String handling using library functions - Step-by-Step Execution

Concept Flow - String handling using library functions

Start

↓

Declare strings

↓

Call library function

↓

Function processes strings

↓

Return result

↓

Use result (print, compare, etc.)

↓

End

This flow shows how C string library functions take strings as input, process them, and return results used in the program.

Execution Sample

C

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

int main() {
  char str1[] = "Hello";
  char str2[] = "World";
  int result = strcmp(str1, str2);
  printf("Compare result: %d\n", result);
  return 0;
}

This code compares two strings using strcmp and prints the comparison result.

Execution Table

Step	Action	Input Values	Function Called	Function Result	Program Output
1	Declare str1	str1 = "Hello"	N/A	N/A	N/A
2	Declare str2	str2 = "World"	N/A	N/A	N/A
3	Call strcmp	str1 = "Hello", str2 = "World"	strcmp(str1, str2)	Negative value (e.g., -15)	N/A
4	Print result	result = -15	printf	N/A	Compare result: -15
5	Return from main	N/A	N/A	N/A	Program ends

💡 Program ends after printing the comparison result.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
str1	undefined	"Hello"	"Hello"	"Hello"	"Hello"
str2	undefined	undefined	"World"	"World"	"World"
result	undefined	undefined	undefined	Negative value (e.g., -15)	Negative value (e.g., -15)

Key Moments - 2 Insights

Why does strcmp return a negative number instead of just true or false?

strcmp returns a negative, zero, or positive number to show if the first string is less than, equal to, or greater than the second string. See step 3 in execution_table where result is a negative value meaning str1 < str2.

Why do we use char arrays for strings instead of just char variables?

Strings in C are arrays of characters ending with a special '\0' character. Each char variable holds one character only. See steps 1 and 2 where str1 and str2 are arrays holding multiple characters.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'result' after step 3?

A -15

B 0

C 15

D Undefined

💡 Hint

Check the 'Function Result' column in row for step 3.

At which step is the output 'Compare result: -15' printed?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'Program Output' column to find when the output appears.

If str1 was "World" and str2 was "Hello", what would strcmp likely return?

A A negative number

B Zero

C A positive number

D An error

💡 Hint

strcmp returns positive if first string is greater than second; see explanation in key_moments.

Concept Snapshot

C strings are char arrays ending with '\0'.
Use library functions like strcmp, strcpy, strlen.
strcmp compares strings and returns <0, 0, or >0.
strcpy copies one string to another.
strlen returns string length excluding '\0'.
Always include <string.h> to use these functions.

Full Transcript

This example shows how C handles strings using library functions. We declare two strings as char arrays: str1 with "Hello" and str2 with "World". Then we call strcmp to compare them. strcmp returns a negative number because "Hello" is less than "World" alphabetically. We store this result in the variable 'result'. Next, we print the result using printf, showing "Compare result: -15". Finally, the program ends. Key points: strcmp returns a number indicating order, not just true/false. Strings are arrays of chars ending with a null character. This trace helps visualize each step and variable change.

Practice

1. Which library function in C is used to find the length of a string (excluding the null character)?

easy

A. strcpy()

B. strcat()

C. strcmp()

D. strlen()

Solution

1. Step 1: Understand the purpose of strlen()

   The function strlen() counts characters in a string until it reaches the null terminator '\0', giving the string length.

2. Step 2: Compare with other functions

   strcpy() copies strings, strcmp() compares strings, strcat() concatenates strings, so they do not measure length.

3. Final Answer:

   strlen() -> Option D

4. Quick Check:

   Length function = strlen() [OK]

Hint: Length means count chars before '\0', use strlen() [OK]

Common Mistakes:

• Confusing strcpy() with strlen()
• Using strcmp() to get length
• Forgetting null terminator in length count

2. Which of the following is the correct syntax to copy string src into dest using a standard library function?

easy

A. strcpy(src, dest);

B. strcpy(dest, src);

C. strcpy(dest);

D. strcpy(src);

Solution

1. Step 1: Recall strcpy() function signature

   strcpy() takes two arguments: destination string first, then source string to copy from.

2. Step 2: Check each option

   Only strcpy(dest, src); matches the correct order and argument count.

3. Final Answer:

   strcpy(dest, src); -> Option B

4. Quick Check:

   Copy syntax = strcpy(dest, src) [OK]

Hint: Destination first, source second in strcpy() [OK]

Common Mistakes:

• Swapping source and destination arguments
• Using only one argument
• Missing semicolon or parentheses

3. What will be the output of the following C code?

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

int main() {
    char str1[20] = "Hello";
    char str2[] = "World";
    strcat(str1, str2);
    printf("%s", str1);
    return 0;
}

medium

A. HelloWorld

B. Hello World

C. WorldHello

D. Hello

Solution

1. Step 1: Understand strcat() behavior

   strcat() appends the second string to the end of the first string without adding spaces.

2. Step 2: Analyze the code output

   str1 starts as "Hello", str2 is "World". After strcat(str1, str2), str1 becomes "HelloWorld".

3. Final Answer:

   HelloWorld -> Option A

4. Quick Check:

   Concatenate without space = HelloWorld [OK]

Hint: strcat joins strings directly, no spaces added [OK]

Common Mistakes:

• Expecting a space between strings
• Confusing strcat() with strcpy()
• Not allocating enough space in destination

4. Identify the error in the following code snippet:

#include <string.h>

int main() {
    char s1[5] = "Data";
    char s2[] = "Science";
    strcat(s1, s2);
    return 0;
}

medium

A. Missing #include <stdio.h>

B. strcat() syntax is incorrect

C. s1 array size is too small for concatenation

D. s2 should be declared as char *s2

Solution

1. Step 1: Check array sizes for strcat()

   s1 has size 5, holding "Data" plus '\0'. Concatenating "Science" (7 chars) will overflow s1.

2. Step 2: Verify other options

   strcat() syntax is correct, #include <stdio.h> is not mandatory here, and s2 declaration is valid as array.

3. Final Answer:

   s1 array size is too small for concatenation -> Option C

4. Quick Check:

   Buffer overflow risk = s1 too small [OK]

Hint: Destination array must fit combined strings [OK]

Common Mistakes:

• Ignoring buffer size for strcat()
• Assuming strcat() adds space automatically
• Confusing array and pointer declarations

5. Given two strings char s1[] = "apple"; and char s2[] = "apricot";, which library function call correctly checks if s1 comes before s2 in dictionary order?

hard

A. strcmp(s1, s2) < 0

B. strcmp(s1, s2) > 0

C. strcmp(s2, s1) < 0

D. strcmp(s2, s1) > 0

Solution

1. Step 1: Understand strcmp() return values

   strcmp() returns a negative value if first string is lexicographically less than second, zero if equal, positive if greater.

2. Step 2: Apply to given strings

   "apple" comes before "apricot", so strcmp(s1, s2) returns negative, meaning strcmp(s1, s2) < 0 is true.

3. Final Answer:

   strcmp(s1, s2) < 0 -> Option A

4. Quick Check:

   Lex order check = strcmp(s1, s2) < 0 [OK]

Hint: strcmp() negative means first string is before second [OK]

Common Mistakes:

• Reversing argument order in strcmp()
• Checking for > 0 instead of < 0
• Assuming strcmp() returns boolean