Java经典基础练习21-30

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【程序21】

题目：求1+2!+3!+…+20!的和

[java] view plain copy print?public class lianxi21 { public static void main(String[] args) { long sum = 0; long fac = 1; for (int i = 1; i <= 20; i++) { fac = fac * i; sum += fac; } System.out.println(sum); } }

public class lianxi21 {
public static void main(String[] args) {
long sum = 0;
long fac = 1;
for (int i = 1; i <= 20; i++) {
fac = fac * i;
sum += fac;
}
System.out.println(sum);
}
}

【程序22】

题目：利用递归方法求5!。

[java] view plain copy print?<pre name=“code” class=“java”>public class lianxi22 {
public static void main(String[] args) {
int n = 5;
rec fr = new rec();
System.out.println(n + ”! = ” + fr.rec(n));
}
}

class rec {
public long rec(int n) {
long value = 0;
if (n == 1) {
value = 1;
} else {
value = n * rec(n - 1);
}
return value;
}
}

<pre name="code" class="java">public class lianxi22 {
public static void main(String[] args) {
int n = 5;
rec fr = new rec();
System.out.println(n + "! = " + fr.rec(n));
}
}

class rec {
public long rec(int n) {
long value = 0;
if (n == 1) {
value = 1;
} else {
value = n * rec(n - 1);
}
return value;
}
}

【程序23】

题目：有5个人坐在一起，问第五个人多少岁？他说比第4个人大2岁。问第4个人岁数，他说比第3个人大2岁。问第三个人，又说比第2人大两岁。问第2个人，说比第一个人大两岁。最后问第一个人，他说是10岁。请问第五个人多大？

[java] view plain copy print?<pre name=“code” class=“java”>public class lianxi23 {
public static void main(String[] args) {
int age = 10;
for (int i = 2; i <= 5; i++) {
age = age + 2;
}
System.out.println(age);
}
}

<pre name="code" class="java">public class lianxi23 {
public static void main(String[] args) {
int age = 10;
for (int i = 2; i <= 5; i++) {
age = age + 2;
}
System.out.println(age);
}
}

【程序24】

题目：给一个不多于5位的正整数，要求：一、求它是几位数，二、逆序打印出各位数字。

//使用了长整型最多输入18位

[java] view plain copy print?import java.util.*;

public class lianxi24 {
public static void main(String[] args) {
Scanner s = new Scanner(System.in);
System.out.print(”请输入一个正整数：”);
long a = s.nextLong();
String ss = Long.toString(a);
char[] ch = ss.toCharArray();
int j = ch.length;
System.out.println(a + ”是一个” + j + “位数。”);
System.out.print(”按逆序输出是：”);
for (int i = j - 1; i >= 0; i–) {
System.out.print(ch[i]);
}
}
}

import java.util.*;

public class lianxi24 {
public static void main(String[] args) {
Scanner s = new Scanner(System.in);
System.out.print("请输入一个正整数：");
long a = s.nextLong();
String ss = Long.toString(a);
char[] ch = ss.toCharArray();
int j = ch.length;
System.out.println(a + "是一个" + j + "位数。");
System.out.print("按逆序输出是：");
for (int i = j - 1; i >= 0; i--) {
System.out.print(ch[i]);
}
}
}

【程序25】

题目：一个5位数，判断它是不是回文数。即12321是回文数，个位与万位相同，十位与千位相同。

[java] view plain copy print?import java.util.*;

public class lianxi25 {
public static void main(String[] args) {
Scanner s = new Scanner(System.in);
int a;
do {
System.out.print(”请输入一个5位正整数：”);
a = s.nextInt();
} while (a < 10000 || a > 99999);
String ss = String.valueOf(a);
&
1688e
nbsp; char[] ch = ss.toCharArray();
if (ch[0] == ch[4] && ch[1] == ch[3]) {
System.out.println(”这是一个回文数”);
} else {
System.out.println(”这不是一个回文数”);
}
}
}

import java.util.*;

public class lianxi25 {
public static void main(String[] args) {
Scanner s = new Scanner(System.in);
int a;
do {
System.out.print("请输入一个5位正整数：");
a = s.nextInt();
} while (a < 10000 || a > 99999);
String ss = String.valueOf(a);
char[] ch = ss.toCharArray();
if (ch[0] == ch[4] && ch[1] == ch[3]) {
System.out.println("这是一个回文数");
} else {
System.out.println("这不是一个回文数");
}
}
}

[java] view plain copy print?//这个更好，不限位数
import java.util.*;

public class lianxi25a {
public static void main(String[] args) {
Scanner s = new Scanner(System.in);
boolean is = true;
System.out.print(”请输入一个正整数：”);
long a = s.nextLong();
String ss = Long.toString(a);
char[] ch = ss.toCharArray();
int j = ch.length;
for (int i = 0; i < j / 2; i++) {
if (ch[i] != ch[j - i - 1]) {
is = false;
}
}
if (is == true) {
System.out.println(”这是一个回文数”);
} else {
System.out.println(”这不是一个回文数”);
}
}
}

//这个更好，不限位数
import java.util.*;

public class lianxi25a {
public static void main(String[] args) {
Scanner s = new Scanner(System.in);
boolean is = true;
System.out.print("请输入一个正整数：");
long a = s.nextLong();
String ss = Long.toString(a);
char[] ch = ss.toCharArray();
int j = ch.length;
for (int i = 0; i < j / 2; i++) {
if (ch[i] != ch[j - i - 1]) {
is = false;
}
}
if (is == true) {
System.out.println("这是一个回文数");
} else {
System.out.println("这不是一个回文数");
}
}
}

【程序26】

题目：请输入星期几的第一个字母来判断一下是星期几，如果第一个字母一样，则继续 判断第二个字母。

[java] view plain copy print?import java.util.*;

public class lianxi26 {
public static void main(String[] args) {
getChar tw = new getChar();
System.out.println(”请输入星期的第一个大写字母：”);
char ch = tw.getChar();
switch (ch) {
case ‘M’:
System.out.println(”Monday”);
break;
case ‘W’:
System.out.println(”Wednesday”);
break;
case ‘F’:
System.out.println(”Friday”);
break;
case ‘T’: {
System.out.println(”请输入星期的第二个字母：”);
char ch2 = tw.getChar();
if (ch2 == ‘U’) {
System.out.println(”Tuesday”);
} else if (ch2 == ‘H’) {
System.out.println(”Thursday”);
} else {
System.out.println(”无此写法！”);
}
}
;
break;
case ‘S’: {
System.out.println(”请输入星期的第二个字母：”);
char ch2 = tw.getChar();
if (ch2 == ‘U’) {
System.out.println(”Sunday”);
} else if (ch2 == ‘A’) {
System.out.println(”Saturday”);
} else {
System.out.println(”无此写法！”);
}
}
;
break;
default:
System.out.println(”无此写法！”);
}
}
}

class getChar {
public char getChar() {
Scanner s = new Scanner(System.in);
String str = s.nextLine();
char ch = str.charAt(0);
if (ch < ‘A’ || ch > ‘Z’) {
System.out.println(”输入错误，请重新输入”);
ch = getChar();
}
return ch;
}
}

import java.util.*;

public class lianxi26 {
public static void main(String[] args) {
getChar tw = new getChar();
System.out.println("请输入星期的第一个大写字母：");
char ch = tw.getChar();
switch (ch) {
case 'M':
System.out.println("Monday");
break;
case 'W':
System.out.println("Wednesday");
break;
case 'F':
System.out.println("Friday");
break;
case 'T': {
System.out.println("请输入星期的第二个字母：");
char ch2 = tw.getChar();
if (ch2 == 'U') {
System.out.println("Tuesday");
} else if (ch2 == 'H') {
System.out.println("Thursday");
} else {
System.out.println("无此写法！");
}
}
;
break;
case 'S': {
System.out.println("请输入星期的第二个字母：");
char ch2 = tw.getChar();
if (ch2 == 'U') {
System.out.println("Sunday");
} else if (ch2 == 'A') {
System.out.println("Saturday");
} else {
System.out.println("无此写法！");
}
}
;
break;
default:
System.out.println("无此写法！");
}
}
}

class getChar {
public char getChar() {
Scanner s = new Scanner(System.in);
String str = s.nextLine();
char ch = str.charAt(0);
if (ch < 'A' || ch > 'Z') {
System.out.println("输入错误，请重新输入");
ch = getChar();
}
return ch;
}
}

【程序27】

题目：求100之内的素数

//使用除sqrt(n)的方法求出的素数不包括2和3

[java] view plain copy print?public class lianxi27 {
public static void main(String[] args) {
boolean b = false;
System.out.print(2 + “ ”);
System.out.print(3 + “ ”);
for (int i = 3; i < 100; i += 2) {
for (int j = 2; j <= Math.sqrt(i); j++) {
if (i % j == 0) {
b = false;
break;
} else {
b = true;
}
}
if (b == true) {
System.out.print(i + ” ”);
}
}
}
}

// 该程序使用除1位素数得2位方法，运行效率高通用性差。
public class lianxi27a {
public static void main(String[] args) {
int[] a = new int[] { 2, 3, 5, 7 };
for (int j = 0; j < 4; j++)
System.out.print(a[j] + ” ”);
boolean b = false;
for (int i = 11; i < 100; i += 2) {
for (int j = 0; j < 4; j++) {
if (i % a[j] == 0) {
b = false;
break;
} else {
b = true;
}
}
if (b == true) {
System.out.print(i + ” ”);
}
}
}
}

public class lianxi27 {
public static void main(String[] args) {
boolean b = false;
System.out.print(2 + " ");
System.out.print(3 + " ");
for (int i = 3; i < 100; i += 2) {
for (int j = 2; j <= Math.sqrt(i); j++) {
if (i % j == 0) {
b = false;
break;
} else {
b = true;
}
}
if (b == true) {
System.out.print(i + " ");
}
}
}
}

// 该程序使用除1位素数得2位方法，运行效率高通用性差。
public class lianxi27a {
public static void main(String[] args) {
int[] a = new int[] { 2, 3, 5, 7 };
for (int j = 0; j < 4; j++)
System.out.print(a[j] + " ");
boolean b = false;
for (int i = 11; i < 100; i += 2) {
for (int j = 0; j < 4; j++) {
if (i % a[j] == 0) {
b = false;
break;
} else {
b = true;
}
}
if (b == true) {
System.out.print(i + " ");
}
}
}
}

【程序28】

题目：对10个数进行排序

[java] view plain copy print?import java.util.*;

public class lianxi28 {
public static void main(String[] args) {
Scanner s = new Scanner(System.in);
int[] a = new int[10];
System.out.println(”请输入10个整数：”);
for (int i = 0; i < 10; i++) {
a[i] = s.nextInt();
}
for (int i = 0; i < 10; i++) {
for (int j = i + 1; j < 10; j++) {
if (a[i] > a[j]) {
int t = a[i];
a[i] = a[j];
a[j] = t;
}
}
}
for (int i = 0; i < 10; i++) {
System.out.print(a[i] + ” ”);
}
}
}

import java.util.*;

public class lianxi28 {
public static void main(String[] args) {
Scanner s = new Scanner(System.in);
int[] a = new int[10];
System.out.println("请输入10个整数：");
for (int i = 0; i < 10; i++) {
a[i] = s.nextInt();
}
for (int i = 0; i < 10; i++) {
for (int j = i + 1; j < 10; j++) {
if (a[i] > a[j]) {
int t = a[i];
a[i] = a[j];
a[j] = t;
}
}
}
for (int i = 0; i < 10; i++) {
System.out.print(a[i] + " ");
}
}
}

【程序29】

题目：求一个3*3矩阵对角线元素之和

[java] view plain copy print?import java.util.*;

public class lianxi29 {
public static void main(String[] args) {
Scanner s = new Scanner(System.in);
int[][] a = new int[3][3];
System.out.println(”请输入9个整数：”);
for (int i = 0; i < 3; i++) {
for (int j = 0; j < 3; j++) {
a[i][j] = s.nextInt();
}
}
System.out.println(”输入的3 * 3 矩阵是:”);
for (int i = 0; i < 3; i++) {
for (int j = 0; j < 3; j++) {
System.out.print(a[i][j] + ” ”);
}
System.out.println();
}
int sum = 0;
for (int i = 0; i < 3; i++) {
for (int j = 0; j < 3; j++) {
if (i == j) {
sum += a[i][j];
}
}
}
System.out.println(”对角线之和是：” + sum);
}
}

import java.util.*;

public class lianxi29 {
public static void main(String[] args) {
Scanner s = new Scanner(System.in);
int[][] a = new int[3][3];
System.out.println("请输入9个整数：");
for (int i = 0; i < 3; i++) {
for (int j = 0; j < 3; j++) {
a[i][j] = s.nextInt();
}
}
System.out.println("输入的3 * 3 矩阵是:");
for (int i = 0; i < 3; i++) {
for (int j = 0; j < 3; j++) {
System.out.print(a[i][j] + " ");
}
System.out.println();
}
int sum = 0;
for (int i = 0; i < 3; i++) {
for (int j = 0; j < 3; j++) {
if (i == j) {
sum += a[i][j];
}
}
}
System.out.println("对角线之和是：" + sum);
}
}

【程序30】

题目：有一个已经排好序的数组。现输入一个数，要求按原来的规律将它插入数组中。

//此程序不好，没有使用折半查找插入

[java] view plain copy print?import java.util.*;

public class lianxi30 {
public static void main(String[] args) {
int[] a = new int[] { 1, 2, 6, 14, 25, 36, 37, 55 };
int[] b = new int[a.length + 1];
int t1 = 0, t2 = 0;
int i = 0;
Scanner s = new Scanner(System.in);
System.out.print(”请输入一个整数：”);
int num = s.nextInt();
if (num >= a[a.length - 1]) {
b[b.length - 1] = num;
for (i = 0; i < a.length; i++) {
b[i] = a[i];
}
} else {
for (i = 0; i < a.length; i++) {
if (num >= a[i]) {
b[i] = a[i];
} else {
b[i] = num;
break;
}
}
for (int j = i + 1; j < b.length; j++) {
b[j] = a[j - 1];
}
}
for (i = 0; i < b.length; i++) {
System.out.print(b[i] + ” ”);
}
}
}

import java.util.*;

public class lianxi30 {
public static void main(String[] args) {
int[] a = new int[] { 1, 2, 6, 14, 25, 36, 37, 55 };
int[] b = new int[a.length + 1];
int t1 = 0, t2 = 0;
int i = 0;
Scanner s = new Scanner(System.in);
System.out.print("请输入一个整数：");
int num = s.nextInt();
if (num >= a[a.length - 1]) {
b[b.length - 1] = num;
for (i = 0; i < a.length; i++) {
b[i] = a[i];
}
} else {
for (i = 0; i < a.length; i++) {
if (num >= a[i]) {
b[i] = a[i];
} else {
b[i] = num;
break;
}
}
for (int j = i + 1; j < b.length; j++) {
b[j] = a[j - 1];
}
}
for (i = 0; i < b.length; i++) {
System.out.print(b[i] + " ");
}
}
}