cpp
#include <iostream>
#include <cstdlib>
using namespace std;

int main(int argc, char *argv[])
{
    char answer = 'y';

    answer = *(argv[1]);  // a dangerous move without checking argc first

    switch(answer)
    {
        case 'N':
        case 'n':
            cout << "You said No" << endl;
            break;
        case 'Y':
        case 'y':
            cout << "You said Yes" << endl;
            break;
        default:
            cout << "This is default" << endl;
            cout << "You said " << answer << endl;
            break;
    }
    return 0;
}

Another version, getting user input as part of the while loop:


#include <iostream>
#include <cstdlib>
using namespace std;

int main()
{
    char answer = 'y';

    while (answer != 'q' && answer != 'Q')
    //while (answer != 'q' || answer != 'Q') // logic error; infinite loop
    {
        cin >> answer;
        switch(answer)
        {
            case 'N':
            case 'n':
                cout << "You said No" << endl;
               // break;
            case 'Y':
            case 'y':
                cout << "You said Yes" << endl;
                break;
            default:
                cout << "This is default" << endl;
                cout << "You said " << answer << endl;
                break;
        } // end switch
    } // end while

    return 0;  // Make sure this is outside of the while loop, otherwise, the program will exit too soon
}

Practice Questions

  1. Write a Makefile so that when we type make hello, the compiler will compile the file main.cpp into an executable named program01

  2. Write the following if/else-if/else chain as a switch statement:

    char vimLetter;
cin >> vimLetter;
if(vimLetter == ‘x’)
 cout << “Save and quit” << endl;
else if(vimLetter == ‘i’)
 cout << “Insert mode” << endl;
else if(vimLetter == ‘q’)
 cout << “Quit” << endl;
else if(vimLetter == 'r')
 cout << “Replace” << endl;
else
 cout << "Other functionality" << endl;