Loop, For...Next

The For Each...Next loop structure is similar to the For...Next loop structure, except that it executes the statements within the loop for each object within a group of objects. Figure 14-12 illustrates the syntax of the statement. 

Often you use a loop structure to search through an array or collection of objects, looking for a certain value or property. Once you find a match, you don t need to cycle through the rest of the loops. You can exit from the loop using the Exit For (from a For...Next loop or For Each...Next loop) or Exit Do (from a Do While... loop). The Exit statement will normally be located within an If statement. For example. 

Duplicate variable names can cause all sorts of problems. You might use a variable called A to hold a value at the beginning of the program, and then inadvertently use the same variable name later on. If the program returns to the beginning, the value has changed. FOR-NEXT loops sometimes lead to duplication. When you re using a variable like A, make sure you don t use it as an index in a FOR-NEXT loop. And remember, only the first two characters of a variable count the computer thinks ALT is the same variable name as ALIEN. 

To avoid doubled variables, it helps to pick certain letters to be used only in loops and as "temporary" variables. For example, decide ahead of time that you will always use J, K, and L in FOR-NEXT loops. 

When BASIC begins a FOR-NEXT loop, it writes a note about where in memory the loop begins, pushes it on the stack, and forgets about it until it comes across a NEXT statement. NEXT tells the computer that somewhere earlier in the program a FOR started a loop. It then pulls the information it needs off the stack and jumps back. 

Don t worry about adding any NEXTs, the computer will never get that far. Run the program and you ll get an OUT OF MEMORY error after only ten loops have begun. A FOR-NEXT loop uses up a lot of space on the stack—for a pointer to beginning of the loop, step size, highest value, and variable names. 

When stack problems pop up, they re often caused by a GOTO in the middle of a subroutine. It can leave some garbage on the stack. The same goes for jumping out of a FOR-NEXT loop. And too many parentheses can give you either a FORMULA TOO COMPLEX error or contribute to an OUT OF MEMORY message. As the garbage on the stack builds up, it eventually reaches the limit. 

Check free memory. If there seems to be a lot left, you may have a full stack, caused by too many unresolved FOR-NEXT loops or GOSUBs. 

Perhaps you ve narrowed it down to a certain FOR-NEXT loop. An important variable, K8, is somehow being changed. So you add a line PRINT K8 STOP, and every time the program reaches that line, it prints the value of K8 and stops. 

If your computer locks up, consider the possibility that your computer is not locked up. A FOR-NEXT loop that counts to a million takes a lot of time. So does POKEing a few thousand numbers into memory. And it s possible to write an inefficient sorting routine that takes hour, even days, to complete. In cases like these, you might want to demonstrate that there s no lockup by printing to the screen or changing border color once in a while. 

The description request flag in line 400 determines whether this section of the program is executed or skipped over. Remember that 0 indicates the latter. If it is 0, then, this entire routine is bypassed. If it is executed, describing the room consists simply of printing the appropriate element of the room descripion array. That s line 410. Then in line 430, a FOR-NEXT loop executes, which goes through each item in the item location array. For each item that s located in the current room (F = 0), it prints the corresponding element of the item description array (done in line 450 and 460). This way the player will see what each room contains. 

Before we use it, we will first explain the working of this function, line by line. The first line defines the function by its name, Smallstep, and specifies what input data the function requires. The last line specifies the end of the function. In between, the value of ais recomputed n times, using a For... next loop in lines 2 through 4, and the result is specified in line 5. These instructions are written in Visual BASIC, the language that Excel uses for its functions and macros. (Visual BASIC is a modern form of compiled BASIC, with additions to make it suitable for spreadsheet use. Section 10.12 will review some features ofVisual BASIC.)... 

Note that the third line of the function is equivalent to (9.2-18) except that Af has been replaced by At/n, which is then repeated n times in the For. .. next loop. And if you wonder where the labels wand w-1 on whave gone, the equal sign in BASIC actually means an assignment, i.e., the computer first evaluates the value of a (i-a k delt/n), then assigns that value to the variable a on the left-hand side of the equal sign. 

Below the assignment Array = Selection.Value we encounter two nested For Next loops. In each loop a calculation is repeated a specified number of times. In this example, each numerical value in the array is in turn raised to its cube power (or to any other power we might have specified instead of 3). Incidentally, note that the indentation used facilitates our reading of the macro, by showing what lines are involved in the two different loops. 

The cause of the problem lies in the numbers 3 and 113 used to raise the individual terms of the array to their cube power, and to take their cube root. When we raised the individual cells in the block (or the individual elements in Array) to their cube power, we reduced the results to single precision. To demonstrate this and, at the same time, fix the problem, add a dimension statement to both macros, reading Dim p As Double , and add a line specifying the value of p (e.g., either p = 3 or p =1/3 ). Finally, in the For Each... Next (or in the nested For... Next loop) refer to p rather than to 3 (or 1/3). The macros now should read like the example shown below, where we have also added a title and some comment lines and, as before, boldfaced the most recent changes. 

There are two types of control loops. When you knowhow many times a particular procedure should be repeated, use a For...Next loop when you do not know this in advance, use a Do loop instead. 

When a For... Next loop contains only a single, short statement, it may be written on one line, with its three parts separated by colons, as in... 

For...Next loops may be nested. In that case it is good practice to use separate counters, as in... 

The syntax of the Do...Until loop is similar except that the term While is replaced by Until . In the above example, the Do Until statement would read Do Until c > N to yield the same result. As with For... Next loops, Do loops can be nested. 

For Each...Next loop 268 For...Next loop 268 form, custom 159 Format Cells dialog box 33 Format Painter toolbutton 34 formats, number... 

The numbers correspond to the letters in the name John Smith in particular, the number 32 corresponds to the space. The FOR-NEXT loop copies the entries from... 

Alternative ways of coding that fragment without using the EXIT DO statement are either longer or more obscure. Standard BASIC also allows exiting from the middle of a FOR-NEXT loop. Standard BASIC includes a variety of constructs for making choices for example ... 

VBA does loops in two different ways. For.Next loops do a set number of iterations of the loop (typically from i = 1 to a higher number [N in loop below] or a variable that is equal to a higher number). A sinple loop to write values on the spreadsheet is. 

We can also write For.Next loops with a logic expression (e.g., If eqnO > 0, Then Exit For ) to exit the loop. 

FOR-NEXT Loops. One of the simplest loops is a for-next loop, essentially a counting loop. Hie program cycles around the loop a prescribed number of times. While in the loop, however, one can do a large number of things. An example of a simple counting loop is... 

We first define five single-precision variables as being equal to zero. Then, using a for-NEXT loop for the N data points, we determine the summations found in the determinants required for a Cramer s rule solution to simultaneous equations for m and b. For example, as A cycles through the loop, it adds 2 each time for N times. A determines 2. Likewise, B determines 2x, and so on. Once the sums are determined, we calculate the determinants D, Dj, and >2, which allows us to calculate m and b. The total program in FUTUREBASICII is ... 

It is possible to put loops inside loops. This process is called nesting. For example, suppose we wished to print out all the possible permutations of Miller indices h, k, and 1 from 0 to 6. We can do this with nested for-next loops. The steps are ... 

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