01
The followingcannot be defined as a user identifier:
A) man
B) _0
C) _int
D) sizeof
Answer: D
Explanation:
Among the given options, sizeof is a keyword in C language used to calculate the number of bytes occupied by a data type or variable. Since keywords cannot be used as user identifiers,
The naming rules for C language identifiers are:
1. Composed of valid characters
Can only contain the following characters:
Letters A-Z, a-z
Digits 0-9
Underscore _
[Note] Cannot start with a digit
2. Case sensitive
C language is strictly case-sensitive
3. Cannot use keywords
Keywords in C language cannot be used as identifiers
4. Length limit
Standard C specifies that the length of identifiers must support at least 31 characters (C99 standard), but the specific length may vary by compiler.
02 (Common Mistake)
Given that i, j, k are int type variables, if the input from the keyboard is: 1,2,3<Enter>, making i’s value 1, j’s value 2, and k’s value 3, which of the following input statements iscorrect?
A) scanf(“%2d%2d%2d”,&i,&j,&k);
B) scanf(“%d %d %d”,&i,&j,&k);
C) scanf(“%d,%d,%d”,&i,&j,&k);
D) scanf(“i=%d,j=%d,k=%d”,&i,&j,&k);
Answer: C
Explanation:
For option A: %2d indicates reading a 2-digit integer, but the input 1,2,3 consists of single-digit numbers and has no spaces.
For option B: %d %d %d requires input to be space-separated (like 1 2 3), but the input is 1,2,3 (comma-separated).
For option C: “%d,%d,%d” strictly matches the input format of 1,2,3 (comma-separated).
For option D: It requires the input format to be i=1,j=2,k=3, but the input is 1,2,3 (without prefixes like i=, j=, etc.).
03
In the following program segment, the number of “*” output is
char *s= “\037\tcac”;
for( *s !=’\0′; s++) printf(” * “);
A) 9
B) 5
C) 6
D) 7
Answer: B
Explanation:
The actual memory representation of the string is:
\037 (1 byte)
\t (1 byte)
c (1 byte)
a (1 byte)
c (1 byte)
\0 (null terminator, not counted)
Total length = 5 valid characters
04 (Common Mistake)
Read the following program:
point(char *p) {p+=3;}
main()
{ char b[4]={‘a’,’b’,’c’,’d’}, *p=b;
point(p);
printf(“%c\n”,*p);
}
The output of the program is
A) a
B) b
C) c
D) d
Answer: A
Explanation:
The function internally moves the pointer p 3 positions forward (p now points to b[3], which is ‘d’); but the key point is that the function parameter is passed by value, so the modifications inside the function do not affect the external p.
The final pointer direction: after the function call, the external p still points to b[0] (because the modification inside the function does not affect the external p).
Passing pointer by value
Characteristics: The function receives a copy of the pointer (i.e., the value of the pointer variable, which is a memory address). The function can modify the data pointed to by the pointer, but cannot modify the pointer itself (i.e., cannot make the external pointer point to a new address). Passing pointer value *p can only modify data, not change the pointer.
Passing pointer by address
Characteristics: The function receives the address of the pointer (i.e., int **p, a pointer to a pointer). The function can modify the data pointed to by the pointer and can also modify the external pointer itself (making it point to a new address). Passing pointer address **p can modify data and change the pointer.
05
Let int x=11; then the value of the expression (x++ * 1/3) is
A) 3
B) 4
C) 11
D) 12
Answer: A
Explanation: Omitted
06
The following options cancorrectly define a one-dimensional array:
A) int a[5]={0,1,2,3,4,5};
B) char a[]={0,1,2,3,4,5};
C) char a={‘A’,’B’,’C’};
D) int a[5]=”0123″;
Answer: B
Explanation:
For option A: The array size is 5, but the initialization list has 6 elements, exceeding the array capacity.
For option B: Omitting the array size, the compiler will automatically infer the size as 6 based on the initialization list.
For option C: a is declared as a single char variable, but attempts to initialize multiple characters with {}. This is a syntax error (a single char variable cannot be initialized with a list).
For option D: “0123” is a string (char[] type) and cannot be directly assigned to an int array.
07
Read the following program segment:
int x,y;
x=13;
y=5;
printf(“%d”,x%=(y/=2));
The output of this program segment is
A) 3
B) 2
C) 1
D) 0
Answer: C
Explanation: Omitted
08
After executing the following program, the value of sum is
main()
{ int i , sum;
for(i=1;i<6;i++) sum+=i;
printf(“%d\n”,sum);}
A) 15
B) 14
C) Uncertain
D) 0
Answer: C
Explanation:
The variable sum is uninitialized, its value is uncertain. It is best to assign an initial value when defining variables to develop good programming habits.
09
Among the following statements, the one that iscorrect is
A) Input items can be a real number, e.g., scanf(“%f”,3.5);
B) Input can be correctly done without input items, e.g., scanf(“a=%d,b=%d”);
C) When inputting a real number, the format control part can specify the number of decimal places, e.g., scanf(“%4.2f”,&f);
D) When inputting data, the variable address must be specified, e.g., scanf(“%f”,&f);
Answer: D
Explanation:
For option A: The second parameter of scanf must be the address of a variable, using & to get the address.
For option B: scanf requires the variable address to store the input data, and no variable address is provided here.
For option C: The format specifier of scanf cannot specify precision (like %4.2f).
For option D: scanf must receive the address of a variable (&f) to correctly store the input value.
Introduction to the scanf function
The scanf function is used in C language to read data from standard input (such as keyboard), belonging to the standard input-output library <stdio.h>.
Its core function is to parse input data according to the specified format and store the results in variables. It is necessary to pass the variable address (&a), otherwise it will lead to undefined behavior. By default, data is separated by spaces, tabs, or newlines during input.
Advanced usage of scanf function
Specify input width:
To prevent buffer overflow
For example, scanf(“%9s”,str);
Skip certain inputs:
%*d indicates reading but not storing that number
For example, scanf(“%d%*d%d”,&a,&b);
Match specific characters
For example, scanf(“%d/%d”,&a,&b);
10
Read the following program segment:
main()
{
char a1=’M’,a2=’m’;
printf(“%c\n”,(a1, a2));
}
Among the following statements, the one that iscorrect is
A) The program outputs the uppercase letter M
B) The program outputs the lowercase letter m
C) Insufficient format specifier, compilation error
D) The program generates an error message at runtime
Answer: B
Explanation:
Comma expression:
Form: (expression1, expression2, …, expressionn).
Rule: Calculated from left to right, the final result is the value of the last expression.
Note: The expressions in parentheses are calculated in order, and the values of the involved variables are dynamically updated.