Lesson 12.3. Structures and pointers

Structure pointers

Structure pointers use the same syntax as variables:

struct vector v;
struct vector * address = &v;

The pointer contains the address of the first field of the structure. When we increment a structure pointer, the pointer increase by the size of the structure (which is not necessarily equal to the sum of the field sizes).

Field access

The access to the fields of the structure can be done with the classical syntax of pointers respecting the priority of the operators :

(*ptr).field

But there is a more user-friendly syntax that uses the -> operator:

ptr->field

This syntax is to be preferred, but can only be used with a structure pointer.

Note: this syntax is important because it is widely used in C++ for class pointers. A class in C++ is an extension of the structures in C that contains functions in addition to fields.

Example

The following code creates a structure pointer and assigns a value to each field using the two syntaxes presented above:

// Declare a vector and a vector pointer
struct vector vec;
struct vector * ptr = &vec;

// Standard pointers
(*ptr).x = 1.2;
// Structure pointer (recommended)
ptr->y = 2.4;

Exercises

Exercise 1

Write the code for the function homothety() which performs a homothety of vector passed by reference (pointer). The second parameter of the function is the homothety factor. As a reminder, the homothety of a vector \( \vec{v} \) with factor \(h\) is given by :

$$ \vec{V} = h \times \vec{v} $$

Here is the expected output:

before: x=1.2 y=3.4
After : x=2.4 y=6.8

#include <stdio.h> // Vector structure struct vector { double x,y; }; // Make a homothety of the vector v of factor h void homothety(struct vector* v, double h); int main(void) { // Vector struct vector vec = {1.2 , 3.4}; // Display the vector before the homothety printf ("Before: x=%.1f y=%.1f\n", vec.x, vec.y); // Homothety of v by factor 2 // COMPLETE HERE // Display the vector after the homothety printf ("After : x=%.1f y=%.1f\n", vec.x, vec.y); return 0; } // Make a homothety of the vector v of factor h // x <= h*x // y <= h*y void homothety(struct vector* v, double h) { // COMPLETE HERE }

Exercise 2

The following code is provided which retrieves the current local time from the timeinfo structure (type struct tm). Referring to the documenation of the tm structure, complete the program so that it displays the time.

// Source : http://www.cplusplus.com/reference/ctime/localtime/
// Get local time from timeinfo
time_t rawtime;
struct tm * timeinfo;

time (&rawtime);
timeinfo = localtime (&rawtime);

Here is a possible display:

15:49 29s

#include <stdio.h> #include <time.h> int main(void) { // Source : http://www.cplusplus.com/reference/ctime/localtime/ // Get local time from timeinfo time_t rawtime; struct tm * timeinfo; time (&rawtime); timeinfo = localtime (&rawtime); // Display current time (on the replit server) // COMPLETE HERE return 0; }

Quiz

A structure pointer points to ...

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Which syntaxes allow to access the field of a structure pointer?

struct article * ptr;

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Which syntaxes allow to access the field of a structure pointer?

struct article * ptr;

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When incrementing a structure pointer ...

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See also

• Learn C programming
• Lesson 1.1. History of C programming language
• Lesson 1.2. First program
• Lesson 1.3. Compilation
• Lesson 1.4. Preprocessor directives
• Lesson 1.5. Which compiler to choose?
• Lesson 1.6. Flowchart
• Lesson 2.1. Types and variables
• Lesson 2.2. Integers
• Lesson 2.3. Floating point numbers
• Lesson 2.4. Characters
• Lesson 2.5. Variables initialization
• Lesson 2.6. Ariane flight 501
• Lesson 3.1. Arithmetic operators
• Lesson 3.2. modulo
• Lesson 3.3. Operators and types
• Lesson 3.4. Type casting
• Lesson 3.5. Bitwise operators
• Lesson 3.6. Bitwise operators details
• Lesson 3.7. Shifting operators
• Lesson 3.8. Assignment operators
• Lesson 3.9. Increment and decrement operators
• Lesson 3.10. Comparison operators
• Lesson 3.11. Logical operators
• Lesson 3.12. Operator precedence
• Lesson 4.1. printf
• Lesson 4.2. scanf
• Lesson 4.3. putchar
• Lesson 5.1. Conditional branching (if...else)
• Lesson 5.2. Nested if and indentation
• Lesson 5.3. Checking intervals
• Lesson 5.4. Ternary Operator ?:
• Lesson 5.5. switch..case statement
• Lesson 5.6. Break keyword in switch..case
• Lesson 6.1. do..while loop in C
• Lesson 6.2. While loop in C
• Lesson 6.3. For loop in C
• Lesson 6.4. How to choose the right loop in C ?
• Lesson 6.5. Exercices on loops in C
• Lesson 7.1. Bit masking
• Lesson 7.2. Bit masking, how to clear a bit in C?
• Lesson 7.3. Bit masking, how to set a bit in C?
• Lesson 7.4. Bit masking, how to toggle a bit in C?
• Lesson 7.5. Bit masking, how to check a single bit in C?
• Lesson 7.6. Overview of bit masking
• Lesson 8.1. Basic syntax of C functions
• Lesson 8.2. Calling C functions
• Lesson 8.3. Void keyword
• Lesson 8.4. Return keyword
• Lesson 8.5. Variable scope
• Lesson 8.6. Global variables
• Lesson 8.7. Static variables
• Lesson 8.8. Random numbers in C
• Lesson 8.9. Mathematical functions in C
• Lesson 9.1. Syntax of C arrays
• Lesson 9.2. Array initialization
• Lesson 9.3. Multidimensional arrays
• Lesson 9.4. How C arrays are stored in memory?
• Lesson 9.5. Arrays and functions
• Lesson 9.6. Exercises on C arrays
• Lesson 10.1. Strings in C
• Lesson 10.2. Null terminated string
• Lesson 10.3. The string.h library
• Lesson 10.4. String and functions
• Lesson 11.1. Introduction to pointers in C
• Lesson 11.2. Syntax of C pointers
• Lesson 11.3. Dynamic memory allocation
• Lesson 11.4. Incrementing pointers
• Lesson 11.5. Pointers as function argument in C
• Lesson 12.1. Introduction to structures in C
• Lesson 12.2. Properties of structures in C
• Lesson 12.4. Structures and functions
• Lesson 13.1. Recursive functions in C
• Lesson 13.2. Depth of recursive functions
• Lesson 13.2. Mutual recursion
• Lesson 14.1. Additional exercises

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