[Example 1-4] Two people, A and B, scored 87 and 76 in a math exam. Please input the codename and scores of A and B, and output the scores.

#include "stdio.h"
main()
{
    char c1,c2;
    int x,y;

    printf("Please input A's score and codename (e.g., 87:A): ");
    scanf("%d:%c",&x,&c1);

    printf("Please input B's score and codename (e.g., 76:B): ");
    scanf("%d:%c",&y,&c2);

    printf("Output A's code and score: \n");
    printf("%c:%d\n",c1,x);

    printf("Output B's code and score: \n");
    printf("%c:%d\n",c2,y);
}
    

Program Execution Result:

Please input A's score and code: 87:A
Please input B's score and code: 76:B
Output A's code and score:
A:87
Output B's code and score:
B:76
    

Knowledge Expansion

1. Escape Characters

The escape character "\n" has been mentioned, which is a newline.

Another common escape character is "\t", which horizontally tabs to the next output field.

【Example 1-5】Using escape characters to control output effect.

#include "stdio.h"
main()
{
    printf("%d\t%d\t%d\n",1,2,3);
}
    

Program Execution Result is shown in the figure:

1       2       3
    

2. Formatted Output (Advanced)

Previously, we learned the general format of the output statement: printf("format string"[, output list]);

Commonly used format string representations are as follows:

• “%5d”: Outputs a signed decimal integer. Field width is 5. If the number is too long, it uses its actual width. If shorter, spaces are padded to the left. “%ld” indicates a long integer.
• “%-5d”: Outputs a signed decimal integer. Field width is 5. If shorter, spaces are padded to the right.
• “%6.1f”: Outputs a signed decimal float/double. Field width is 6, including one decimal place. If shorter, spaces are padded to the left.
• “%-7.2f”: Outputs a signed decimal float/double. Field width is 7, including two decimal places. If shorter, spaces are padded to the right.

【Example 1-6】Use of the Type Conversion Character 'd'.

#include 
main()
{
    int a=123;
    long b=123456;

    /* Output value of int type data 'a' using four different formats */
    printf("a=%d,a=%5d,a=%-5d,a=%2d\n",a,a,a,a);

    /* Output value of long type data 'b' using four different formats */
    printf("b=%ld,b=%8ld,b=%-8ld,b=%2ld\n",b,b,b,b);

    printf("a=%ld\n",a); // Output int type data 'a' using %ld
    printf("b=%d\n",b); // Output long type data 'b' using %d
}
    

Program Execution Result:

a=123,a=  123,a=123  ,a=123
b=123456,b=  123456,b=123456  ,b=123456
a=123
b=123456
    

Analysis of Example 1-6

• a=%d: Outputs 123 according to its actual length.
• a=%5d: 123 takes 3 columns, so two spaces are padded to the front. Output:   123.
• a=%-5d: 123 takes 3 columns, with a "-" sign, so two spaces are padded to the right. Output: 123  .
• a=%2d: 123 takes 3 columns, so it outputs according to its actual width. Output: 123.
• b=%ld: Outputs 123456 according to its actual length. Output: 123456.
• b=%8ld: 123456 takes 6 columns, so two spaces are padded to the front. Output:   123456.
• b=%-8ld: 123456 takes 6 columns, with a "-" sign, so two spaces are padded to the right. Output: 123456  .
• b=%2ld: 123456 takes 6 columns, so it outputs according to its actual width. Output: 123456.
• a=%ld: Outputs a=123. (Correctly uses %ld for an int)
• b=%d: Outputs b=123456. (Correctly uses %d for a long, though %ld is preferred)

【Example 1-7】Use of the Type Conversion Character 'f'.

#include "stdio.h"
main()
{
    float f=123.456;
    double d1,d2;
    d1=111111.11111111;
    d2=222222.22222222;

    printf("f=%f,f=%12f,f=%12.2f,f=%-12.2f,f=%.0f,f=%.2f\n",f,f,f,f,f,f);
    printf("d1+d2=%f\n",d1+d2);
}
    

Analysis:

• f=%f: Keeps 6 decimal places. Output is 123.456001 (due to float precision, the last digit is 1).
• f=%12f: Total width 12, 6 decimal places (default). Total digits are 3(int) + 1(dot) + 6(dec) = 10. Two spaces are padded to the left.
• f=%12.2f: Total width 12, two decimal places (rounding up/down). Total digits 3(int) + 1(dot) + 2(dec) = 6. Six spaces are padded to the left. Output:       123.46.
• f=%-12.2f: Same as above, but spaces are padded to the right. Output: 123.46      .
• f=%.0f: Integer part only (0 decimal places, rounding). Output: 123.
• f=%.2f: Actual width, 2 decimal places (rounding). Output: 123.46.

3. Single Character Input and Output

(1) Single Character Output — putchar() Function

【Example 1-8】Format and usage of the putchar() function.

#include "stdio.h"
main()
{
    char ch1,ch2,ch3;
    ch1='S';
    ch2='u';
    ch3='n';

    putchar(ch1); putchar(ch2);  // Output ch1, ch2 values
    putchar(ch3); putchar('\n'); // Output ch3 value and newline

    putchar(ch1); putchar('\n'); // Output ch1 value and newline

    putchar('u'); putchar('\n'); // Output character 'u' and newline

    putchar(ch3); putchar('\n'); // Output ch3 value and newline
}
    

Program Execution Result:

Sun
S
u
n
    

(2) Single Character Input — getchar() Function

【Example 1-9】Format and usage of the getchar() function.

#include "stdio.h"
main()
{
    char ch;

    printf("Please input a character: ");
    ch=getchar();           // Input a character from the keyboard and assign it to variable ch

    putchar(ch);putchar('\n');  // Output the value of ch and a newline

    putchar(getchar());     // Input a character from the keyboard and output it
    putchar('\n');
}
    

4. String Constants

A string constant is a sequence of characters enclosed in double quotes. For example, "How do you do." is a string constant. In the C language, the system automatically appends a null character '\0' at the end of every string constant as a marker for the end of the string.

Please note the difference between a character constant and a string constant. For instance, 'a' is a character constant, occupying one byte in memory; while "a" is a string constant, occupying two bytes of storage space, one of which is used to store '\0'. Two consecutive double quotes "" is also a string constant, called the "null string," but it still occupies one byte of storage space for '\0'.

【Example 1-10】Which of the following data is a string constant?

(A) 'A'      (B) "house"      (C) How do you do      (D) $asd

Practice Exercise

【Example 1-10】If the variables are of type float, and you use the statement scanf("%f%f%f", &a, &b, &c); to assign 10.0 to a, 22.0 to b, and 33.0 to c, which of the following four answers (A, B, C, D) is incorrect?

Please write a complete C program to make the execution result as follows:

A. 10<Enter> 22<Enter> 33

B. 10.0,22.0,33.0<Enter>

C. 10.0<Enter> 22.0 33.0<Enter>

D. 10 22<Enter> 33<Enter>

#include "stdio.h"
main()
{
    float a,b,c;
    scanf("%f%f%f", &a, &b, &c);
    printf("a=%.1f,b=%.1f,c=%.1f\n", a, b, c);
}
        

Practice Exercise

【Example 1-11】What is the reason why the input function scanf(“%d”,k) cannot correctly obtain a value for the float type variable k?

【Example 1-12】Read the following program. If the input data format is: 12,34, what is the correct output result ( )?

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

A. a+b=46      B. Syntax Error      C. a+b=12      D. Undefined Value

Practice Exercise

【Example 1-13】Given the following program, if the required values for x1, x2, y1, y2 are 10, 20, A, B, respectively, what is the correct data input? Please complete the program by adding printf() statements after the given program segment.

// Reference Program:
#include "stdio.h"
main()
{
    int x1, x2;
    char y1, y2;
    scanf("%d%d", &x1,&x2);
    scanf("%c%c", &y1,&y2);
    printf("x1=%d,x2=%d\n", x1,x2);
    printf("y1=%c,y2=%c\n", y1,y2);
}
    

Task 2: Calculation of Total Score and Average Score

I. Task Presentation

A class took an exam. Now, the scores of several students need to be inputted into the computer, their average and total scores calculated, and then outputted as required.

II. Specific Implementation

#include "stdio.h"
main()
{
    int x,y,z;
    float sum,avg;      // Define float variables sum and avg

    printf("Please input the scores of three students: ");
    scanf("%d%d%d",&x,&y,&z); /* Input */

    sum=x+y+z;          // Assign the value of x+y+z to a variable sum
    avg=sum/3;          // Assign the value of sum/3 to variable avg

    printf("Please output the total score and average score of the three students: ");
    printf("sum=%.2f,avg=%.2f\n",sum,avg); // Output sum and avg values (2 decimal places)
}
    

Program Execution Result:

Please input the scores of three students 78 81 93
Please output the total score and average score of the three students sum=252.00,avg=84.00
    

III. Relevant Knowledge

(I) Arithmetic Operations and Arithmetic Expressions

1. Five Basic Arithmetic Operators

+ (Addition), - (Subtraction), * (Multiplication), / (Division), % (Modulo/Remainder)

Here, it is necessary to specifically mention:

(1) Regarding the Division Operation /

• The C language stipulates that when two integers are divided, the quotient is an integer, and the fractional part is truncated (discarded). Example: 5/2=2.
• If the quotient is negative, the direction of rounding varies by system. However, most systems adopt the "truncate toward zero" principle. Example: -5/3=-1.

(2) Regarding the Modulo/Remainder Operation %

• Both operands are required to be integer data, otherwise an error occurs. Examples: 5%3=2, 3%5=3, 5%(-3)=2, -5%3=-2.
• However, 5.2%3 is a syntax error.

(I) Arithmetic Operations and Arithmetic Expressions

2. Expressions and Arithmetic Expressions

(1) Concept of an Expression

A sequence of operands (constants, variables, and functions) connected by operators and parentheses, conforming to the C language syntax rules, is called an expression.

A single constant or variable can be regarded as a special case of an expression. Expressions consisting of single constants or variables are called simple expressions; others are called complex expressions.

(2) Concept of an Arithmetic Expression

The operators in the expression are all arithmetic operators. Examples: 3+2*5, (x+y)/2+3, 5%2+3 are all arithmetic expressions.

3. Operator Precedence and Associativity

(1) The precedence of arithmetic operators is: first *, /, %, then +, -.

(2) Operations within parentheses are performed first.

(I) Arithmetic Operations and Arithmetic Expressions

4. Data Type Conversion

High    double ← float
        ↑
        long
        ↑
        unsigned
        ↑
Low     int ← char,short
    

Example:

• (double)x is equivalent to (double)(x) // Convert the value of variable x to double type
• (int)(x+y) // Convert the result of x+y to int type
• (float)7/2 is equivalent to (float)(7)/2 // Convert 7 to float type, then divide by 2 (=3.5)
• (float)(7/2) // Convert the result of integer division 7/2 (which is 3) to float type (3.0)

(II) Assignment Operations and Assignment Expressions

1. Assignment Operation

The assignment symbol "=" is the assignment operator, and its function is to assign the value of an expression to a variable.

The general form of the assignment operator is: Variable = Assignment_Expression

Examples of incorrect usage:

• 5=x is incorrect
• x+y=z is also incorrect. (Left side must be a modifiable lvalue - a variable)

2. Compound Assignment Operations

x+=3      is equivalent to x=x+3
x+=5+8    is equivalent to x=x+(5+8)
x*=y+2    is equivalent to x=x*(y+2)
x/=x+y    is equivalent to x=x/(x+y)
x/=8      is equivalent to x=x/8
x%=7      is equivalent to x=x%7
x%=(4-2)  is equivalent to x=x%(4-2)
    

【Example 1-16】Read the following program.

#include "stdio.h"
main()
{
    int x,y,z;
    float a,b,c; // Define three single-precision float variables a,b,c

    x=1; // Assign 1 to variable x, so x's value is 1
    y=2;
    z=3;

    a=1.1; // Assign 1.1 to variable a, so a's value is 1.1
    b=2.1;
    c=3.5;

    x=x+y+z; // Assign x+y+z value to variable x. x = (1+2+3), so x=6
    printf("x=%d\n",x);

    y*=y+1; // Assign y*(y+1) to y. y = 2*(2+1), so y=6
    printf("y=%d\n",y);

    z=(int)a%(int)b; // z=1%2, so z=1
    printf("z=%d\n",z);

    a+=a+b+c; // a=a+(a+b+c). a = 1.1+(1.1+2.1+3.5), so a=9.9
    printf("a=%f\n",a);
}
    

Program Execution Result:

x=6
y=6
z=1
a=9.900000
    

(III) C Language Unique Operations and Operators

1. Increment (++) and Decrement (--) Operations

Function: The increment operation increases the value of a single variable by 1; the decrement operation decreases it by 1.

Usage and Rules:

Increment and decrement operators have two forms of usage:

1) Prefix Operation: The operator is placed before the variable: ++x or --x.

First, the variable's value is incremented (or decremented) by 1, and then the changed value participates in the subsequent expression calculation.

#include "stdio.h"
main()
{
    int x=2,y,z;
    printf("x=%d\n",x); // Output: x=2

    y=++x; // x is incremented first (x=3), then assigned to y (y=3)
    printf("x=%d y=%d\n",x,y); // Output: x=3 y=3

    ++x; // x is incremented (x=4)
    printf("x=%d\n",x); // Output: x=4

    y=++x+2; // x is incremented first (x=5), then x+2 (5+2=7), then assigned to y (y=7)
    printf("x=%d y=%d\n",x,y); // Output: x=5 y=7

    z=--x; // x is decremented first (x=4), then assigned to z (z=4)
    printf("x=%d z=%d\n",x,z); // Output: x=4 z=4

    --x; // x is decremented (x=3)
    printf("x=%d\n",x); // Output: x=3
}
    

Postfix Operation — Variable++ and Variable--

2) Postfix Operation: The operator is placed after the variable: x++ or x--.

First, the variable's *original value* participates in the expression calculation, and then the variable's value is incremented (or decremented) by 1.

#include "stdio.h"
main()
{
    int x=2,y,z;
    printf("x=%d\n",x); // Output: x=2

    y=x++; // x's original value (2) is assigned to y, then x is incremented (x=3)
    printf("x=%d y=%d\n",x,y); // Output: x=3 y=2

    x++; // x is incremented (x=4)
    printf("x=%d\n",x); // Output: x=4

    y=(x++)+2; // x's original value (4) + 2 = 6 is assigned to y, then x is incremented (x=5)
    printf("x=%d y=%d\n",x,y); // Output: x=5 y=6

    z=x--; // x's original value (5) is assigned to z, then x is decremented (x=4)
    printf("x=%d z=%d\n",x,z); // Output: x=4 z=5

    x--; // x is decremented (x=3)
    printf("x=%d\n",x); // Output: x=3
}
    

Practice Exercise

【Example 1-19】Input the radius of a circle, and calculate its area and circumference. Use π = 3.14.

// C Code
#include "stdio.h"
main()
{
    float r,s,c;

    printf("Please input the radius of the circle r: ");
    scanf("%f",&r);

    s=3.14*r*r;
    c=2*3.14*r;

    printf("The area of the circle s is: %f\n",s);
    printf("The circumference of the circle c is: %f\n",c);
}
    

Program Execution Result (Input r: 1):

Please input the radius of the circle r: 1
The area of the circle s is: 3.140000
The circumference of the circle c is: 6.280000
    

Symbolic Constants

#define PI 3.14

This defines a symbolic constant PI with a value of 3.14. The naming rules for symbolic constants are the same as for variable names, but conventionally, symbolic constants are represented by uppercase letters.

#include "stdio.h"
#define PI 3.14 // Define a symbolic constant PI with a value of 3.14
main()
{
    float r,s,c;

    printf("Please input the radius of the circle r: ");
    scanf("%f",&r);

    s=PI*r*r;
    c=2*PI*r;

    printf("The area of the circle s is: %f\n",s);
    printf("The circumference of the circle c is: %f\n",c);
}
    

【Example 1-20】Input the lengths of the three sides of a triangle, and calculate its perimeter and area.

    #include "stdio.h"
    #include "math.h" // For the square root function sqrt()
    main()
    {
            int a,b,c;
            float cc,l,s; // cc for perimeter, l for semi-perimeter, s for area

            printf("Please input the lengths of the three sides a, b, c: ");
            scanf("%d%d%d",&a,&b,&c);

            cc = a + b + c;
            l  = cc / 2.0f; // use 2.0f to ensure float division

            s = sqrt(l * (l - a) * (l - b) * (l - c));

            printf("The perimeter of the triangle cc is: %f\n", cc);
            printf("The area of the triangle S is: %f\n", s);
    }
        

Program Execution Result (Input: 3 4 5):

Please input the lengths of the three sides a,b,c: 3 4 5
The perimeter of the triangle cc is: 12.000000
The area of the triangle S is: 6.000000
    

【Example 1-21】Find the real roots of the equation ax² + bx + c = 0. Please input a, b, c from the keyboard, where a ≠ 0 and b² - 4ac > 0.

#include "stdio.h"
#include "math.h"
main()
{
    float a,b,c,disc,x1,x2;

    printf("Please input the coefficients a, b, c of the equation: ");
    scanf("%f%f%f",&a,&b,&c); // Input a, b, c

    disc=b*b-4*a*c;           // Calculate the discriminant and assign it to disc

    x1=(-b+sqrt(disc))/(2*a); // Calculate the roots
    x2=(-b-sqrt(disc))/(2*a);

    printf("The roots of the equation are x1=%f, x2=%f\n",x1,x2); // Output x1, x2 values
}
    

Program Execution Result (Input: 1 2 1):

Please input the coefficients a,b,c of the equation: 1 2 1
The roots of the equation are x1=-1.000000, x2=-1.000000
    

Computational Approximation for Quadratic Equations: Iterative Root Finding

Numerical Approximation Approach

We'll use a simple incremental search method:

1. Define a range of x values to test (e.g., from -1000 to 1000)
2. For each x value, calculate f(x) = ax² + bx + c
3. Look for x values where f(x) is close to zero (within a small tolerance)
4. The x values that produce f(x) ≈ 0 are approximate roots of the equation

#include "stdio.h"
#include "math.h"

main()
{
    float a, b, c;
    float x, fx, tolerance = 0.01;  // Tolerance for approximation
    float start = -100.0, end = 100.0, step = 0.001;  // Search parameters
    int found = 0;

    printf("Please input the coefficients a, b, c of the equation: ");
    scanf("%f%f%f", &a, &b, &c);

    printf("Searching for roots in range [%.1f, %.1f] with step %.3f\n", start, end, step);

    // Iterative search for roots
    for(x = start; x <= end; x += step) {
        fx = a*x*x + b*x + c;  // Calculate f(x) = ax^2 + bx + c

        if(fabs(fx) < tolerance) {  // Check if f(x) is close to 0
            printf("Approximate root found at x = %.3f, f(%.3f) = %.6f\n", x, x, fx);
            found = 1;
        }
    }

    if(!found) {
        printf("No roots found within the specified range and tolerance.\n");
        printf("Try adjusting the range or tolerance parameters.\n");
    }
}
    

Program Execution Result (For equation: x² + 2x + 1 = 0):

Please input the coefficients a, b, c of the equation: 1 2 1
Searching for roots in range [-100.0, 100.0] with step 0.001
Approximate root found at x = -1.000, f(-1.000) = 0.000000
    

【Example 1-22】Input a lowercase letter from the keyboard, and output the letter in uppercase form and its corresponding ASCII code.

#include "stdio.h"
main()
{
    char c1,c2; // Define two character variables c1, c2

    printf("Please input a lowercase letter: ");
    scanf("%c",&c1); // Input the lowercase letter

    c2=c1-32; // Calculate the corresponding uppercase letter

    // Output the original letter and its corresponding uppercase letter and ASCII value
    printf("Original letter is %c, Uppercase letter is %c\n",c1,c2);
    // To output the ASCII code, cast the char to an int with %d format specifier
    printf("ASCII code of %c is %d, ASCII code of %c is %d\n", c1, c1, c2, c2);
}
    

Program Execution Result (Input: a):

Please input a lowercase letter: a
Original letter is a, Uppercase letter is A
ASCII code of a is 97, ASCII code of A is 65