Catch a number from the keyboard, if it s equal to one, print out test 1 ; if it s equal to two, print out test 2 ; if it s equal to three, print out

Transcription

1 Catch a number from the keyboard, if it s equal to one, print out test 1 ; if it s equal to two, print out test 2 ; if it s equal to three, print out test 3 ; for all other values, print out test 4.

2 { { #include <stdio.h> int main(void) int myint; scanf("%d", &myint); switch(myint) case 1: printf("test 1"); break; case 2: printf("test 2"); break; case 3: printf("test 3"); break; default:printf("test 4"); } return 0; }

3 { Example: char ch; ch = getchar();// get one character input, equivalent to scanf( %c, &ch). Correspondingly, we have putchar(ch) to print a char value on the screen, which functions the same as printf(). But printf() can be used for printing a sentence. switch(ch) case 'A': printf("a\n"); // or use 65 instead of 'A' break; case 'B': printf("b\n"); // or use 66 instead of 'B' break; default: printf("not A or B\n"); }

4 The previous example doesn t mean switch is not powerful than if-else. The limitation of switch is we can only compare whether a given integer value matches a label, which is similar to ==

5 The format is condition_is_true? do_if_true : do_if_false; condition_is_true is an expression. Nonzero value means true, and zero indicates false. do_if_true should be simple expression or printf(), but not statement. an example x > y? printf("%d", x) : printf("%d", y); //print out the larger number Ternary operator three operands

6 It s an expression, so must have a value in C programming. To test the value of the expression, use the following example. (1) and (2) are the same Example (1) int a, b; scanf("%d", &a); b = a < 10? 2 : a +1; printf("the value of b is %d", b); Example (2) int a, b; scanf("%d", &a); if (a < 10) b = 2; else b = a + 1; printf("the value of b is %d", b);

7 Here we have a conditional expression inside a function call: #include <stdio.h> int main(void) { int x; scanf("%d", &x); printf("x is %s.\n", (x < 5)? "small" : "large"); } // printf("%s", "Hello world!\n"); to print a string, the format specifier is %s.

8 The following operators are available in C for incrementing and decrementing variables by a value of one: they are unary operator with single operand ++ increment -- decrement

9 These operators can be placed before (prefix) or after (postfix) a variable: int x = 5; x--; /* x has a value of 4 here */ or int x = 5; --x; /* x has a value of 4 here */ --x, x - are the same as x = x 1 ++x, x++ are the same as x = x +1 They are exactly the same from the perspective of increasing the value of x.

10 But they make difference in the values of their expressions. #include <stdio.h> int main(void) { int a, b; a = 4; b = a++;// b = ++a; printf("the value of a is %d.\n", a); printf("the value of the expression a++ is %d.\n", b); return 0; }

11 #include <stdio.h> int main(void) { int a, b; a = 4;//a = 3; if (a++!= 5)// (++a!=5) printf("test 1.\n"); else return 0; } printf("test 2.\n");

12 C has the following loop constructs: while do-while for

13 Loop helps repeat a task. (exactly the same or a formula to follow) Need to determine when to terminate the loop. How? after a predetermined number of iterations when some condition is met

14 To terminate a loop after a predetermined number of iterations, we need: A counting variable should be defined and initialized The counter should be compared with some limiting value The counter is incremented each time the loop is traversed

15 The basic form of the while loop is while(test_condition) do_something; As long as test is true, the loop will repeat. test should be an expression. If it is nonzero, it s a true condition, and do something. If it is zero, the while statement will be skipped. To do multiple things in each iteration, we use curly braces: while(test) { do_something; }

16 while loop that stops after predetermined number of iterations: #include <stdio.h> int main(void) { int i = 1, sum = 0; while( i <= 4 ) { sum = sum + i;// sum += I; i++; } printf("sum of the integers 1 to %d is %d\n", i-1, sum); } /* sum of the integers 1 to 4 is 10 */

17 i = 1 printf false i <= 4 true sum = sum + i i++

18 There are many occasions in which we don t know in advance how many times the loop should repeat, but we do know under what conditions the loop should terminate. E.g., read a file line by line, until reaching the end of the file.

19 To repeat the same task (exactly), simply do it. To repeat a dynamic task, try to link the task with the counter, which will dynamically change in the loop E.g., calculate the sum of all integers smaller than 5;

20 Print out a triangle. * ** *** ****

21 { #include <stdio.h> int main(void) int j, i = 1; while (i <= 4) { j = 1; while (j <= i) { printf("*"); j ++; } printf("\n"); i ++; } return 0; }

22 The basic form of the while loop is while(test_condition) do_something; As long as test is true, the loop will repeat. test_condition should be an expression. If it is nonzero, it s a true condition, and do something. If it is zero, the while statement will be skipped. Give the test_condition a chance to change in do_something, otherise, the loop will run for ever In general, (1) define a variable outside while loop and (2) use it to compose the test_condition, and then (3) change the variable in do_something

23 { #include <stdio.h> int main(void) int n = 5; while (n < 7) {// Without the curly braces, the loop will printf("n = %d\n", n); n++; printf("now n = %d\n", n); } printf("the loop has finished.\n"); return 0; }

24 { #include <stdio.h> int main(void) char ch; int count = 0; scanf("%c", &ch); while (ch!= '$') // To check the end of the input sentence. { printf("%c\n", ch); scanf("%c", &ch); count++; } printf("the total number of input characters is %d.\n", count); return 0; }

25 Print out all even integers smaller or equal to a bound Input: 17 Output: #include <stdio.h> int main(void) { int upbound; int i = 0; scanf("%d",&upbound); while(i <= upbound) { if (i % 2 == 0) printf("%d ", i); i++; } printf("\n"); return 0; }