1 Unit 7a 'while' and 'for' Loop Syntax and Semantics
2 Control Structures • We need ways of making decisions in our program – To repeat code until we want it to stop – To only execute certain code if a condition is true – To execute one segment of code or another • Language constructs that allow us to make decisions are referred to as control structures • The common ones are: – if statements – switch statements – while loops – for loops
3 Loops • Loops are structures of code that may be repeated some number of times • Examples: – Sum each student's grades (for all students in the class) – Search through a sequence of numbers for a particular value – Attend lecture ☺ • We need some condition to tell us when to stop looping, otherwise we'll repeat our code forever and never stop (a.k.a. an infinite loop) • Several kinds of loops: 'while', 'do..while', and 'for'
4 Generalizing and repeating code MOTIVATION FOR LOOPS
5 Motivation for Loops #include <iostream> using namespace std; • Take a simple task such int main() { cout << 1 << endl; as outputting the first cout << 2 << endl; cout << 3 << endl; 1000 positive integers // hundreds more cout statements – We could write 1000 cout << 999 << endl; cout << 1000 << endl; cout statements return 0; } – Yikes! We could do it but it would be painful! #include <iostream> using namespace std; int main() • Or we could use a loop { for(int i=1; i <= 1000; i+=1 ) { cout << i << endl; } return 0; }
6 Why We Need Loops (1) #include <iostream> using namespace std; • Suppose we are writing int main() { a program for a simple int guess; int secretNum = /* some code */ cin >> guess; turn-based guessing if(guess != secretNum) { /* What should we do here? */ game where the user must guess a secret } else { cout << "You got it!" << endl; number } return 0; • If they guess incorrectly } what should we do?
7 Why We Need Loops (2) #include <iostream> using namespace std; • What if they guess int main() { wrong a second time? int guess; int secretNum = /* some code */ cin >> guess; What should we do? if(guess != secretNum) { cin >> guess; if(guess != secretNum) { /* What should we do here? */ } else { cout << "You got it!" << endl; } } else { cout << "You got it!" << endl; } return 0; }
8 Why We Need Loops (3) #include <iostream> using namespace std; • We can never write int main() { enough if statements int guess; int secretNum = /* some code */ cin >> guess; because someone might if(guess != secretNum) { cin >> guess; always use one more if(guess != secretNum) { cin >> guess; turn than we have if if(guess != secretNum) { /* What should we do here? */ } statements else { cout << "You got it!"<< endl; • But we see there is a } } else { repetitive structure in cout << "You got it!" << endl; } this code } else { • Let's use a loop cout << "You got it!" << endl; }
9 4 Necessary Parts of a Loop • Loops involve writing a task to be repeated • Regardless of that task, there must be Initialization 4 parts to a make a loop work (e.g. i = 1) • Initialization Loop – Initialization of the variable(s) that will control how many iterations (repetitions) Condition the loop will executed ( e.g. i <= 1000 ) • Condition – Condition to decide whether to repeat the True task or stop the loop Body False • (cout << i << endl;) Body – Code to repeat for each iteration Update Statement • Update (e.g. i += 1) – Modify the variable(s) related to the condition Code after the loop
10 Types of Loops • There are 2 (and a half) kinds of loops • for loops and while ( do..while ) loops There is a variant of the while loop which is the do..while loop which we'll cover later. int i; int i = 1; 4 parts: for ( i = 1; i <= 1000; i++ ) while ( i <= 1000 ) • Initialization { { • cout << i << endl; // repetitive task Condition } cout << i << endl; • Body // following statements i++; // update • Update } // following statements There is a variant of the while loop which is the do..while loop which we'll cover later.
11 Type 1: while Loops • A while loop is essentially a repeating 'if' statement // initialization // initialization if (condition) while ( condition ) { { // executed if condition1 is true // executed if condition1 is true } // update statement // following statements } // go to top, eval cond1 again // following statements Initialization Initialization False False condition condition True True If Block while Block Statements Statements Update Statement Following Following statements statements
12 Type 1: while Loops • A while loop is essentially a repeating 'if' statement 1 initialization Initialization 2 5 8 while ( condition1 ) (e.g. i = 1) T T F { 3 6 // Body: if condition1 is true Loop 4 7 Condition } // go to top, eval cond1 again ( e.g. i <= 1000 ) 9 // following statements True // only gets here when cond1 is false Loop task False (cout << i << endl;) Update Statement (e.g. i += 1) Code after the loop
13 Type 1: Applying the 4 Parts • Examine the guessing game code #include <iostream> using namespace std; • When using a loop there are int main() { almost always FOUR parts: int guess; • Initialization int secretNum = /* some code */ – 1 or more variables that are part of cin >> guess; while(guess != secretNum) the loop { cout << "Wrong, guess again: " << endl; • Body cin >> guess; } • Condition cout << "You got it!" << endl; – Condition for continuing return 0; } • Update: – 1 or more variables involved in the Always make sure you have the 4 parts condition (without the update, the (it's easy to forget initialization and/or update) condition could be TRUE forever leading to an "infinite loop")
14 What Goes In a Loop Body • What do we put in a #include <iostream> using namespace std; int main() while or for loop { int guess; body? int secretNum = /* some code */ • ANYTHING! cin >> guess; while(guess != secretNum) { – Expressions & variable cout << "Enter guess: " << endl; cin >> guess; assignment } – Function calls cout << "You got it!" << endl; return 0; } – if..else statements – Even other loops!
15 Hand Tracing (1) int main() { • Ensure you understand int x, y; cin >> x; the meaning (semantics) while( (x % 2) == 0){ x = x/2; of a while loop by tracing } through the code to the cin >> y; while(y > 0){ right if( y >= 10 ){ y -= 5; • Show all changes to x and } else if( y >= 5 ){ y for: y -= 3; } – x = 24 else { y -= 1; – y = 18 } } return 0; }
16 Hand Tracing (2) int main() { • Trace through the code int x, y; cin >> x; and show all changes to x while( (x % 2) == 0){ x = x/2; and y for: } – x = 27 cin >> y; while(y > 0){ – y = 6 if( y >= 10 ){ y -= 5; } else if( y >= 5 ){ y -= 3; } else { y -= 1; } } return 0; }
17 Type 2: for Loops • 'for' loops have the same ability as a 'while' loop but makes the 4 parts of a loop EXPLICIT // initialization for( init ; condition ; update ) while ( condition ) { { // executed if condition is true // executed if condition is true } // go to top, do update, eval cond. again // Update statement } // following statements // following statements init init condition condition False False True True while Block For Block Update Statement Example Statements for( int i=1; i < 1000; i++) Following { Update cout << i << endl; statements Following }
18 Type 2: 'for' Loop Sequencing Condition: T T F • 'for' loop 1 2 5 8 4 7 for( init; condition; update ) { – performs initialization 3 6 // executed if condition is true } // go to top, do update, eval cond. again statement once // following statements 9 – checks the condition // only gets here when cond. is false each iteration before init False deciding to execute the condition body or end the loop True For Block – performs the update Statements statement after each Update execution of the body Following
19 Some Examples #include <iostream> #include <iostream> using namespace std; using namespace std; int main() int main() { { int i; int i; for(i=0; i < 5; i++) for(i=8; i > 0; i=i/2 ) { { cout << i << endl; cout << i << endl; } } return 0; return 0; } } Program Output: Program Output: 0 8 1 4 2 2 3 1 4 The initial value, condition, and update statement can be any valid expression!
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