Fundamental Programming 310201 Fundamental Programming for Loops.

Fundamental Programming 310201

Fundamental Programming

for Loops


Repetition Statements

we have now used two C++ repetition statement – while and do-while

while tests the loop condition at the start of the loop – allowing for the possibility that we may not need to perform the loop

do-while tests the loop condition at the end of the loop – useful when we need to perform a loop at least once


Repetition Statements

in this class we introduce one more repetition statement - for

for is just a convenience, you don’t need it - anything you can do with for, you can do with while

for is convenient when you know in advance how many times you need to perform a loop – instead of…


for vs while cout << “Number of marks in exam ==> “;

cin >> NbrMarks; cout >> “Number of students ==> “ cin >> NbrStudents; NbrLoops = 0;while (NbrLoops < NbrStudents){

cout << “Student’s mark ==> “;cin >> StudentMark;Percentage = 100 * StudentMark / NbrMarks;cout << “ Student’s percentage: “;cout << Percentage;NbrLoops = NbrLoops +1;

}

notes: initialise loop control variable




cout << “Student’s mark ==> “;cin >> StudentMark;Percentage = 100 * StudentMark / NbrMarks;cout << “ Student’s percentage: “;cout << Percentage;NbrLoops = NbrLoops +1;

}

notes: loop condition




cout << “Student’s mark ==> “;cin >> StudentMark;Percentage = 100 * StudentMark / NbrMarks;cout << “ Student’s percentage: “;cout << Percentage;NbrLoops++;

}

notes: modify loop control variable (to avoid looping forever)


cout << “Number of marks in exam ==> “; cin >> NbrMarks; cout >> “Number of students ==> “ cin >> NbrStudents; for (NbrLoops = 0; NbrLoops < NbrStudents; NbrLoops++){

cout << “Student’s mark ==> “;cin >> StudentMark;Percentage = 100 * StudentMark / NbrMarks;cout << “ Student’s percentage: “;cout << Percentage;

}

for vs while

notes: initialise loop control variable




}

for vs while

notes: loop condition




}

for vs while

notes: modify loop control variable (to avoid looping forever)


for ( < loop initialisation statement > ; < loop condition > ; < loop completion statement > ){

< for statements >}

for Loop Syntax

notes: parentheses around for clause



< for statements >}

for Loop Syntax

notes: statement performed once before entering loop for first time



< for statements >}

for Loop Syntax

notes: semi-colons after loop initialisation and loop condition



< for statements >}

for Loop Syntax

notes: condition tested at the start of each loop – including the very first loop



< for statements >}

for Loop Syntax

notes: statement performed at the end of each loop


for Loop Operation

for

loopstatements

loopconditio

n

false

true

loopinitialisestateme

nt

loopcompletio

nstatemen

t


Activity

for ( int Counter = 0; Counter < 5; Counter++ ) {

cout << “Counter = “ << Counter << endl;}

what output is produced by the following code:


Activity Break


Activity Feedback

Counter = 0Counter = 1Counter = 2Counter = 3Counter = 4

the output produced by this code is:


Loop Control Variables

usually an integer, but can be a character

can be declared within the for clause – instead of...


int NbrLoops;cout << “Number of marks in exam ==> “; cin >> NbrMarks; cout >> “Number of students ==> “ cin >> NbrStudents; for (NbrLoops = 0; NbrLoops < NbrStudents; NbrLoops++){


}



cout << “Number of marks in exam ==> “; cin >> NbrMarks; cout >> “Number of students ==> “ cin >> NbrStudents; for (int NbrLoops = 0; NbrLoops < NbrStudents; NbrLoops++){


}



int NbrLoops;cout << “Number of marks in exam ==> “; cin >> NbrMarks; cout >> “Number of students ==> “ cin >> NbrStudents; for (int NbrLoops = 0; NbrLoops < NbrStudents; NbrLoops++){


}


notes: if you try to declare the same variable twice, you get a compilation error



loop control variables are not normally modified within the loop

it’s a common source of error…


Activity

for ( int Counter = 0; Counter != 5; Counter++ ) {

cout << “Counter = “ << Counter << endl; Counter = Counter + 1;}

what output is produced by the following code:


Activity Break


Activity Feedback

Counter = 0Counter = 2Counter = 4Counter = 6Counter = 8:( until you terminate the program! )

the output produced by this code is:


Activity

write the for clause of a for loop to produce this output:

Counter = 15Counter = 14Counter = 13Counter = 12Counter = 11Counter = 10


Activity Break


Activity Feedback

the for clause of a for loop to produce this output is:

for ( int Counter = 15; Counter >= 10; Counter-- )


Nested Loopswe saw that an if-else statement can be embedded inside another if-else statement

likewise, a for statement can be nested inside another for statement

note: in fact, any selection statement (if-else, switch) or repetition statement (while, do-while, for) can be embedded inside another selection or repetition statement


Nested Loops

with a nested loop, one performs one or more trips around the inner loop for each trip around the outer loop

here’s an example...


Nested Loops

for ( int RowNbr = 1; RowNbr <= 5; RowNbr++ ){ cout << endl << “Row “ << RowNbr << “:“; for ( int ColNbr = 1; ColNbr <= 3; ColNbr++ ) { cout << " Col " << ColNbr ; }}

inner loop


Activity


activity: what does this code produce as output


Activity Break


Activity Feedback


feedback: this code produces the following output… Col 1 Col 2 Col3


Nested Loops


outer loop

inner loop


Nested Loops


note: here, three trips around the inner loop are performed for each trip around the outer loop


Activity


activity: what is the output produced by this code?


Activity Break


Activity Feedback


feedback: the output produced by this code isRow 1: Col 1 Col 2 Col 3 Row 2: Col 1 Col 2 Col 3 Row 3: Col 1 Col 2 Col 3Row 4: Col 1 Col 2 Col 3


Looping Examples

Chapter 7 of the textbook provides many examples of looping

there are also many examples in the Study Guide

use tracing to check that you understand the mechanics of looping statements


Increment and Decrement

the textbook would use ++RowNbr instead of RowNbr++ in a for loopfor ( int RowNbr = 1; RowNbr <= 5; ++RowNbr )

the above for clause has the exact same effect as the one below for ( int RowNbr = 1; RowNbr <= 5; RowNbr++ )


Increment and Decrement

the difference between ++RowNbr and RowNbr++ is quite subtle – it can be seen from:RowNbr = 1;cout << RowNbr++ << endl;cout << RowNbr;

RowNbr = 1;cout << ++RowNbr << endl;cout << RowNbr;

12

22

output


Increment and Decrementrecall the syntax of cout:cout << <expression>;

below, the expression is simply the value of variable RowNbr cout << RowNbr;

below, the value of RowNbr is incremented after it’s value is used in the expression

RowNbr = 1;cout << RowNbr++ << endl;

1 output


Increment and Decrementbelow, the value of RowNbr is incremented before it’s value is used in the expression

RowNbr = 1;cout << ++RowNbr << endl;

the statements below have exactly the same effect – they simply increment the value of RowNbrRowNbr++;

++RowNbr;

2 output


Summarythe for loop is a convenience, it’s not needed - anything you can do with a for loop you can do with a while loop

the for loop is useful when the number of required trips around a loop is known before entering the loop

consequently, the for loop is useful when using arrays – the topic of a future class

Fundamental Programming 310201 Fundamental Programming for Loops.

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