For-loop statement

The for-loop statement is the one typically supported for synthesis. A for-loop is implemented by unrolling the for-loop, that is, all statements within the for-loop are replicated, once for each value of the for-loop index. This puts a restriction on the for-loop bounds, which must therefore evaluate to constants. Here is an example of a for-loop statement. 

After in-line expansion of the function call and further in-line expansion of the for-loop statement, the following code is obtained. 

We now consider a structure node in which the set of nodes is partitioned in a number of subsets. Nodes in a subset have identical behavior. These subsets are called node domains. Let D be a domain and iV be a type of node. Then a node domain V)vd is characterized by the pair (iV, D) and consists of an indexed set of nodes Vnd = i I D. The node domain of the Floyd-Steinberg algorithm is given in figure 5. The boundary planes of the domain are derived from the upper and lower bound expressions of the FOR-loop statements of its nested loop program. 

A For-Loop statement enables a set of other sequential statements to be repeated for a fixed number of iterations. It has the following syntax. 

By acting on the present value of the iteration parameter or any other condition, fee present iteration of the loop or the whole For Loop statement can be terminated. See Box 6.12... 

Remember that multiple calls to the same procedure will result in logic being generated each time. For a concurrent procedure, multiple calls can be made through a For-Generate statement. For a sequential procedure, multiple calls can be made with a For-Loop statement. 

Since many digital waveforms are periodic, a GOTO statement is provided for looping. The attributes in Table 8-3 and shown in the screen capture produce a clock frequency of 1 kHz for four cycles and then 500 Hz for an infinite number of cycles ... 

When the for-loop is expanded, the following four i f statements are obtained. 

There is no correlation between the gates produced and the number of lines of Verilog HDL code. A 2500-gate circuit could have been synthesized from a 10-line Verilog HDL code (may have a for-loop and/or vectors) or from 10,000 lines of Verilog HDL code (maybe from a large case statement with simple assignments). 

From a Do or For loop, control is transferred to the statement following the Loop or Next statement, or, in the case of nested loops, to the loop that is one level above the loop containing the Exit statement. From a Function or Sub procedure, control is transferred to the statement following the one that called the procedure. 

Use for construct when the number of loop iterations is known beforehand. Likewise, use while construct when the number of loop iterations is not known beforehand. Cases include reading data from a file or from user input line by line until the end is encountered. Though this can be achieved by using a for loop with a conditional break statement, the while statement conveys the logic clearly. Some special cases require using do-while (when the first statement has to be executed before the conditional). 

The For Each...Next loop is similar to a For... Next statement except that it will automatically apply the statements to each element of the specified group. The syntax is... 

C has two additional flow of control statements, break and continue, that augment the capabilities of the loops that have just been described. The break statement is used to break out of (to immediately terminate) the enclosing do, while, for, or switch statement. Its use is routine in switch statements, to terminate the switch statement at the conclusion of each individual case. In do, while, and for loops, break is often used to terminate the loop prematurely when an error or special condition occurs. Using a break in the body of a loop often simplifies the control expression of the loop, because it allows special case code to be placed elsewhere. 

Local formulas correspond to each type of statement in the VHDL subset. In general, the formalization of a statement consists of two parts flow and function. The flow component captures the program flow implicit in the statement. In traditional procedural languages, flow proceeds from one statement to the next, except for loops and selection statements. In VHDL, the situation is complicated somewhat by the presence of a time dimension for certain statements (e.g. WAIT). The second component of a statement formalization is the functional component, and captures the semantics of the variable values affected by the statement. The functional component is complicated by the presence of statements that affect variable values at a different time (e.g. signal assignment). 

The following subsections give the semantics for three kinds of statement wait scheduled assignment, and their sequential and parallel compositions. A process consists of a looped statement with at least one wait statement inside the loop. Parallelism is only one level deep in VHDL and in VHDL-AMS. 

A solution is to divide up the maximum allowable execution time of a program segment into a budget for each individual method call and a remainder for other statements. This can be done in such a way that the total execution time (taking account of any loops involved) will meet the deadline, as long as no individual method call exceeds its budgeted time. 

It is also possible to create functions in Mathcad. The authors leave it up to the reader to explore function options more fully but review control structures for functions here, including/or loops, while loops, and /statements. The general format for control statements is as follows ... 

To insert a/or loop, type the name of the variable, use a definition equals sign, then select Add Line on the Programming toolbar, and choose for. For loops will continue to execute a line of code for a given range variable therefore, as a minimum, a range variable and a loop statement are required. A simple example is shown as follows ... 

While loops are entered in a similar way to for loops however, while is chosen. The condition statements will continue to be executed so long as the while criteria is met.. Tloops are also from the same toolbar, choosing if. condition statements are executed only if the condition statement is evaluated as the Boolean true. Control statements can be made increasingly complex by adding lines. It is important to remember that the user cannot type while, if or for but must select the operator from the programming toolbox. It is also important to note that Mathcad indexes from zero and a subscript operator (found in the matrix toolbar) must be used to call a specific element. Please see Section 5.3.1 for the implementation of a for loop. 

Example 6 An endless loop Mathcad provides programming operators for creating for loops and while loops. To create until loop or interrupt a loop during execution, we should create an endless loop and insert statements break, continue, or return. To create an endless loop we use infinite operand in a while statement. The infinity symbol (it may be any non-zero number) is typed in white on white (Fig. 6.60). 

The Process statement part may only contain sequential statements such as If-Else, Case and For Loop (Chapter 6). These are executed in the order in which they appear and the behaviour will be modelled with combinational and/ or sequential (synchronous or asynchronous) logic. 

Any sequential statements can be used inside the loop except a Walt statement. Therefore a For Loop can only be used to create combinational logic. This is constructed by iinfolding the loop during compilation. 

Program control statements include those for decision making (branching) and for looping. Many such statements available in VBA are discussed below. Ones less frequently used (or ones that are repetitious) are not covered. 

There are four looping statements in VBA (For...Next, Do While...Loop, Do...Loop While, For Each...Next). Only the first two of these are presented since the other two are repetitious (or can be confusing). 

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