C語言英文ch03

1、Chapter 3Processing and Interactive InputAssignment（賦值）（賦值）The general syntax（語法）（語法） for an assignment statement（賦值語句）（賦值語句） isvariable（變量）（變量） = operand （操作數(shù)）（操作數(shù)）; The operand to the right of the assignment operator (=) can be a constant（常量）（常量）, a variable, or an expression（表達(dá)式）（表達(dá)式）The equal si

2、gn in C does not have the same meaning as an equal sign in algebra（代數(shù)學(xué)）（代數(shù)學(xué)） length=25; is read “l(fā)ength is assigned the value 25”Assignment（賦值）（賦值）Subsequent（后來的）（后來的） assignment statements can be used to change the value assigned to a variablelength = 3.7;length = 6.28;Assignment (continued)The ope

3、rand（操作數(shù)）（操作數(shù)） to the right of the equal sign（等號）（等號） in an assignment statement（賦值語句）（賦值語句） can be a variable（變量）（變量） or any valid（有效的）（有效的） C expressionsum = 3 + 7;product = .05 * 14.6;TThe value of the expression to the right of = is computed（計(jì)算）（計(jì)算） first and then the calculated value（計(jì)算結(jié)果）（計(jì)算結(jié)果

4、） is stored （存儲）（存儲）in the variable to the left of =Assignment (continued)Variables used in the expression to the right of the = must be initialized（初始化）（初始化） if the result is to make sense （有（有意義）意義）amount + 1892 = 1000 + 10 * 5 is invalid（無效的）（無效的）!因?yàn)楸磉_(dá)式因?yàn)楸磉_(dá)式的值存儲在等號左邊的變量中，所以必須把變量直接列在等號的左邊。的值存儲在等號左

5、邊的變量中，所以必須把變量直接列在等號的左邊。Assignment (continued)If width was not initialized（初始化）, the computer uses the value that happens to occupy that memory space previously (compiler（編譯器） would probably issue a warning)Assignment (continued)= has the lowest precedence（優(yōu)先級）（優(yōu)先級） of all the binary（二元）（二元） and unar

6、y（一元）（一元） arithmetic operators（算術(shù)（算術(shù)操作符）操作符）Assignment (continued)Multiple assignments（多次賦值）（多次賦值） are possible in the same statementa = b = c = 25;TAll = operators have the same precedenceTOperator has right-to-left Associativity（結(jié)合性）（結(jié)合性）c = 25;b = c;a = b;Implicit Type Conversions（隱式類型轉(zhuǎn)換）（隱式類型轉(zhuǎn)換）

7、Data type conversions（數(shù)據(jù)類型轉(zhuǎn)換）（數(shù)據(jù)類型轉(zhuǎn)換） take place across assignment operatorsdouble result;result = 4; /integer 4 is converted to 4.0Implicit Type Conversions（隱式類型轉(zhuǎn)換）（隱式類型轉(zhuǎn)換）The automatic（自動的）（自動的） conversion across an assignment operator is called an implicit type conversion（隱式類型轉(zhuǎn)（隱式類型轉(zhuǎn)換）換）int answe

8、r;answer = 2.764; /2.764 is converted to 2 Here the implicit conversion is from a higher precision（精度）（精度） to a lower precision data type; the compiler（編譯器）（編譯器） will issue a warning（警告）（警告）Explicit Type Conversions (Casts)（顯式類型轉(zhuǎn)換（顯式類型轉(zhuǎn)換-強(qiáng)制類型轉(zhuǎn)換）強(qiáng)制類型轉(zhuǎn)換）The operator used to force（強(qiáng)制）（強(qiáng)制） the conversio

9、n（轉(zhuǎn)換）（轉(zhuǎn)換） of a value to another type is the cast operator（強(qiáng)制類型轉(zhuǎn)（強(qiáng)制類型轉(zhuǎn)換運(yùn)算符）換運(yùn)算符）(dataType) expressionTwhere dataType is the desired（期望的）（期望的） data type of the expression following the castExplicit Type Conversions (Casts)（顯式類型轉(zhuǎn)換（顯式類型轉(zhuǎn)換-強(qiáng)制類型轉(zhuǎn)換）強(qiáng)制類型轉(zhuǎn)換）Example: If sum is declared as double sum; , (int)

10、sum is the integer value determined by truncating（截去）（截去）sums fractional part（小數(shù)部分）（小數(shù)部分）Assignment Variations（復(fù)合賦值）（復(fù)合賦值）sum = sum + 10 is not an equation（等式） it is an expression that is evaluated in two major steps（它是一個(gè)用兩個(gè)步驟計(jì)算的表達(dá)式）Assignment Variations (continued)Assignment Variations (continued)A

11、ssignment Variations (continued)Assignment expressions like sum = sum + 25 can be written using the following operators: += - -= *= /= %=sum = sum + 10 can be written as sum += 10price *= rate is equivalent（等價(jià)）（等價(jià)） to price = price * rateprice *= rate + 1 is equivalent to price = price * (rate + 1)A

12、ccumulating （累加）（累加）Accumulating (continued)Counting（計(jì)數(shù)）（計(jì)數(shù)）A counting statement（計(jì)數(shù)語句）（計(jì)數(shù)語句） is very similar to the accumulating statementvariable = variable + fixedNumber; （變量（變量=變量變量+固定數(shù)值；）固定數(shù)值；）Examples: i = i + 1; and m = m + 2; Increment operator（自增運(yùn)算符）（自增運(yùn)算符） (+): variable = variable + 1 can b

13、e replaced by variable+ or +variableCounting (continued)Counting (continued)Counting (continued)When the + operator appears before a variable, it is called a prefix increment operator（前綴自增運(yùn)算符）（前綴自增運(yùn)算符）; when it appears after a variable, it is called postfix increment operator（后綴自增運(yùn)算符）（后綴自增運(yùn)算符） k = +

14、n; is equivalent toTn = n + 1; / increment n firstTk = n; / assign ns value to k k = n+; is equivalent toTk = n; / assign ns value to kTn = n + 1; / and then increment nCounting (continued)Prefix decrement operator （前綴自減運(yùn)算符）（前綴自減運(yùn)算符） : the expression k = -n first decrements the value of n by 1 befor

15、e assigning the value of n to kPostfix decrement operator （后綴自減運(yùn)算符）（后綴自減運(yùn)算符） : the expression k = n- first assigns the current value of n to n and then reduces the value of n by 1Counting (continued)Mathematical Library Functions（數(shù)學(xué)庫函數(shù)）（數(shù)學(xué)庫函數(shù)）Mathematical Library Functions (continued)The argument to

16、 sqrt must be floating-point value; passing an integer value results in a compiler error Return value is double-precisionMust include #include Mathematical Library Functions (continued)Mathematical Library Functions (continued)Mathematical Library Functions (continued)3.0 * sqrt(5 * 33 - 13.91) / 5I

17、nteractive Input（交互式輸入）（交互式輸入）Interactive Input (continued)This program must be rewritten to multiply different numbersscanf() is used to enter data into a program while it is executing（用于正在（用于正在運(yùn)行程序時(shí)輸入數(shù)據(jù)）運(yùn)行程序時(shí)輸入數(shù)據(jù)）; the value is stored in a variable It requires a control string as the first argumen

18、t inside the function name parentheses（要求一個(gè)控制字符串作為這個(gè)函數(shù)名括號內(nèi)的第一個(gè)要求一個(gè)控制字符串作為這個(gè)函數(shù)名括號內(nèi)的第一個(gè)參數(shù)）參數(shù)）Interactive Input (continued)The control string passed to scanf() typically consists of conversion control sequences only（傳遞給傳遞給scanf()函數(shù)的控制字符函數(shù)的控制字符串典型地只由轉(zhuǎn)換控制序列符組成）串典型地只由轉(zhuǎn)換控制序列符組成）scanf() requires that a list

19、 of variable addresses follow the control string（ scanf()函數(shù)要求變量地址列表跟隨控制字符串函數(shù)要求變量地址列表跟隨控制字符串） scanf(%d, &num1);Interactive Input (continued)Interactive Input (continued)This statement produces a prompt（提示）Address operator (&)Interactive Input (continued)scanf() can be used to enter many value

20、sscanf(%f %f,&num1,&num2); /%f%f is the sameA space（空格）（空格） can affect what the value being entered is when scanf() is expecting a character data type（字符數(shù)據(jù)類型）（字符數(shù)據(jù)類型）Interactive Input (continued)scanf(%c%c%c,&ch1,&ch2,&ch3); stores the next three characters typed in the variables

21、 ch1, ch2, and ch3; if you type x y z, then x is stored in ch1, a blank is stored in ch2, and y is stored in ch3scanf(“%c %c %c”,&ch1,&ch2,&ch3); causes scanf() to look for three characters, each character separated by exactly one space（尋找正好被一（尋找正好被一個(gè)空格分開的三個(gè)字符）個(gè)空格分開的三個(gè)字符）scanf() does not

22、 test the data type of the values being enteredIn scanf(%d %f, &num1, &num2), if user enters 22.87, 22 is stored in num1 and .87 in num2Formatted Output（格式化輸出）（格式化輸出）618124-148Output is not alignedFormatted Output (continued) 6 18124-148Field width specifier（域?qū)捴付ǎ〧ormatted Output (continued)

23、Format Modifiers（格式修飾符）（格式修飾符）Left justification（字段左對齊）（字段左對齊）: printf(%-10d,59); produces the display 59?Explicit sign display（顯式符號的顯示）（顯式符號的顯示）: printf(%+10d,59); produces the display ?+59Format Modifiers（格式修飾符）（格式修飾符）Format modifiers may be combined（格式修飾符可以組合（格式修飾符可以組合 ） %-+10d would cause an integer number to both display it