C语言疑难问题探讨

本详细介绍了C语言教学实践中遇到的各种疑难问题及其解决方案。其中涉及自加，自减运算符的使用和表达式的求解顺序，数组与指针数组，scanf(),gets(),…等函数的使用。     

UPML吸收边界及其内部区域的统一建模方法

利用C＋＋语言的多态性实现了在FDTD计算中单轴各向异性完全匹配层（UPML）吸收边界与吸收边界内部计算区域的统一建模。首先构造基类一Yee元胞类及其继承类来分别封装UPML内部介质和UPML的电磁特性；然后分别创建基于以上两个类的对象数组来给UPML及其内部计算区域开辟计算空间;再构造基类类型的指针数组，并用以上数组的地址赋值；最后，所有的计算在指针数组空间完成。该方法避免了UPML与其内部计算区域间的数据传递，简化了编程。数值实验验证了UPML的吸收效果，证明了方法的有效性。     

函数间数据通信方式的分析与应用

对利用全局变量和参数来实现C语言函数间数据通信问题进行了分析 ,着重阐述了参数传递中的传值方式和传地址方式的工作原理及功能 ,在此基础上讨论了传地址方式的应用———数组形参的相关问题。     

FOXPRO2.5数组应用浅谈

木文介绍了数据库管理系统Ｆｏｘｐｒｏ２．５版中有关数组建立、赋值、作为参数传递等方面独特的功能，以及在ＳＱＬ语言中数组的应用。     

嵌入式处理器P2020机器码程序到C语言源程序的溯源方法

处理器PowerPC P2020在航空航天等嵌入式领域被广泛应用,以RTCA/DO-178C中A级软件的适航要求作为研究的出发点,提出了从文件、函数声明、函数体代码3层次实现P2020机器码程序到C源程序的溯源方法。在获取C语言源程序与PowerPC P2020机器码程序文件名列表的基础上,实现PowerPC P2020机器码文件主名的溯源;通过遍历C源程序抽象语法树和PowerPC P2020机器码获取函数名列表,实现PowerPC P2020机器码函数声明的溯源;通过定义C语言程序抽象语法树节点生成期望汇编指令序列的规则,实现PowerPC P2020机器码函数体的溯源。通过设计245个C源程序文件与345个PowerPC P2020机器码程序文件,1 111个C语言函数声明与1 273个PowerPC P2020机器码函数声明,以及覆盖C语言程序23类语法结构的460个测试用例,验证了PowerPC P2020机器码程序到C源程序的自动化溯源方法的有效性。结果表明：文件溯源和函数声明溯源的追溯匹配率达100%,程序函数体代码的平均溯源匹配率达97.22%。溯源匹配结果可以检查PowerPC P2020机器码程序是否在编译过程插入例外异常代码,以防止其带来的安全隐患,保证航空航天嵌入式安全关键软件机器码的安全性和可靠性。     

NC车削自动编程系统中信息输入建模方法的研究

信息输入是数控车床自动编程系统的第一个环节，提出信息输入建模方法的研究，旨在解决NC本削自动加工所需数据的输入，存储和快建传递的问题。利用VISUAL C 6．0语言中的模板数组类，实现零件、毛坯数据信息的输入，完成各种具体算法的实现，并将输入和计算所得的零件、毛坯的数据信息保存于计算机内部，以实现数据的传递。     

涡轮增压器监控系统与PIC单片机的串行通讯

依托Borland C 5.0集成开发环境和MPLAB IDE（PIC集成开发环境），使用Windows应用程序接口（API）函数和PIC单片机14位的精简指令（RISC）系统，开发了涡轮增压器监控系统和PIC单片机民控单元。监控系统采用C语言模块化程度设计；电控单元采用PIC精简指令系统汇编语言编程。简要介绍了涡轮增压器控制系统的组成，并给出了监控系统同PIC电控单元的双机通讯软件设计实例。     

基于FDTD与C＋＋的金属目标RCS省内存计算

利用C＋＋语言的面向对象特性,实现了金属目标雷达散射截面（Radar Cross Section,RCS）的省内存计算。一般用时域有限差分（Finite—Difference Time-Domain,FDTD）法计算金属目标的RCS时,没有考虑到金属目标内部电磁场量为零的特性,对金属目标内部也分配了大量的内存。将金属目标分为内外两部分,对内部区域分配内存消耗少的数组,利用C＋＋语言的多态性,将各个不同的区域连接成一个整体来计算,从而减少了内存消耗。数值计算结果表明,与普通计算方法相比,对计算精度没有影响,同时可以大幅节省内存消耗。     

动态负载平衡的二维非结构网格DSMC并行算法研究

基于PC-CLUSTER群机并行体系结构与消息传递库MPI并行环境,研究了二维非结构网格DSMC并行算法。提出一类非结构网格动态分区策略,保证各子区域的分子数量大致相等,实现计算进程间的动态负载平衡。利用MPI库函数构造了符合DSMC并行原理的单步通讯法。采用单控制多数据流(SPMD)以及Master/Slave并行模式,设计了二维非结构网格DSMC整体并行算法。在程序的编制过程中,充分展现了Fortran90高级语言的主要特性,引人动态数组、指针、链表及派生类型数据。最后对过渡流域高超声速绕流进行了并行计算,数值试验的结果表明本文设计的并行算法可以取得较为理想的加速比。     

利用AutoLISP绘制单管径保护管敷设断面图

一、AutoLISP简介 AutoLISP是一种针对扩充及自订AutoCAD函数机能而产生，以LISP为基础的程序设计语言。AutoLisp语言建立在基于普通的LISP语言上，并扩充了许多适用于CAD的特殊功能而形成。它是一种不需先行编译就可在AutoCAD内部执行的直译性程序语言。所谓LISP程序就是一组相关的AutoLISP函数，包括这样一组函数的ASC码文件就成为LISP文件。在一个文件中，函数的执行顺序是从上到下的。在计算每个函数时，     

Control-Loop Noise in Adaptive Array Antennas

Adaptive array receiving antennas can be designed to sense the external noise field and to optimize the array illumination function. A substantial improvement in signal-to-noise ratio can be obtained with adaptive arrays when the external noise field is nonuniformly distributed in angle. The external noise process may be time varying and contain both discrete sources and continuously distributed sources. Two adaptive array implementations which maximize the signal-to-noise ratio are described in this paper. Expressions are derived for control-loop noise, i.e., the variance of the array element weights, and for the additional noise in the array output due to this element weight noise. It is shown that both the element weight noise and the array convergence rate are determined by the eigenvalues of the noise covariance matrix.     

The Effect of Random Steering Vector Errors in the Applebaum Adaptive Array

The effect of random errors in the steering vector of an Applebaum adaptive array is examined. Each component of the steering vector is assumed to have a random error component uncorrelated between elements. The array output signal-to-interferenceplus-noise ratio (SINR) is computed as a function of the error variance. It is shown that the array output SINR becomes more sensitive to steering vector errors as more elements are added to the array and as the received desired signal power becomes larger. The variance of the steering vector error that may be tolerated depends on the required desired signal dynamic range. The larger the dynamic range that must be accommodated, the smaller the error variance must be.     

Error Analysis of the Optimal Antenna Array Processors

The optimal weights of an antenna array processor, which maximizes the output signal-to-noise ratio (SNR) in the absence of errors, are computed using the noise-alone matrix inverse (NAMI) and the steering vector in the look direction or the signal-plus-noise matrix inverse (SPNMI) and the steering vector. In practice the estimated steering vector as well as the estimated optimal weights are corrupted by random errors. This paper has analyzed the effects of these errors on the performance of the NAMI processor and the SPNMI processor by deriving analytic expressions for the output signal power, output noise power, output SNR, and the array gain as a function of the error variance. The treatment is for a general array configuration and no assumption about a particular array geometry is made.     

Limitations of Series-Fed Arrays in Broadband Communications

The purpose of this paper is to investigate the signal distortion which results when long, series-fed arrays are used for communication. It is shown that under certain conditions the antenna array can be considered a linear filter, the response of which is a function of frequency as well as spatial coordinates. The distortion accompanying the transmission as measured by the least mean-square difference between the transmitted and the received signals is derived. This expression is evaluated for the case of a uniformly excited array and a rectangular band-pass signal spectrum. In the absence of noise the transmitted signal can be completely recovered in the receiver by employing the proper linear filter. In the presence of noise the signal cannot, of course, be completely recovered. However, a technique for minimizing the distortion, in this case using a pre-filter and a post-filter, is investigated.     

The hit array: an analysis formalism for multiple access frequencyhop coding

A formalism is presented for the analysis of general frequency hop waveforms, such as those suitable for use in coherent active radar and sonar echolocation systems as well as multiple-access spread-spectrum communications. This formalism is based on the concept of coincidence, or `hit', between two frequency hopping patterns. The collection of all possible hits, together with their locations, is recorded in time-frequency space, which produces the high array associated with the two patterns considered. If the code length is sufficiently small with respect to the time-bandwidth product chosen, the hit array can be viewed as a digital representation of the corresponding ambiguity function. Salient properties of the hit array formalism are derived, including simple relationships between hit arrays resulting from basic symmetry-preserving transformations. These properties make it possible to predict the performance of a given set of frequency hop waveforms directly from the associated set of frequency hopping patterns     

Equivalent transformation for a partially adaptive array

A partially adaptive array is one in which elements of a phased array are controlled or adaptively weighted in groups or in which certain elements, called auxiliary elements, are made controllable. Mathematically, this type of array is formed by transforming all of the elements of an array by a nonsquare matrix such that the resulting output vector has a length less than the number of array elements. It is shown that there is an equivalent matrix transform that can effectively be utilized in analyzing the partially adaptive array's performance when a small number of external jammers are present. Processor implementation and convergence rate considerations lead to the desirability of reducing the dimensionality of the cancellation processor while maintaining good sidelobe interference protection. A meaningful measure of canceller performance is to compute the optimal output signal-to-noise ratio. This expression is a function of the jammer, direction-of-arrival vectors (DOAVs), jammer powers, the array steering vector, and internal noise. It is shown that if this expression is computed for the fully adaptive array then it is easily computed for the partially adaptive array by transforming the jammer DOAVs and the steering vector by the orthogonal projection matrix defined by the rows of the subarray transformation matrix and substituting these vectors back into the original expression for the fully adaptive array     

Robust space-time adaptive processing for airborne radar in nonhomogeneous clutter environments

Space-time adaptive processing (STAP) holds tremendous potential for the new generation airborne surveillance radar, in which the phased array antennas and pulse Doppler processing mode are adopted. A new STAP approach using the multiple-beam and multiple Doppler channels is presented here for airborne phased array radar. The approach with space-time multiple-beam (STMB) architecture is robust to array errors and has very low system degrees of freedom (DOFs). Hence, it has low sample support requirement and it is very suitable for the practical planar phased array radar under nonhomogeneous clutter environments. Meanwhile, a new nonhomogeneous detector (NHD) based on the correlation dimension (CD) is also proposed here, which is used as an effective method to screen tracing data prior to detection processing. It can further improve the performance of the STAP approach in the severely nonhomogeneous clutter environments. Therefore, a scheme that incorporates the correlation dimension nonhomogeneity detector (CD-NHD) with the STMB is recommended, which we term CD-NHD-STMB. The experimental simulation results indicate that: 1) the STMB processor is robust to array element error and has high performance under nonhomogeneous clutter environments; 2) the CD-NHD is also effective on the nonhomogeneous clutter. As a result, the CD-NHD-STMB scheme is robust to array element error and nonhomogeneous clutter, and therefore available for airborne phased array radar applications.     

Estimating the Angles of Arrival of Multiple Plane Waves

The problem of estimating the angles of arrival of M plane waves incident simultaneously on a line array with L + 1 (L?M) sensors utilizing the special eigenstructure of the covariance matrix C of the signal plus noise at the output of the array is addressed. A polynomial D(z) with special properties is constructed from the eigenvectors of C, the zeros of which give estimates of the angle of arrival. Although the procedure turns out to be essentially the same as that developed by Reddi, the development presented here provides insight into the estimation problem.     

Distortion Generated in Angle-Modulation Systems by Phased Arrays

The case of linear, uniformly weighted phased arrays is examined via time and frequency domain analyses. Bounds that must be placed on array length, modulation frequency, modulation index ?, and scan angle relationships if excessive distortion is to be avoided in wideband angle-modulated communication systems are established. Distortion is shown to consist of odd harmonic terms. It is also shown that, for one class of equivalent RF signals, phased arrays produce approximately three times as much distortion in frequency-modulation (FM) systems as in phase-modulation (PM) systems. Graphs of distortion plotted as functions of signal and array parameters show that, for practical array sizes, distortion is a monotonically increasing function of the product of L and sin ? where L is the length of the array expressed in modulation wavelengths ?m and ? is the scan angle. In PM systems, distortion also increases monotonically with the modulation index ?. Plots of distortion versus L sin ? show that even relatively small arrays can produce intolerable distortion levels in wideband systems; e.g., an FM system having ? = 3,L = 0.35?m, and ? = 60 degrees exhibits approximately 20 percent distortion.     

Effect of Sampling Time Jitter on Array Performance

The effect on array SNR of errors in sampling times (jitter) isan analyzed for the "Bryn processor." Jitter is modeled as a discrete-paramter er random process, and expressions for array SNR are determined as a function of signal and noise spectra and jitter characteristics. For a plane wave signal, it is found that jitter in the data used to design the processor has no effect on output SNR if the noise is independent between channels. However, jitter in the input (evaluation) n) data can cause a substantial loss in peak array SNR and in the array's frequency selectivity. These losses can be expected to increase as the high-frequency content of the data increases. The Bryn processor also loses its interpretation as a likelihood ratio processor in the presence of jitter. The effect of jitter can be reduced in many cases by increasing the sampling rate.