C Programming Cheat Sheet

CSE240 Homework 4 Concepts & Patterns

🧠 Programming Methodology

The Three-Step Process

1. INSTRUCTION
Read what the homework says to do

2. TRANSLATION
Convert requirement to C syntax

3. WHY
Understand the reasoning behind the choice

Key Insight: Instruction order ≠ Implementation order. Always think about logical flow, not comment order.

⚡ Essential Function Design Pattern

int myFunction() {
    // 1. VALIDATE INPUTS FIRST
    if (somethingWrong) {
        return errorCode;
    }
    
    // 2. PREPARE DATA
    convertInputsToProperFormats();
    
    // 3. PERFORM MAIN OPERATION
    doTheActualWork();
    
    // 4. CLEAN UP / ORGANIZE
    sortOrOrganizeResults();
    
    // 5. REPORT SUCCESS
    return successCode;
}
            

Remember: Always validate before doing work. Check conditions BEFORE attempting operations.

📊 Structs & Arrays of Structs

Basic Struct Syntax

struct studentRecord {
    char studentName[MAX_NAME_LENGTH];
    char major[MAX_NAME_LENGTH];
    schoolYear schoolYear;
    unsigned int studentID;
};

// Create array of structs
struct studentRecord list[MAX_STUDENTS];
            

Accessing Struct Elements

// Access elements using dot notation
list[i].studentName
list[i].major
list[i].schoolYear
list[i].studentID

// Copy strings into struct
strcpy(list[count].studentName, studentName_input);

// Assign direct values
list[count].studentID = studentID_input;
            

🔢 Enums (Enumeration Types)

typedef enum { 
    freshman = 0, 
    sophomore, 
    junior, 
    senior 
} schoolYear;
            

String to Enum Conversion

schoolYear yearEnum;
if (strcmp(schoolYear_input, "freshman") == 0) {
    yearEnum = freshman;
}
else if (strcmp(schoolYear_input, "sophomore") == 0) {
    yearEnum = sophomore;
}
// ... etc
            

Enum to String Conversion

switch (list[i].schoolYear) {
    case freshman:
        printf("freshman\n");
        break;
    case sophomore:
        printf("sophomore\n");
        break;
    // ... etc
}
            

🔤 String Operations

✅ CORRECT

// Compare strings
if (strcmp(str1, str2) == 0) {
    // strings are equal
}

// Copy strings
strcpy(destination, source);
                    

❌ WRONG

// Can't use == with strings!
if (str1 == str2) {
    // This compares addresses!
}

// Can't assign strings directly!
destination = source;
                    

strcmp() Return Values

0 → strings are identical
> 0 → first string comes after second alphabetically
< 0 → first string comes before second alphabetically

🔄 Loop Patterns

Basic Array Traversal

// Loop through all existing elements
for (int i = 0; i < count; i++) {
    // Process list[i]
}
            

Bubble Sort Pattern

// Nested loops for bubble sort
for (int i = 0; i < count - 1; i++) {
    for (int j = 0; j < count - 1 - i; j++) {
        if (strcmp(list[j].studentName, list[j + 1].studentName) > 0) {
            // Swap using temp variable
            temp = list[j];
            list[j] = list[j + 1];
            list[j + 1] = temp;
        }
    }
}
            

Why count - 1? If you have 5 elements, you only need 4 comparisons (pairs: 0-1, 1-2, 2-3, 3-4)

✅ Input Validation Patterns

Essential Validation Checks

// 1. Check if array/list is full
if (count >= MAX_STUDENTS) {
    return 2; // or appropriate error code
}

// 2. Check for duplicates
for (int i = 0; i < count; i++) {
    if (strcmp(list[i].name, newName) == 0) {
        return 0; // duplicate found
    }
}

// 3. Check if list is empty (for display functions)
if (count == 0) {
    printf("List is empty.\n");
    return;
}
            

📁 File I/O Patterns

Reading Files

FILE* file;
file = fopen(fileName, "rb"); // "rb" = read binary

// Always check if file exists!
if (file == NULL) {
    printf("%s not found\n", fileName);
    return;
}

// Read data in same order it was saved
fread(&count, sizeof(count), 1, file);
fclose(file);
            

Writing Files

FILE* file;
file = fopen(fileName, "wb"); // "wb" = write binary

// Save data in specific order
fwrite(&count, sizeof(count), 1, file);
fclose(file);
            

Critical: Read and write data in the EXACT same order, or your program will crash!

🎯 Common Return Value Patterns

// Standard return codes for add functions
return 0; // Error: item already exists
return 1; // Success: item added
return 2; // Error: list is full
            

How Calling Functions Use Return Values

int result = addSort(name, major, year, id);
if (result == 0)
    printf("Student is already on the list!\n");
else if (result == 1)
    printf("Student successfully added!\n");
else
    printf("Unable to add. List is full!\n");
            

💡 Key Programming Insights

count tracks how many items exist, also points to next empty spot
Arrays are zero-indexed: first element is [0], last is [count-1]
Always validate inputs before performing operations
String operations require special functions (strcmp, strcpy)
Enum conversion requires explicit if/else or switch statements
File I/O order matters: read in same sequence as write
Struct swapping: use temp variable to swap entire structs

⚠️ Common Mistakes to Avoid

❌ Don't Do This

Use == to compare strings
Assign strings with = operator
Forget to check if list is full
Try to add before validating
Use count as loop limit when it can change
Forget to increment count after adding

✅ Do This Instead

Use strcmp() for string comparison
Use strcpy() for string copying
Check count >= MAX_SIZE first
Validate all inputs before processing
Use count-1 for loop limits
Always count++ after successful add

Keep this cheat sheet handy for future C programming assignments!

Remember: Validate → Prepare → Process → Organize → Return