Expanded the function to work with negative balance changes.

src/CustomerData.txt

@@ -3,4 +3,4 @@ ID=1234
 forename=Max
 Surname=Mustermann
 password=example
-balance=1000
+balance=2000

src/lineReplacer.h

@@ -0,0 +1,6 @@
+#include <stdio.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <string.h>
+
+void replaceLineInFile(const char* file_name, int line, const char* new_line); //replaces the line at "line" on the file "file_name", with the new line "new_line".

src/updateCustomerAccountBalance.c

@@ -1,7 +1,8 @@
 #include "updateCustomerAccountBalance.h"
 #include "currentCustomerAccountBalance.c"
+#include "lineReplacer.h"
 
-void deleteLineFromFile(const char* file_name, int line, const char* new_line){
+void replaceLineInFile(const char* file_name, int line, const char* new_line){
     FILE* file = fopen(file_name, "r");
     if (file == NULL) {
         printf("Error opening file!\n");
@@ -39,10 +40,10 @@ void replaceBalanceInString(float replacementBalance, int currentLine) {
     char balance_as_string[MAX_LENGTH];
     sprintf(balance_as_string, "%g", replacementBalance); //converts replacement balance to string
     strcat(newBalanceLine, balance_as_string);
-    deleteLineFromFile("CustomerData.txt",currentLine,newBalanceLine);
+    replaceLineInFile("CustomerData.txt",currentLine,newBalanceLine);
 }
 
-bool updateAvailableAccountBalance(int user_id, float changeInBalance){
+bool updateAvailableAccountBalance(int user_id, float changeInBalance, bool positive){
 
     bool keep_reading = true;
     float availableBalance = 0;
@@ -71,19 +72,19 @@ bool updateAvailableAccountBalance(int user_id, float changeInBalance){
             fgets(buffer, MAX_LENGTH, file);
             strcpy(balance_String, buffer);
             currentLine+=4;
-            availableBalance = fetchBalanceFromBalanceString(balance_String);
-            
+            availableBalance = fetchBalanceFromBalanceString(balance_String); 
             keep_reading = false;
         }
 
     }
     fclose(file);
-    replaceBalanceInString(availableBalance+changeInBalance,currentLine);
+    float newBalance = (positive) ? availableBalance+changeInBalance : availableBalance-changeInBalance;
+    replaceBalanceInString(newBalance,currentLine);
 }
 
 int main(){
 
-    updateAvailableAccountBalance(1234,500);
+    updateAvailableAccountBalance(1234,500, true);
     return 0;
 
 }

src/updateCustomerAccountBalance.h

@@ -4,6 +4,8 @@
 #include <string.h>
 
 #define MAX_LENGTH 100
-bool updateAvailableAccountBalance(int user_id, float changeInBalance);
-void deleteLineFromFile(const char* file_name, int line, const char* new_line);
+
+
+bool updateAvailableAccountBalance(int user_id, float changeInBalance, bool positive);
+
 void replaceBalanceInString(float replacementBalance, int currentLine);