#include <stdio.h>
#include <string.h>

//        Escolha uma opcao:
//          1. Validar CPF
//          2. Validar ISBN
//          3. sair
//        Exemplo: 1
//        enter
//        Digite o CPF
//        enter
//        digite 3 para sair

int validarCPF(char *cpf) {
    int soma = 0, resto, i, j;

    // (11 digitos)
    if (strlen(cpf) != 11) {
        printf("Erro: O CPF deve conter exatamente 11 digitos.\n");
        return 0;
    }

    
    for (i = 0; i < 9; i++) {
        soma += (cpf[i] - '0') * (10 - i);
    }
    resto = (soma * 10) % 11;
    if (resto == 10) resto = 0;

    if (resto != (cpf[9] - '0')) {
        return 0; // CPF invalido
    }

    // Segundo digito
    soma = 0;
    for (i = 0; i < 10; i++) {
        soma += (cpf[i] - '0') * (11 - i);
    }
    resto = (soma * 10) % 11;
    if (resto == 10) resto = 0;

    if (resto != (cpf[10] - '0')) {
        return 0; // CPF invalido
    }

    return 1; // CPF valido
}

int validarISBN(char *isbn) {
    int soma = 0, i;

    //(10 digitos)
    if (strlen(isbn) != 10) {
        printf("Erro: O ISBN deve conter exatamente 10 digitos.\n");
        return 0;
    }

    for (i = 0; i < 9; i++) {
        soma += (isbn[i] - '0') * (i + 1);
    }

    //  digito de controle
    char digitoControle = (soma % 11 == 10) ? 'X' : (soma % 11) + '0';
    if (isbn[9] != digitoControle) {
        return 0; // ISBN invalido
    }

    return 1; // ISBN valido
}

int main() {
    char cpf[12], isbn[11];
    int opcao;

    while (1) {
        printf("\n=== Validacao de CPF e ISBN ===\n");
        printf("1. Validar CPF\n");
        printf("2. Validar ISBN\n");
        printf("3. Sair\n");
        printf("Escolha uma opcao: ");
        scanf("%d", &opcao);

        switch (opcao) {
            case 1:
                printf("Digite o CPF (somente numeros): ");
                scanf("%s", cpf);
                if (validarCPF(cpf)) {
                    printf("CPF valido!\n");
                } else {
                    printf("CPF invalido!\n");
                }
                break;

            case 2:
                printf("Digite o ISBN (somente numeros, sem hifens): ");
                scanf("%s", isbn);
                if (validarISBN(isbn)) {
                    printf("ISBN valido!\n");
                } else {
                    printf("ISBN invalido!\n");
                }
                break;

            case 3:
                printf("Saindo...\n");
                return 0;

            default:
                printf("Opção invalida! Tente novamente.\n");
        }
    }

    return 0;
}
by

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#include <stdio.h>
int main()
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    scanf("%s", name);
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    return 0;
    
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About C

C language is one of the most popular general-purpose programming language developed by Dennis Ritchie at Bell laboratories for UNIX operating system. The initial release of C Language was in the year 1972. Most of the desktop operating systems are written in C Language.

Key features:

  • Structured Programming
  • Popular system programming language
  • UNIX, MySQL and Oracle are completely written in C.
  • Supports variety of platforms
  • Efficient and also handle low-level activities.
  • As fast as assembly language and hence used as system development language.

Syntax help

Loops

1. If-Else:

When ever you want to perform a set of operations based on a condition if-else is used.

if(conditional-expression) {
   // code
} else {
   // code
}

You can also use if-else for nested Ifs and if-else-if ladder when multiple conditions are to be performed on a single variable.

2. Switch:

Switch is an alternative to if-else-if ladder.

switch(conditional-expression) {    
case value1:    
 // code    
 break;  // optional  
case value2:    
 // code    
 break;  // optional  
...    
    
default:     
 // code to be executed when all the above cases are not matched;    
}

3. For:

For loop is used to iterate a set of statements based on a condition.

for(Initialization; Condition; Increment/decrement){  
  // code  
}

4. While:

While is also used to iterate a set of statements based on a condition. Usually while is preferred when number of iterations are not known in advance.

while(condition) {  
 // code 
}

5. Do-While:

Do-while is also used to iterate a set of statements based on a condition. It is mostly used when you need to execute the statements atleast once.

do {
  // code 
} while (condition);

Arrays

Array is a collection of similar data which is stored in continuous memory addresses. Array values can be fetched using index. Index starts from 0 to size-1.

Syntax

One dimentional Array:

data-type array-name[size];

Two dimensional array:

data-type array-name[size][size];

Functions

Function is a sub-routine which contains set of statements. Usually functions are written when multiple calls are required to same set of statements which increases re-usuability and modularity.

Two types of functions are present in C

  1. Library Functions:

Library functions are the in-built functions which are declared in header files like printf(),scanf(),puts(),gets() etc.,

  1. User defined functions:

User defined functions are the ones which are written by the programmer based on the requirement.

How to declare a Function

return_type function_name(parameters);

How to call a Function

function_name (parameters)

How to define a Function

return_type function_name(parameters) {  
  //code
}