String to Integer atoi in DSA C - Build from Scratch

String to Integer atoi

📖 Scenario: Imagine you are building a simple calculator app that takes user input as text. You need to convert the typed number strings into actual numbers to perform calculations.

🎯 Goal: Build a small program that converts a string representing a number into an integer using the atoi logic.

📋 What You'll Learn

Create a string variable with a number as text

Create a variable to hold the converted integer

Write code to convert the string to an integer manually (without using library atoi)

Print the converted integer

💡 Why This Matters

🌍 Real World

Converting user input strings into numbers is common in calculators, form inputs, and data parsing.

💼 Career

Understanding how to manually convert strings to integers helps in debugging, writing parsers, and working with low-level data processing.

1

Create the input string

Create a character array called str and initialize it with the string "1234".

DSA C

char str[] = "1234";  // Your code here

Hint

Use double quotes to create a string in C.

2

Create the integer variable

Create an integer variable called result and initialize it to 0.

DSA C

char str[] = "1234";
int result = 0;  // Your code here

Hint

Initialize result to zero before conversion.

3

Convert string to integer

Write a for loop using variable i to iterate over str until the null character '\0'. Inside the loop, update result by multiplying it by 10 and adding the integer value of str[i] minus '0'.

DSA C

char str[] = "1234";
int result = 0;

// Your code here

Hint

Each character digit can be converted to an integer by subtracting '0'.

4

Print the converted integer

Write a printf statement to print the value of result.

DSA C

char str[] = "1234";
int result = 0;

for (int i = 0; str[i] != '\0'; i++) {
    result = result * 10 + (str[i] - '0');
}

// Your code here

Hint

Use printf("%d\n", result); to print the integer.