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1. Formatted Output with printf Function

Basic Usage of printf Function

<span>printf</span> is responsible for outputting information to the screen.

Basic Syntax:

printf("format control string", output item list);

Detailed Explanation of Format Specifiers

Format Specifier	Meaning	Example	Output Result
<span>%d</span>	Decimal Integer	<span>printf("%d",100);</span>	100
<span>%f</span>	Floating Point Number	<span>printf("%f",3.14);</span>	3.140000
<span>%c</span>	Character	<span>printf("%c",'A');</span>	A
<span>%s</span>	String	<span>printf("%s","hello");</span>	hello
<span>%.2f</span>	Keep 2 decimal places	<span>printf("%.2f",3.14159);</span>	3.14

Common Mistakes Reminder

Format Specifier Must Match Data Type

int age = 18;printf("Age: %d years\n", age);      // Correctprintf("Age: %f years\n", age);      // Error! Outputs garbage

2. Formatted Input with scanf Function

Basic Usage of scanf Function

<span>scanf</span> is responsible for receiving input from the keyboard.

Basic Syntax:

scanf("format control string", variable address list);

Input of Various Data Types

#include<stdio.h>int main() {    int age;    float score;    char grade;    printf("Please enter age, score, grade: ");    scanf("%d %f %c", &age, &score, &grade);    printf("Age:%d, Score:%.1f, Grade:%c\n", age, score, grade);    return 0;}

Common Questions and Answers

Q: Why do we need to add<span>&</span> before the variable in scanf?

A:<span>&</span> is the address-of operator, which tells scanf where to store the data in memory. Forgetting to add<span>&</span> is a common mistake for beginners.

Q: What is the role of spaces in the format control string during input?

A: Spaces can match any number of whitespace characters (spaces, tabs, newlines), making input more flexible.

3. Character Input and Output: getchar, putchar

Input and Output of Single Characters

#include <stdio.h>int main() {    char ch;    printf("Please enter a character: ");    ch = getchar();     // Get a character    printf("The character you entered is: ");    putchar(ch);        // Output a character    putchar('\n');      // Output newline    return 0;}

Clearing Input Buffer

// When mixing scanf and getchar, you need to clear the bufferint num;char ch;printf("Please enter a number: ");scanf("%d", &num);getchar();  // Absorb the newline characterprintf("Please enter a character: ");ch = getchar();

4. Common Input and Output Errors and Debugging

Common Error Types

1. Format Mismatch Error

float f;scanf("%d", &f);  // Error: should use %f

1. Forgot to Use Address Operator

int x;scanf("%d", x);   // Error: should be &x

1. Buffer Residue Issues

int a;char c;scanf("%d", &a);scanf("%c", &c);  // Will read the previous input's newline

Debugging Tips

Use Debug Output:

int a, b;scanf("%d%d", &a, &b);printf("Debug: a=%d, b=%d\n", a, b);  // Check if input is correct

In-Class Test

Practical Exercise Guidance

Task 1: Personal Information Entry System

#include <stdio.h>int main() {    char name[20];    int age;    float height;    char gender;    printf("=== Personal Information Entry ===\n");    printf("Please enter your name: ");    scanf("%s", name);    printf("Please enter your age: ");    scanf("%d", &age);    printf("Please enter your height (meters): ");    scanf("%f", &height);    printf("Please enter your gender (M/F): ");    scanf(" %c", &gender);  // Note the space before %c to absorb the newline    printf("\n=== Entry Complete ===\n");    printf("Name: %s\n", name);    printf("Age: %d years\n", age);    printf("Height: %.2f meters\n", height);    printf("Gender: %c\n", gender);    return 0;}

Task 2: Simple Calculator

#include <stdio.h>int main() {    double num1, num2;    char operator;    printf("Simple Calculator (format: number1 operator number2): ");    scanf("%lf %c %lf", &num1, &operator, &num2);    switch(operator) {        case '+': printf("Result: %.2f\n", num1 + num2); break;        case '-': printf("Result: %.2f\n", num1 - num2); break;        case '*': printf("Result: %.2f\n", num1 * num2); break;        case '/':             if(num2 != 0) printf("Result: %.2f\n", num1 / num2);            else printf("Error: Divisor cannot be 0!\n");            break;        default: printf("Error: Unsupported operator!\n");    }    return 0;}

Common Mistake Detection

Identify the errors in the following code:

#include <stdio.h>int main() {    int a, b;    float c;    char d;    printf("Enter two integers: ");    scanf("%d %d", a, b);  // Error 1    printf("Enter a floating point number: ");    scanf("%f", c);        // Error 2    printf("Enter a character: ");    d = getchar();    scanf("%c", &d);       // Error 3    return 0;}

Homework

Basic Questions (Required) – Luogu Problem Numbers

1. P1001: A+B Problem – Input two integers and output their sum

2. P1421: Xiaoyu Buys Stationery – Input amount and product price, calculate how many can be bought

3. P1085: Unhappy Jinjing – Input study time for 7 days, determine which day was the least happy

Advanced Questions (Optional) – Luogu Problem Numbers

1. P1046: Taotao Picks Apples – Input apple height and Taotao’s reach height, calculate how many apples can be picked

2. P1055: ISBN Number – Input ISBN number, verify and output the correct result

Thought Questions

1. Write a program to input a three-digit number and output its units, tens, and hundreds digits separately

Homework Answers

P1001: A+B Problem Answer

#include <stdio.h>int main() {    int a, b;    scanf("%d %d", &a, &b);    printf("%d\n", a + b);    return 0;}

P1421: Xiaoyu Buys Stationery Answer

#include <stdio.h>int main() {    int a, b;    scanf("%d %d", &a, &b);    int total = a * 10 + b;  // Convert to jiao    printf("%d\n", total / 19);  // Each stationery costs 19 jiao    return 0;}

P1085: Unhappy Jinjing Answer

#include <stdio.h>int main() {    int school, home, total;    int max_day = 0, max_hours = 8;    for(int i = 1; i <= 7; i++) {        scanf("%d %d", &school, &home);        total = school + home;        if(total > max_hours) {            max_hours = total;            max_day = i;        }    }    printf("%d\n", max_day);    return 0;}

Thought Question Answer

#include <stdio.h>int main() {    int num;    printf("Please enter a three-digit number: ");    scanf("%d", &num);    int hundred = num / 100;    int ten = (num % 100) / 10;    int one = num % 10;    printf("Hundreds: %d\n", hundred);    printf("Tens: %d\n", ten);    printf("Units: %d\n", one);    return 0;}

In the next class, we will learn:

Relational Operations and Logical Operations

Remember to practice more on Luogu, these are great practical training!

If you have any questions, feel free to comment!

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