Computer Science 1 Top Lectures Assignments Noticeboard For loops

• One common loop pattern involves initializing a variable, and then updating it each iteration, until a condition is reached.
• Because this loop is so common, Java (and other languages) provide a special loop statement

The for loop

• The structure of a for loop is as follows:

for (initialization; loop condition; increment) {
  body
}

• The initialization part is performed once before the loop condition is checked for the first time.
• The loop condition and body play the same role as for the while statement.
• The increment is executed each time after the body is executed, but before the loop condition is checked.
• We can translate a for loop into a while loop in the following way:

initialization;
while (loop condition) {
  body
  increment
}

Example:

• Sum all the numbers between 1 and 10

int i;
int sum = 0;
for (i=1; i<=10; i++) {
  sum += i;
}

• Note that i++ is equivalent to i += 1
• Note that any number of initializations and increments can occur in the appropriate parts of the for loop
• each separate initialization or increment must be separated by a comma

int i;
int sum;
for (i=1, sum=0; i<=10; sum+=i, i++)

• The above loop also sums the numbers between 1 and 10.

Exercises

• Write a for loop that calculates 2 to the power of x (i.e., 2 * 2 * 2 * ... * 2, x times).
• Write a for loop that reads in 10 integers from the keyboard, and sums them.