Overview of Loops

End of File Loop

while (infile.hasNext()) // true if more unread data on file{

score = infile.nextInt(); // now input is at beginning of loop

// actions

}

Sentinel-controlled Loop

Sop (“Enter a score (-1 to quit)”);score = kb.nextInt(); // “priming” readwhile (score != -1) // sentinel value is -1{

// actions

Sop (“Enter another score (-1 to quit));score = kb.nextInt(); // remember to read another value at end of

loop}

Count-controlled Loops

int i = 1; // initializationwhile (i <= 10) // test{

Sop ("i = " + i); // action(s)...

i++; // increment}

for ( int i=1; i<=10; i++ ){

Sop ("i = " + i); // action(s)

.

.

.}

When to use while vs for?

while• When you don’t know ahead of

time how many times something needs to happen

for• When you do know ahead of

time how many times something needs to happen• Note: could use while in this situation

but most programmers prefer for

We have 3 files that contain a list of golf scores that we want to average.

golf1.txt

7865827091887568

golf2.txt

7865827091887568-1

golf3.txt

8 7865827091887568

File has scores only File has sentinel value First number in file tells us number of scores on file

golf1.txt (has scores only) – use an “end of file loop”

int score;int total = 0;int numScores = 0;double avg;

while (infile.hasNext()){ score = infile.nextInt(); total = total + score; numScores++;}

avg = (double) total / numScores;SOP (avg);

golf1.txt

7865827091887568

golf2.txt (has sentinel value) – use a “sentinel-controlled loop”

int score;int total = 0;int numScores = 0;double avg;

score = infile.nextInt();while (score != -1){ total = total + score; numScores++; score = infile.nextInt();}

avg = (double) total / numScores;SOP (avg);

golf2.txt

7865827091887568-1

golf3.txt (has number of scores at beginning of file)version 1 – use a “count-controlled” while loop

int score;int total = 0;int numScores;double avg;int count = 0;

numScores = infile.nextInt();while (count < numScores){ score = infile.nextInt(); total = total + score; count++;}

avg = (double) total / numScores;SOP (avg);

golf3.txt

8 7865827091887568

golf3.txt (has number of scores at beginning of file)version 2 – use a “count-controlled” for loop

int score;int total = 0;int numScores;double avg;

numScores = infile.nextInt();for (int i=0; i < numScores; i++){ score = infile.nextInt(); total = total + score;}

avg = (double) total / numScores;SOP (avg);

golf3.txt

8 7865827091887568