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Introduction to
C Language

C language
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Read it right next to your computer.
Book version bitter C
Read it right next to your computer.
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Read it right next to your computer.
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Read it right next to your computer.
Book version bitter C
learn through sufferingC Language
learn through sufferingC Language

String manipulation functions

Conversion to numbers
C has various functions available for processing strings.
By using them effectively, you can freely manipulate strings.

The `atoi` function assigns the result of converting a string to a number to a variable.
Here's how to use the atoi function.
Please note that you need to include stdlib.h to use the atoi function.

Source code 
variable = atoi(文字列array名);

The following program is an example of using the atoi function to convert a number.

Source code 
#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    char str[] = "145";
    int suuti = atoi(str);
    printf("%d\n", suuti);
    return 0;
}

The output of this program is as follows:

Execution results
145

The `atoi` function can also convert signed numbers with plus or minus signs.
If a non-numeric string is specified, it will be converted to 0.
"When converting to a real number, use the atof function."It's the same way to use.
String copy
You can use the `strcpy` function to copy strings.
The following is how to use the strcpy function.
Note that you need to include string.h to use the strcpy function.

Source code 
strcpy(copy先文字列array名, copy元文字列array名);

Originally, this function was designed for copying between string arrays.
In practice, it seems to be frequently used for string assignments.
The following program is an example of using the strcpy function to assign a string.

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

int main(void)
{
    char str[10];
    strcpy(str, "MARIO");
    printf("%s\n", str);
    return 0;
}

The output of this program is as follows:

Execution results
MARIO

"As we described the method of substitution one by one in the previous chapter, this approach is simpler."

Furthermore, there's also the `strncpy` function, which copies only the specified number of characters from the beginning.
Here's how to use the strncpy function.

Source code 
strncpy(copy先文字列array名, copy元文字列array名, copyするnumber of characters);

This function simply copies a number of characters equal to the input length.
Sometimes, the copied string may not have an EOS at the end.
"It will continue to display indefinitely if left as is, so it's essential to add EOS."

Source code 
strncpy(copy先文字列array名, copy元文字列array名, copyするnumber of characters);
copy先文字列array名[copyするnumber of characters] = '\0';

The following program is an example of extracting and displaying the first three characters of a string.

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

int main(void)
{
    char str1[] = "MARIO", str2[10];
    strncpy(str2, str1, 3);
    str2[3] = '\0'; /* EOSを付加 */
    printf("%s\n", str2);
    return 0;
}

The output of this program is as follows:

Execution results
MAR


strncpy is dangerous.
You must always add an EOS (End Of String) when using the `strncpy` function.
If you forget the EOS, you're going to get a bunch of irrelevant characters and an error.
You must use it with extreme caution.

String concatenation
If you're just concatenating string literals, a function isn't necessary.
Because string literals are simply concatenated when placed next to each other.
The following program demonstrates string literal concatenation.

Source code 
#include <stdio.h>
int main(void)
{
    char str[] = "DRAGON""QUEST";
    printf("%s\n", str);
    return 0;
}

The output of this program is as follows:

Execution results
DRAGONQUEST

I think you'll see that DRAGON and QUEST are linked.

However, you cannot concatenate strings stored in an array by simply listing the array name.
In that case, you would use the `strcat` function.The following is how to use the strcat function.
"Note that you need to include string.h to use the strcat function."

Source code 
strcat(元の文字列が記憶されたarray, 追加する文字列の記憶されたarray);

The following program is an example of using the strcat function to concatenate strings.

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

int main(void)
{
    char str1[12] = "DRAGON";
    char str2[] = "QUEST";
    strcat(str1, str2);
    printf("%s\n", str1);
    return 0;
}

The output of this program is as follows:

Execution results
DRAGONQUEST

Please be careful when using this function, as the array that stores the original string...
"It's only necessary to consider the length of the original string plus the string being added."
If the character array's length is insufficient, it can cause a buffer overflow error.

C is prone to bugs.
この章に入ってから何度も出てきているように
"C can easily crash if you use the array length incorrectly."
It's a language that's more prone to bugs than other programming languages.

The ultimate string concatenation function
I'd like to introduce you to the ultimate string synthesis function now.
As far as the author's research indicates, this function is not introduced in most introductory texts.
This function is a versatile one that can be used for any string concatenation, so it's worth learning.

The sprintf function is similar to the printf function in that it...
In the case of the sprintf function, the result is stored within an array.
You can freely utilize the various functions of the printf function.

The following is how to use the sprintf function.
"Note that you need to include stdio.h to use the sprintf function."

Source code 
sprintf(結果を記憶するarray, 書式文字列, 各種variable・・・);

The following program is an example of using the sprintf function.

Source code 
#include <stdio.h>

int main(void)
{
    char str[16];
    char str1[12] = "DRAGON";
    char str2[] = "QUEST";
    int i = 3;
    sprintf(str, "%s%s%d\n", str1, str2, i);
    printf(str);
    return 0;
}

The output of this program is as follows:

Execution results
DRAGONQUEST3

This function allows for most string concatenations.

Direct character array
先ほどのプログラムではString Displayに、
printf(str);
"You are passing the character array str directly without using the %s specifier."
そもそも printf functionは文字列を表示するfunctionなので、
You don't need to use the %s specifier to display it.
ただし、文字列中に % が含まれていると、
"Because it malfunctions due to misunderstanding the output format specifier, we use %s in that case."

String input
You can also use the scanf function to input strings, just like numbers.
When entering a string, use the %s specifier with the scanf function.
However, do not include "&" before the array name.

&をつけない理由
実は、&をつけないことには非常に重大な理由があります。
Arrays are essentially pointers in C.
これはC languageの根底をなす重要なことであるため、
I will explain it thoroughly and at length later on.

The following program is an example that displays the input string as is.

Source code 
#include <stdio.h>

int main(void)
{
    char str[32];
    scanf("%s", str);
    printf("%s\n", str);
    return 0;
}

The results of this program's execution are as follows:

Execution results
MARIO　キーボードから入力した文字列
MARIO

Now you can enter strings freely, but there are a few things to keep in mind.

One issue is that providing more input than the number of elements in the array can cause an error.
"You'll experience the same phenomenon as described in Chapter 6, the fear of input errors."

To resolve this issue, specify the number of elements in the character array between % and s.
For example, if the number of elements is 32, specifying %32s will truncate any characters beyond that.
The following program is an example of string truncation.

Source code 
#include <stdio.h>

int main(void)
{
    char str[32];
    scanf("%32s", str);
    printf("%s\n", str);
    return 0;
}

The results of this program's execution are as follows:

Execution results
0123456789012345678901234567890123456789　キーボードから入力した文字列
01234567890123456789012345678901

The string is incomplete, but we've managed to avoid the incident.

Secondly, this method doesn't allow for spaces to be entered.
This is because the space is recognized as a delimiter.
Unfortunately, this issue is difficult to resolve.
Count characters.
Counting strings isn't difficult.
It's simply a matter of counting the number of characters from the beginning of the character array until an EOS appears.
The following program is an example of displaying the number of characters in an input string.

Source code 
#include <stdio.h>

int main(void)
{
    int i;

    char str[256];
    scanf("%s", str);

    for (i = 0; str[i] != '\0'; i++);

    printf("%d\n", i);

    return 0;
}

The results of this program's execution are as follows:

Execution results
ABCDEF　　キーボードから入力した文字列
6

You might find the meaning of the for loop a little confusing.
"I'm just incrementing the variable 'i' until an EOS character appears within the array elements."
"The variable 'i' can increase as much as needed, and since a loop isn't necessary, I'm omitting it."

It's just tedious to have to write a `for` loop every time I need to count the characters in a string.
Therefore, a function called strlen is provided to count the number of characters in a string.
Note that you need to include string.h to use the strlen function.

Source code 
variable = strlen(文字array);

The following program is an example rewritten using the strlen function.

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

int main(void)
{
    int i;

    char str[256];
    scanf("%s", str);

    i = strlen(str);

    printf("%d\n", i);

    return 0;
}

The result will be the same as before.
String comparison
I suspect the following program would work well when comparing the contents of character arrays.

Source code 
str1 == str2;

However, you cannot use the == operator to compare character arrays.
The specific reasons will become clear in the next chapter, but I'll briefly explain them here.
In this case, we are comparing whether the arrays are exactly the same (using the same memory),
It's not comparing the contents of the arrays.

To compare the contents of a character array, you need to compare all elements using a for loop.
The following program is an example of comparing an input string to "DRAGONQUEST".

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

int main(void)
{
    int len, i;
    char str1[256], str2[] = "DRAGONQUEST";

    scanf("%s", str1);

    len = strlen(str2);

    for (i = 0; i < len + 1; i++) {
        if (str1[i] != str2[i]) break;
    }
    
    if (i == len + 1) {
        printf("同じ\n");
    } else {
        printf("違う\n");
    }

    return 0;
}

The output of this program is as follows:

Execution results
DRAGONQUEST　キーボードから入力した文字列
同じ


Execution results
ABCDEF　キーボードから入力した文字列
違う


Execution results
DRAGONQUEST3　キーボードから入力した文字列
違う

"Because string comparisons require the strings to be identical up to the end-of-string (EOS) marker, we compare one character longer than the length of the string being compared."
It's just tedious to write a for loop every time I need to compare strings.
Therefore, there is a function called strcmp to compare strings.
Note that you need to include string.h to use the strcmp function.

Source code 
variable = strcmp(文字array1, 文字array2);

This function returns 0 if the contents of the two character arrays are the same.
The following program is an example of rewriting the previous program using the strcmp function.

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

int main(void)
{
    char str1[256], str2[] = "DRAGONQUEST";

    scanf("%s", str1);

    if (strcmp(str1, str2) == 0) {
        printf("同じ\n");
    } else {
        printf("違う\n");
    }

    return 0;
}

The result will be the same as before.


About This Site

Learning C language through suffering (Kushi C) is
This is the definitive introduction to the C language.
It systematically explains the basic functions of the C language.
The quality is equal to or higher than commercially available books.

Part 0: Program Overview
  1. What is a program?
Chapter 1: The World's Smallest Program
  1. A do-nothing program.
  2. A compiler is a translation software.
  3. Practice Problem 1
Chapter 2: How to Write Programs
  1. Rules of Writing
  2. Conventions of writing
  3. Practice Problem 2
Chapter 3: Displaying on the Screen
  1. String Display
  2. newline character
  3. Practice Problem 3
Chapter 4: Displaying and Calculating Numbers
  1. Display of numbers
  2. Basic calculations
  3. Numeric types
  4. Practice Problem 4
Chapter 5: Numerical Memory and Calculation
  1. Memorizing numbers.
  2. Types of Variables
  3. Type conversion
  4. Number formatting
  5. Practice Problem 5
Chapter 6: Input from the Keyboard
  1. input function
  2. The fear of input
  3. Practice Problem 6
Chapter 7: Comparison and Judgment
  1. Comparative sentences
  2. Comparison operators
  3. Execution of multiple processes
  4. Problem 7
Chapter 8: Performing Case-by-Case Processing
  1. Two cases
  2. Cases with three or more possibilities.
  3. Case by case basis
  4. Practice Problem 8
Chapter 9: Repeating a Fixed Number of Times
  1. Iterative sentence
  2. How Loops Work
  3. Practice Problem 9
Chapter 10: Repeating Without Knowing the Number of Times
  1. Unspecified loop
  2. Input validation
  3. Practice Problem 10
Chapter 11: How to Create Functions
  1. Creating custom functions
  2. Passing a number to a function.
  3. Return a number from a function.
  4. Practice Problem 11
Chapter 12: Variable Lifetime
  1. Variables with limited scope
  2. The last variable standing
  3. Variables that survive within a function
  4. Practice Problem 12
Chapter 13: Handling Multiple Variables at Once
  1. Handling multiple variables collectively.
  2. Arrays
  3. Practice Problem 13
Chapter 14: How to handle strings
  1. Handling Characters
  2. Ways to handle strings
  3. String manipulation functions
  4. Practice Problem 14
Chapter 15: How Pointer Variables Work
  1. The Mechanism of Memory
  2. Variables and Memory
  3. &付けが必要なvariableの正体
  4. Variable to store an address
  5. Let's try using pointer variables.
  6. Passing information through arguments
  7. &The Peculiar Relationship Between Arrays and Pointers
  8. Practice Problem 15
Chapter 16: Combining Multiple Types
  1. Combining variables of different types.
  2. Structure arguments
  3. Array of Structures
  4. Practice Problem 16
Chapter 17: Handling Files
  1. Reading and writing text files
  2. Reading and writing binary files
  3. Drug response
  4. Practice Problem 17
Chapter 18: Macro Functions
  1. Handling Immutable Values
  2. Constants by other means
  3. Implementing simple functions.
  4. Practice Problem 18
Chapter 19: Dynamic Arrays
  1. Create arrays freely.
  2. Practice Problem 19
Chapter 20: Multiple Source Files
  1. Minimal partitioning
  2. The Divide Principle
  3. Practice Problem 20