CSE 20232 Lecture 5 – Decisions and Loops

CSE 20232Lecture 5 – Decisions and Loops • Switch Statement • While… loop • For… loop • Do…While Loop • Samples • fstr • sineplot • switchit2

Switch Statement • Allows execution of one of several sequences of actions, depending on which constant matches the value of the expression switch ( <expression> ) { case <constant1> : <statements> // for <expression> == <constant1> break; case <constant2> : <statements> // for <expression> == <constant2> break; … default : <statements> // for <expression> != any <constant> }

Switch Statement Example // Evaluate expression x op y (where op is +, -, * or /) int x, y; char op; cout << “enter any expression, such as 4 + 5: “; cin >> x >> op >> y; switch ( op ) { case ‘+’ : cout << “result = “ << x + y; // op == ‘+’ break; case ‘-’ : cout << “result = “ << x – y; // op == ‘-’ break; case ‘*’ : cout << “result = “ << x * y; // op == ‘*’ break; case ‘/’ : cout << “result = “ << x / y; // op == ‘/’ break; default : cout << “unknown operation “ // op != any known operation }

if…else can do the same thing // Evaluate expression x op y (where op is +, -, * or /) int x, y; char op; cout << “enter any expression, such as 4 + 5: “; cin >> x >> op >> y; if ( op == ‘+’) cout << “result = “ << x + y; // op == ‘+’ else if (op == ‘-’) cout << “result = “ << x – y; // op == ‘-’ else if (op == ‘*’) cout << “result = “ << x * y; // op == ‘*’ else if (op == ‘/’) cout << “result = “ << x / y; // op == ‘/’ else cout << “unknown operation “ // op != any known operation

Switch Statement • Warning • “break;” statements must be at the end of each sequence of statements • Otherwise, execution of statements continues into the next case • The “break;” forces execution to jump from there to the end of the switch statement

Switch Statement w/o break; • The problematic switch below produces this output for input: 5 * 3 • result = 15result = 1unknown operation // Evaluate expression x op y (where op is +, -, * or /) int x, y; char op; cout << “enter any expression, such as 4 + 5: “; cin >> x >> op >> y; switch ( op ) { case ‘+’ : cout << “result = “ << x + y; // op == ‘+’ case ‘-’ : cout << “result = “ << x – y; // op == ‘-’ case ‘*’ : cout << “result = “ << x * y; // op == ‘*’ case ‘/’ : cout << “result = “ << x / y; // op == ‘/’ default : cout << “unknown operation “ // op != any known operation }

A Use for a Switch Statement w/o some breaks // this switch works FINE // Evaluate expression x op y (where op is +, -, * or /) double x, y; char op; cout << “enter any expression, such as 4 + 5: “; cin >> x >> op >> y; switch ( op ) { case ‘-’ : y = -y; // negate y and then add it to x below case ‘+’ : cout << “result = “ << x + y; // op == ‘+’ or ‘-’ break; case ‘/’ : y = 1.0 / y; // invert y and multiply this inverse time x below case ‘*’ : cout << “result = “ << x * y; // op == ‘/’ or ‘*’ break; default : cout << “unknown operation “ // op != any known operation }

While… Loop • Indefinite iteration • Allows repetition of a statement, as long as the expression is true while ( < expression> ) <statement>

While… Loop Example • Program that prints table of squares and cubes #include <iostream> using namespace std; int main () { int n = 0; // intitalize n cout << “ Number Square Cube“ << endl; while (n <= 10) // test n against limit { cout << ‘\t’ << n << ‘\t’ << n*n << ‘\t’ << n*n*n << endl; n = n + 1; // increment n } return 0; }

For… Loop • Definite Iteration (usually) • Allows repetition a predetermined number of times • Initializes loop counter(s) in the init section • Repeats statement as long as expression is true • Executes statements in the post section after each execution of the statement for ( <init>; <expression>; <post> ) <statement>

For… Loop Example • Program that prints table of squares and cubes #include <iostream> using namespace std; int main () { cout << “ Number Square Cube“ << endl; // initilize n once, compare and postincrement each time for (int n = 0; n <= 10; n++) { cout << ‘\t’ << n << ‘\t’ << n*n << ‘\t’ << n*n*n << endl; } return 0; }

Do…While Loop • Essentially and upside down while … loop • Executes the statement and then tests the expression to see if it continues to repeat do { <statement> } while ( <expression> );

Do…While Loop Example • Program segment to ensure entering a value that makes sense int age; do { cout << “Please enter your age: “; cin >> age; if (age < 0) cout < “Age cannot be less than 0 years” << endl; } while (age < 0); cout << “Thanks, you are “ << age << “ year”; if (age > 1) cout << ‘s’; cout << “ old.” << endl;

Implementing a Main Menu • Do • Output a list of user options • Read the user’s selection • Use switch statement to perform the correct selected action • While the user still wants to continue do { cout << ... cin >> selection; switch (selection) { ... } } while (selection != ‘q’);

Quick Intro to Strings • The C++ Standard Library defines a string type • #include <string> • Strings can be assigned values, sent to cout, obtained from cin, concatenated together (+), etc. • Some examples: string name = “dan”; name = name + “ the man”; cout << “I am “ << name;

Example 1 – fstr.cpp // File: fstr.cpp // Author: J H Stewman // Date: 3/5/2001 (modified 9/4/2003) // Purpose: // Things move faster in the world today. This program // reads text lines and removes vowels for fstr rdng. // Discussion: // Demo of the use of loops and the switch statement. #include <iostream> #include <string> #include <cctype> using namespace std;

Example 1 – fstr.cpp int main () { char charIn; string sentenceOut; cout << "The world is a faster place these days!" << endl; cout << "Enter a short sentence and the extraneous characters" << " will be removed." << endl; cout << "-> "; sentenceOut = ""; // inititlizes it to empty string

Example 1 – fstr.cpp // gets characters one at a time until newline is found while ((charIn = cin.get()) != '\n') { switch (toupper(charIn)) { case 'A': case 'E': case 'I': case 'O': case 'U': // there is really nothing to do but SKIP THIS VOWEL break; default: // concatenate this NON-VOWEL with the output string sentenceOut = sentenceOut + charIn; } }

Example 1 – fstr.cpp cout << endl; cout << "Here is your sentence \"th fstr wy!\"" << endl << endl; cout << sentenceOut << endl << endl; return 0; }

Fstr Sample Use The world is a faster place these days! Enter a short sentence and the extraneous characters will be removed. -> this is a test of the system. if this was a real emergency ... Here is your sentence "th fstr wy!" ths s tst f th systm. f ths ws rl mrgncy ...

Example – switchit2.cpp // File: switchit2.cpp // Author: J H Stewman // Date: 3/2/2001 (modified 9/4/2003) // Purpose: Evaluation of binary expressions (integer arithmetic) // Discussion: // Illustrates use of the switch statement and the do-while // loop #include <iostream> using namespace std; int main ( ) { int left_operand, right_operand, result; char operation, response; bool error;

Example – switchit2.cpp do { error = false; // get an input expression cout << "\nEnter a simple binary expression " << "(Ex: 3 + 5)" << endl; cout << "--> "; cin >> left_operand >> operation >> right_operand;

Example – switchit2.cpp // perform specified operation (can only be one) switch (operation) { case '+': // addition result = left_operand + right_operand; break; case '-': // subtraction result = left_operand - right_operand; break; case'*': // multiplication result = left_operand * right_operand; break; case '/': // division result = left_operand / right_operand; break; case '%': //remainder result = left_operand % right_operand; break; default: // oops error = true; }

Example – switchit2.cpp // show results to user if (!error) { cout << "(" << left_operand << " " << operation; cout << " " << right_operand << ") = " << result << endl; } else cout << "This is not a recognized binary expression." << endl; // find out whether to continue cout << "\nWould you like to enter another? (y/n)"; cin >> response; } while ((response == 'y') || (response == 'Y')); return 0; }

Switchit2 Sample Use Enter a simple binary expression (Ex: 3 + 5) --> 3 + 5 (3 + 5) = 8 Would you like to enter another? (y/n)y Enter a simple binary expression (Ex: 3 + 5) --> 4 * 9 (4 * 9) = 36 Would you like to enter another? (y/n)y Enter a simple binary expression (Ex: 3 + 5) --> 23 - 5 (23 - 5) = 18 Would you like to enter another? (y/n)y Enter a simple binary expression (Ex: 3 + 5) --> 45 / 3 (45 / 3) = 15 Would you like to enter another? (y/n)n

Example – SinePlot.cpp // File: sinePlot.cpp // Author: J H Stewman // Date: 9/7/2003 // Purpose: Demonstrate plot of sione function using loops // Discussion: // This program calculates the value of the sine function // over a range of 360 degrees and plots it (sideways) // on the screen #include <iostream> #include <iomanip> #include <cmath> using namespace std; int main ( ) { const double rads_per_degree = 2 * 3.14159 / 360.0; double thetaD, thetaR, sineTheta; int s; cout.setf(ios::fixed); // same as cout << fixed; // show header for table of values of theta & sin(theta) cout << "Theta(deg) | sin(theta)\n"; cout << "-----------------------\n";

Example – SinePlot.cpp for (thetaD = 0; thetaD <= 360; thetaD += 10) { thetaR = thetaD * rads_per_degree; // convert to radians // output theta and sin(theta) for this display line cout << " " << setw(4) << setprecision(0) << thetaD; sineTheta = sin(thetaR); cout << " " << setw(7) << setprecision(4) << sineTheta; // compute integer position of function on display line s = 50 + (int)(25 * sineTheta); // draw function plot for this display line for (int x=25; x<=75; x++) { if (x == s) cout << "*"; // plot the function itself else if (x == 50) cout << "|"; // plot the 0-axis else if (((s < x) && (x < 50)) || ((50 < x) && (x < s))) cout << "-"; // show area between axis and function else cout << " "; // show blank spaces outside function } cout << endl; // go to next line on screen } return 0; }

SinePlot Output Theta(deg) | sin(theta) ----------------------- 0 0.0000 * 10 0.1736 |---* 20 0.3420 |-------* 30 0.5000 |-----------* 40 0.6428 |---------------* 50 0.7660 |------------------* 60 0.8660 |--------------------* 70 0.9397 |----------------------* 80 0.9848 |-----------------------* 90 1.0000 |-----------------------* 100 0.9848 |-----------------------* 110 0.9397 |----------------------* 120 0.8660 |--------------------* 130 0.7660 |------------------* 140 0.6428 |---------------* 150 0.5000 |-----------* 160 0.3420 |-------* 170 0.1737 |---* 180 0.0000 * 190 -0.1736 *---| 200 -0.3420 *-------| 210 -0.5000 *-----------| 220 -0.6428 *---------------| 230 -0.7660 *------------------| 240 -0.8660 *--------------------| 250 -0.9397 *----------------------| 260 -0.9848 *-----------------------| 270 -1.0000 *-----------------------| 280 -0.9848 *-----------------------| 290 -0.9397 *----------------------| 300 -0.8660 *--------------------| 310 -0.7660 *------------------| 320 -0.6428 *---------------| 330 -0.5000 *-----------| 340 -0.3420 *-------| 350 -0.1737 *---| 360 -0.0000 *

CSE 20232 Lecture 5 – Decisions and Loops