For Loop. Variations on Format & Specific Examples

Transcription

1 For Loop The for loop is an iterative loop. You determine how many times it executes, using 3 expressions and a loop control variable (lcv). The first expression initializes the loop control variable (lcv) to a beginning value. This first expression executes only once, when your program first reaches the for loop. The second expression tests to see if the control variable is within limits. The second expression is evaluated before each repeat of the controlled statement(s). This means before the statements controlled by the loop are executed, the condition is checked. This happens on each iteration (repetition) of the loop. When the expression finally evaluates to false, the controlled statements will NOT be executed again. The third expression increments (or decrements) the control variable so that it eventually fails the second test, stopping the loop. If the loop s controlled statement was executed, then the for loop loops back up and evaluates the third expression either increasing or decreasing the value of the loop control variable (lcv). Note the 2 semicolons that separate the expressions inside the parentheses. Variations on Format & Specific Examples Format 1: Example 1: int i=0; for(i=0; i<10; i=i+1) cout<<i<<endl; Example 1 Output: This loop prints out the current value of the loop control variable (LCV) on each iteration. The LCV starts at 0 and continues to be incremented by one after each iteration so long as it is still LESS THAN 10. Note how the increment is done with a regular assignment statement in which 1 is added to the old value of i and the resulting new value assigned to i.

2 Format 2: //for Example 2: int j=0; for(j=0; j<10; j++) cout<<"j: "; cout<<j<<endl; //for Example 2 Output: This loop prints out the current value of the loop control variable (LCV) on each iteration along with a label. The LCV starts at 0 and continues to be incremented by one after each iteration so long as it is still LESS THAN 10. Note how the increment is done using the special increment operator. The result is equivalent to the statement: j=j+1 Format 3: for(declare_n_initiale_lcv; test_expression; increment_expression) Example 3: for(int n=10; n>0; n--) cout<<n<<endl; cout<<"lift Off!"; Example 3 Output: This loop starts the LCV at a higher value (10) and counts down instead of up. It prints the value of the LCV so long as that value is GREATER THAN 0. It then exits the loop and prints the message Lift Off! Notice that the LCV can be declared within the loop heading. This makes it only have meaning inside the loop (its SCOPE is inside the loop body only). Notice that the special decrement operator is used. It is equivalent to the statement: n=n-1

3 Format 4: for(declare_n_initiale_lcv; test_expression; increment_expression) Example 4: for(int p=20; p>0; p-=2) cout<<"p: "; cout<<p<<endl; Example 4 Output: This loop starts the LCV at 20 and counts down BY TWOS so long as the LCV is GREATER THAN 0. It prints a label and the current value of the LCV on each iteration. Notice the LCV is declared inside the loop heading. Notice the special assignment notation is used here. It is equivalent to writing p=p-2

4 Nested Loops When you put one loop statement inside another loop statement, you have created a nested loop. A nested loop is under the control of another loop. Example: for(int i=0; i<5; i++) for(int j=0; j<5; j++) cout<<"i: "<<i<<" j: "<<j<<endl; Example Output: Notice: The inner loop s control variable starts over (gets re-initialized) every time the outer loop s control variable increments.

5 The outer loop is like a big sprocket, and the inner loop like a little sprocket: for each turn of the outer loop, the inner loop turns many times.